3rdParty/boost/1.78.0/libs/parameter/doc/index.rst
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ The Boost Parameter Library ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
|(logo)|__
.. |(logo)| image:: ../../../../boost.png :alt: Boost
__ ../../../../index.htm
:Abstract: Use this library to write functions and class templates that can accept arguments by name:
.. parsed-literal::
new_window(
"alert"
, **_width=10**
, **_titlebar=false**
);
smart_ptr<
Foo
, **deleter<Deallocate<Foo> >**
, **copy_policy<DeepCopy>**
> p(new Foo);
Since named arguments can be passed in any order, they are especially useful when a function or template has more than one parameter with a useful default value. The library also supports deduced parameters: that is to say, parameters whose identity can be deduced from their types.
.. @jam_prefix.append(''' project test : requirements <include>. <implicit-dependency>/boost//headers ; ''')
.. @example.prepend(''' #include <boost/parameter.hpp>
namespace test {
BOOST_PARAMETER_NAME(title)
BOOST_PARAMETER_NAME(width)
BOOST_PARAMETER_NAME(titlebar)
BOOST_PARAMETER_FUNCTION(
(int), new_window, tag, (required (title,*)(width,*)(titlebar,*))
)
{
return 0;
}
BOOST_PARAMETER_TEMPLATE_KEYWORD(deleter)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copy_policy)
template <typename T>
struct Deallocate
{
};
struct DeepCopy
{
};
namespace parameter = boost::parameter;
struct Foo
{
};
template <typename T, typename A0, typename A1>
struct smart_ptr
{
smart_ptr(Foo*);
};
}
using namespace test;
int x =
''');
.. @test('compile')
:Authors: David Abrahams, Daniel Wallin
:Contact: [email protected], [email protected]
:organization: BoostPro Computing_
:date: $Date: 2005/07/17 19:53:01 $
:copyright: Copyright David Abrahams, Daniel Wallin 2005-2009. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
.. _BoostPro Computing: http://www.boostpro.com
.. _concepts: http://www.boost.org/more/generic_programming.html#concept
[Note: this tutorial does not cover all details of the library. Please see
also the reference documentation__\ ]
__ reference.html
.. contents:: Table of Contents :depth: 2
.. role:: concept :class: concept
.. role:: vellipsis :class: vellipsis
.. section-numbering::
In C++, arguments_ are normally given meaning by their positions with respect to a parameter_ list: the first argument passed maps onto the first parameter in a function's definition, and so on. That protocol is fine when there is at most one parameter with a default value, but when there are even a few useful defaults, the positional interface becomes burdensome:
.. compound::
Since an argument's meaning is given by its position, we have to choose an (often arbitrary) order for parameters with default values, making some combinations of defaults unusable:
.. parsed-literal::
window* new_window(
char const* name
, **int border_width = default_border_width**
, bool movable = true
, bool initially_visible = true
);
bool const movability = false;
window* w = new_window("alert box", movability);
In the example above we wanted to make an unmoveable window with a default
border_width, but instead we got a moveable window with a
border_width of zero. To get the desired effect, we'd need to write:
.. parsed-literal::
window* w = new_window(
"alert box", **default_border_width**, movability
);
.. compound::
It can become difficult for readers to understand the meaning of arguments at the call site::
window* w = new_window("alert", 1, true, false);
Is this window moveable and initially invisible, or unmoveable and initially visible? The reader needs to remember the order of arguments to be sure.
The author of the call may not remember the order of the arguments either, leading to hard-to-find bugs.
.. @ignore(3)
.. compound::
This library addresses the problems outlined above by associating each
parameter name with a keyword object. Now users can identify arguments by
name, rather than by position:
.. parsed-literal::
window* w = new_window(
"alert box"
, **movable_=**\ false
); // OK!
.. @ignore()
.. compound::
A **deduced parameter** can be passed in any position *without* supplying
an explicit parameter name. It's not uncommon for a function to have
parameters that can be uniquely identified based on the types of arguments
passed. The ``name`` parameter to ``new_window`` is one such
example. None of the other arguments, if valid, can reasonably be
converted to a ``char const*``. With a deduced parameter interface, we
could pass the window name in *any* argument position without causing
ambiguity:
.. parsed-literal::
window* w = new_window(
movable_=false
, **"alert box"**
); // OK!
window* w = new_window(
**"alert box"**
, movable_=false
); // OK!
Appropriately used, a deduced parameter interface can free the user of the
burden of even remembering the formal parameter names.
.. @ignore()
.. compound::
The reasoning we've given for named and deduced parameter interfaces
applies equally well to class templates as it does to functions. Using
the Parameter library, we can create interfaces that allow template
arguments (in this case ``shared`` and ``Client``) to be explicitly named,
like this:
.. parsed-literal::
smart_ptr<
**ownership<shared>**
, **value_type<Client>**
> p;
The syntax for passing named template arguments is not quite as natural as
it is for function arguments (ideally, we'd be able to write
``smart_ptr<ownership = shared, …>``). This small syntactic deficiency
makes deduced parameters an especially big win when used with class
templates:
.. parsed-literal::
// *p and q could be equivalent, given a deduced*
// *parameter interface.*
smart_ptr<**shared**, **Client**> p;
smart_ptr<**Client**, **shared**> q;
.. @ignore(2)
This tutorial shows all the basics—how to build both named- and deduced-parameter interfaces to function templates and class templates—and several more advanced idioms as well.
In this section we'll show how the Parameter library can be used to
build an expressive interface to the Boost Graph library__\ 's
|dfs|_ algorithm. [#old_interface]_
.. Revisit this
After laying some groundwork and describing the algorithm's abstract
interface, we'll show you how to build a basic implementation with keyword
support. Then we'll add support for default arguments and we'll gradually
refine the implementation with syntax improvements. Finally we'll show
how to streamline the implementation of named parameter interfaces,
improve their participation in overload resolution, and optimize their
runtime efficiency.
__ ../../../graph/doc/index.html
.. _dfs: ../../../graph/doc/depth_first_search.html
.. |dfs| replace:: depth_first_search
Most components of the Parameter library are declared in a header named for the component. For example, ::
#include <boost/parameter/keyword.hpp>
will ensure boost::parameter::keyword is known to the compiler. There
is also a combined header, boost/parameter.hpp, that includes most of
the library's components. For the the rest of this tutorial, unless we
say otherwise, you can use the rule above to figure out which header to
#include to access any given component of the library.
.. @example.append(''' using boost::parameter::keyword; ''')
.. @test('compile')
Also, the examples below will also be written as if the namespace alias ::
namespace parameter = boost::parameter;
.. @ignore()
has been declared: we'll write parameter::xxx instead of
boost::parameter::xxx.
The Graph library's |dfs| algorithm is a generic function accepting from one to four arguments by reference. If all arguments were required, its signature might be as follows::
template <
typename Graph
, typename DFSVisitor
, typename Index
, typename ColorMap
>
void
depth_first_search(
Graph const& graph
, DFSVisitor visitor
, typename graph_traits<g>::vertex_descriptor root_vertex
, IndexMap index_map
, ColorMap& color
);
.. @ignore()
However, most of the parameters have a useful default value, as shown in the table below.
.. _parameter table:
.. _default expressions:
.. table:: depth_first_search Parameters
+-----------------+------+-------------------------+------------------------------------+
| Parameter | Data | Type | Default Value |
| Name | Flow | | (if any) |
+=================+======+=========================+====================================+
| ``graph`` | in | Model of | none - this argument is required. |
| | | |IncidenceGraph|_ and | |
| | | |VertexListGraph|_ | |
+-----------------+------+-------------------------+------------------------------------+
| ``visitor`` | in | Model of |DFSVisitor|_ | ``boost::dfs_visitor<>()`` |
+-----------------+------+-------------------------+------------------------------------+
| ``root_vertex`` | in | ``graph``'s vertex | ``*vertices(graph).first`` |
| | | descriptor type. | |
+-----------------+------+-------------------------+------------------------------------+
| ``index_map`` | in | Model of | ``get(boost::vertex_index,graph)`` |
| | | |ReadablePropertyMap|_ | |
| | | with key type := | |
| | | ``graph``'s vertex | |
| | | descriptor and value | |
| | | type an integer type. | |
+-----------------+------+-------------------------+------------------------------------+
| ``color_map`` | in / | Model of | a ``boost::iterator_property_map`` |
| | out | |ReadWritePropertyMap|_ | created from a ``std::vector`` of |
| | | with key type := | ``default_color_type`` of size |
| | | ``graph``'s vertex | ``num_vertices(graph)`` and using |
| | | descriptor type. | ``index_map`` for the index map. |
+-----------------+------+-------------------------+------------------------------------+
.. |IncidenceGraph| replace:: :concept:Incidence Graph
.. |VertexListGraph| replace:: :concept:Vertex List Graph
.. |DFSVisitor| replace:: :concept:DFS Visitor
.. |ReadablePropertyMap| replace:: :concept:Readable Property Map
.. |ReadWritePropertyMap| replace:: :concept:Read/Write Property Map
.. _IncidenceGraph: ../../../graph/doc/IncidenceGraph.html
.. _VertexListGraph: ../../../graph/doc/VertexListGraph.html
.. _DFSVisitor: ../../../graph/doc/DFSVisitor.html
.. _ReadWritePropertyMap: ../../../property_map/doc/ReadWritePropertyMap.html
.. _ReadablePropertyMap: ../../../property_map/doc/ReadablePropertyMap.html
Don't be intimidated by the information in the second and third columns above. For the purposes of this exercise, you don't need to understand them in detail.
The point of this exercise is to make it possible to call
depth_first_search with named arguments, leaving out any
arguments for which the default is appropriate:
.. parsed-literal::
graphs::depth_first_search(g, **color_map_=my_color_map**);
.. @ignore()
To make that syntax legal, there needs to be an object called
“\ color_map_\ ” whose assignment operator can accept a
my_color_map argument. In this step we'll create one such
keyword object for each parameter. Each keyword object will be
identified by a unique keyword tag type.
.. Revisit this
We're going to define our interface in namespace ``graphs``. Since users
need access to the keyword objects, but not the tag types, we'll define
the keyword objects so they're accessible through ``graphs``, and we'll
hide the tag types away in a nested namespace, ``graphs::tag``. The
library provides a convenient macro for that purpose.
We're going to define our interface in namespace graphs. The
library provides a convenient macro for defining keyword objects::
#include <boost/parameter/name.hpp>
namespace graphs {
BOOST_PARAMETER_NAME(graph) // Note: no semicolon
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)
}
.. @test('compile')
The declaration of the graph keyword you see here is equivalent to::
namespace graphs {
namespace tag {
// keyword tag type
struct graph
{
typedef boost::parameter::forward_reference qualifier;
};
}
namespace // unnamed
{
// A reference to the keyword object
boost::parameter::keyword<tag::graph> const& _graph
= boost::parameter::keyword<tag::graph>::instance;
}
}
.. @example.prepend('#include <boost/parameter/keyword.hpp>') .. @test('compile')
It defines a keyword tag type named tag::graph and a keyword object
reference named _graph.
This “fancy dance” involving an unnamed namespace and references is all done
to avoid violating the One Definition Rule (ODR) [#odr]_ when the named
parameter interface is used by function templates that are instantiated in
multiple translation units (MSVC6.x users see this note__).
__ Compiler Can't See References In Unnamed Namespace_
Now that we have our keywords defined, the function template definition
follows a simple pattern using the BOOST_PARAMETER_FUNCTION macro::
#include <boost/parameter/preprocessor.hpp>
namespace graphs {
BOOST_PARAMETER_FUNCTION(
(void), // 1. parenthesized return type
depth_first_search, // 2. name of the function template
tag, // 3. namespace of tag types
(required (graph, *) ) // 4. one required parameter, and
(optional // four optional parameters,
// with defaults
(visitor, *, boost::dfs_visitor<>())
(root_vertex, *, *vertices(graph).first)
(index_map, *, get(boost::vertex_index,graph))
(color_map, *,
default_color_map(num_vertices(graph), index_map)
)
)
)
{
// ... body of function goes here...
// use graph, visitor, index_map, and color_map
}
}
.. @example.prepend(''' #include <boost/parameter/name.hpp>
BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(in(root_vertex))
BOOST_PARAMETER_NAME(in(index_map))
BOOST_PARAMETER_NAME(in_out(color_map))
namespace boost {
template <typename T = int>
struct dfs_visitor
{
};
int vertex_index = 0;
}
''')
.. @test('compile')
The arguments to BOOST_PARAMETER_FUNCTION are:
The return type of the resulting function template. Parentheses around the return type prevent any commas it might contain from confusing the preprocessor, and are always required.
The name of the resulting function template.
The name of a namespace where we can find tag types whose names match the function's parameter names.
The function signature.
Function signatures are described as one or two adjacent parenthesized terms
(a Boost.Preprocessor_ sequence_) describing the function's parameters in the
order in which they'd be expected if passed positionally. Any required
parameters must come first, but the (required … ) clause can be omitted
when all the parameters are optional.
.. _Boost.Preprocessor: ../../../preprocessor/doc/index.html .. _sequence: http://boost-consulting.com/mplbook/preprocessor.html#sequences
.. compound::
Required parameters are given first—nested in a ``(required … )``
clause—as a series of two-element tuples describing each parameter name
and any requirements on the argument type. In this case there is only a
single required parameter, so there's just a single tuple:
.. parsed-literal::
(required **(graph, \*)** )
Since ``depth_first_search`` doesn't require any particular type for its
``graph`` parameter, we use an asterix to indicate that any type is
allowed. Required parameters must always precede any optional parameters
in a signature, but if there are *no* required parameters, the
``(required … )`` clause can be omitted entirely.
.. @example.prepend(''' #include <boost/parameter.hpp>
BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_FUNCTION((void), f, tag,
''')
.. @example.append(') {}') .. @test('compile')
.. compound::
Optional parameters—nested in an ``(optional … )`` clause—are given as a
series of adjacent *three*\ -element tuples describing the parameter name,
any requirements on the argument type, *and* and an expression
representing the parameter's default value:
.. parsed-literal::
(optional
**(visitor, \*, boost::dfs_visitor<>())
(root_vertex, \*, \*vertices(graph).first)
(index_map, \*, get(boost::vertex_index,graph))
(color_map, \*,
default_color_map(num_vertices(graph), index_map)
)**
)
.. @example.prepend(''' #include <boost/parameter.hpp>
namespace boost {
int vertex_index = 0;
template <typename T = int>
struct dfs_visitor
{
};
}
BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(in(root_vertex))
BOOST_PARAMETER_NAME(in(index_map))
BOOST_PARAMETER_NAME(in_out(color_map))
BOOST_PARAMETER_FUNCTION((void), f, tag,
(required (graph, \*))
''')
.. @example.append(') {}') .. @test('compile')
.. compound::
By default, Boost.Parameter treats all parameters as if they were
*forward* `parameters`_, which functions would take in by rvalue reference
and only ``std::forward`` or ``boost::forward`` to other functions. Such
parameters can be ``const`` lvalues, mutable lvalues, ``const`` rvalues,
or mutable rvalues. Therefore, the default configuration grants the most
flexibility to user code. However:
* Users can configure one or more parameters to be *in* `parameters`_,
which can fall into the same categories as *forward* `parameters`_ but
are now passed by ``const`` lvalue reference and so must only be read
from. Continuing from the previous example, to indicate that
``root_vertex`` and ``index_map`` are read-only, we wrap their names
in ``in(…)``.
* Users can configure one or more parameters to be either *out*
`parameters`_, which functions would strictly write to, or *in-out*
`parameters`_, which functions would both read from and write
to. Such parameters can only be mutable lvalues. In the example, to
indicate that ``color_map`` is read-write, we wrap its name in
``in_out(…)``. Note that Boost.Parameter sees no functional
difference between ``out(…)`` and ``in_out(…)``, so you may choose
whichever makes your interfaces more self-documenting.
* Users can configure one or more parameters to be *consume* or
*move-from* `parameters`_, which functions would take in by mutable
rvalue reference and ``std::move`` or ``boost::move`` as the last
access step. Such parameters can only be mutable
rvalues. Boost.Parameter supports wrapping the corresponding names in
``consume(…)`` or ``move_from(…)``.
.. parsed-literal::
BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(**in(root_vertex)**)
BOOST_PARAMETER_NAME(**in(index_map)**)
BOOST_PARAMETER_NAME(**in_out(color_map)**)
In order to see what happens when parameters are bound to arguments that
violate their category constraints, attempt to compile the |compose_cpp|_
test program with either the ``LIBS_PARAMETER_TEST_COMPILE_FAILURE_0``
macro or the ``LIBS_PARAMETER_TEST_COMPILE_FAILURE_1`` macro
``#defined``. You should encounter a compiler error caused by a specific
constraint violation.
.. @example.prepend(''' #include <boost/parameter.hpp>
namespace boost {
int vertex_index = 0;
template <typename T = int>
struct dfs_visitor
{
};
}
''')
.. @example.append(''' BOOST_PARAMETER_FUNCTION((void), f, tag, (required (graph, *)) (optional (visitor, *, boost::dfs_visitor<>()) (root_vertex, *, *vertices(graph).first) (index_map, *, get(boost::vertex_index, graph)) (color_map, *, default_color_map(num_vertices(graph), index_map) ) ) ) { } ''')
.. @test('compile')
.. _parameters: http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters
.. |compose_cpp| replace:: compose.cpp
.. _compose_cpp: ../../test/compose.cpp
When arguments are passed positionally (without the use of keywords), they will be mapped onto parameters in the order the parameters are given in the signature, so for example in this call ::
graphs::depth_first_search(x, y);
.. @ignore()
x will always be interpreted as a graph and y will always be
interpreted as a visitor.
.. compound::
Note that in our example, the value of the graph parameter is used in the
default expressions for ``root_vertex``, ``index_map``, and ``color_map``.
.. parsed-literal::
(required (**graph**, \*) )
(optional
(visitor, \*, boost::dfs_visitor<>())
(root_vertex, \*, \*vertices(**graph**).first)
(index_map, \*, get(boost::vertex_index, **graph**))
(color_map, \*,
default_color_map(num_vertices(**graph**), index_map)
)
)
.. @ignore()
A default expression is evaluated in the context of all preceding
parameters, so you can use any of their values by name.
.. compound::
A default expression is never evaluated—or even instantiated—if an actual
argument is passed for that parameter. We can actually demonstrate that
with our code so far by replacing the body of ``depth_first_search`` with
something that prints the arguments:
.. parsed-literal::
#include <boost/graph/depth_first_search.hpp> // for dfs_visitor
BOOST_PARAMETER_FUNCTION(
(bool), depth_first_search, tag
*…signature goes here…*
)
{
std::cout << "graph=" << graph;
std::cout << std::endl;
std::cout << "visitor=" << visitor;
std::cout << std::endl;
std::cout << "root_vertex=" << root_vertex;
std::cout << std::endl;
std::cout << "index_map=" << index_map;
std::cout << std::endl;
std::cout << "color_map=" << color_map;
std::cout << std::endl;
return true;
}
#include <boost/core/lightweight_test.hpp>
int main()
{
char const\* g = "1";
depth_first_search(1, 2, 3, 4, 5);
depth_first_search(
g, '2', _color_map = '5',
_index_map = "4", _root_vertex = "3"
);
return boost::report_errors();
}
Despite the fact that default expressions such as
``vertices(graph).first`` are ill-formed for the given ``graph``
arguments, both calls will compile, and each one will print exactly the
same thing.
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream>
BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)
''')
.. @example.replace_emphasis(''' , (required (graph, *) (visitor, *) (root_vertex, *) (index_map, *) (color_map, *) ) ''') .. @test('run')
In fact, the function signature is so general that any call to
depth_first_search with fewer than five arguments will match our function,
provided we pass something for the required graph parameter. That might
not seem to be a problem at first; after all, if the arguments don't match the
requirements imposed by the implementation of depth_first_search, a
compilation error will occur later, when its body is instantiated.
There are at least three problems with very general function signatures.
By the time our depth_first_search is instantiated, it has been
selected as the best matching overload. Some other depth_first_search
overload might've worked had it been chosen instead. By the time we see a
compilation error, there's no chance to change that decision.
Even if there are no overloads, error messages generated at instantiation
time usually expose users to confusing implementation details. For
example, users might see references to names generated by
BOOST_PARAMETER_FUNCTION such as
graphs::detail::depth_first_search_with_named_params (or worse—think
of the kinds of errors you get from your STL implementation when you make
a mistake). [#ConceptsTS]_
The problems with exposing such permissive function template signatures
have been the subject of much discussion, especially in the presence of
unqualified calls__. If all we want is to avoid unintentional
argument-dependent lookup (ADL), we can isolate depth_first_search in
a namespace containing no types [#using]_, but suppose we want it to
found via ADL?
__ http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#225
It's usually a good idea to prevent functions from being considered for
overload resolution when the passed argument types aren't appropriate. The
library already does this when the required graph parameter is not
supplied, but we're not likely to see a depth first search that doesn't take a
graph to operate on. Suppose, instead, that we found a different depth first
search algorithm that could work on graphs that don't model
|IncidenceGraph|_? If we just added a simple overload, it would be
ambiguous::
// new overload
BOOST_PARAMETER_FUNCTION((void), depth_first_search, (tag),
(required (graph,*))( … )
)
{
// new algorithm implementation
}
…
// ambiguous!
depth_first_search(boost::adjacency_list<>(), 2, "hello");
.. @ignore()
Predicate Requirements ......................
We really don't want the compiler to consider the original version of
depth_first_search because the root_vertex argument, "hello",
doesn't meet the requirement__ that it match the graph parameter's vertex
descriptor type. Instead, this call should just invoke our new overload. To
take the original depth_first_search overload out of contention, we first
encode this requirement as follows:
__ parameter table_
.. parsed-literal::
struct vertex_descriptor_predicate
{
template <typename T, typename Args>
struct apply
: boost::mpl::if_<
boost::is_convertible<
T
, typename boost::graph_traits<
typename boost::parameter::value_type<
Args
, graphs::graph
>::type
>::vertex_descriptor
>
, boost::mpl::true\_
, boost::mpl::false\_
>
{
};
};
This encoding is an MPL Binary Metafunction Class__, a type with a nested
apply metafunction that takes in two template arguments. For the first
template argument, Boost.Parameter will pass in the type on which we will
impose the requirement. For the second template argument, Boost.Parameter
will pass in the entire argument pack, making it possible to extract the
type of each of the other depth_first_search parameters via the
value_type metafunction and the corresponding keyword tag type. The
result type of the apply metafunction will be equivalent to
boost::mpl::true_ if T fulfills our requirement as imposed by the
boost::is_convertible statement; otherwise, the result will be
equivalent to boost::mpl::false_.
__ ../../../mpl/doc/refmanual/metafunction-class.html
At this point, we can append the name of our metafunction class, in
parentheses, to the appropriate * element of the function signature.
.. parsed-literal::
(root_vertex
, \*(**vertex_descriptor_predicate**)
, \*vertices(graph).first
)
.. @ignore()
Now the original depth_first_search will only be called when the
root_vertex argument can be converted to the graph's vertex descriptor
type, and our example that was ambiguous will smoothly call the new
overload.
We can encode the requirements on other arguments using the same concept; only
the implementation of the nested apply metafunction needs to be tweaked
for each argument. There's no space to give a complete description of graph
library details here, but suffice it to show that the next few metafunction
classes provide the necessary checks.
.. parsed-literal::
struct graph_predicate
{
template <typename T, typename Args>
struct apply
: boost::mpl::eval_if<
boost::is_convertible<
typename boost::graph_traits<T>::traversal_category
, boost::incidence_graph_tag
>
, boost::mpl::if_<
boost::is_convertible<
typename boost::graph_traits<T>::traversal_category
, boost::vertex_list_graph_tag
>
, boost::mpl::true\_
, boost::mpl::false\_
>
>
{
};
};
struct index_map_predicate
{
template <typename T, typename Args>
struct apply
: boost::mpl::eval_if<
boost::is_integral<
typename boost::property_traits<T>::value_type
>
, boost::mpl::if_<
boost::is_same<
typename boost::property_traits<T>::key_type
, typename boost::graph_traits<
typename boost::parameter::value_type<
Args
, graphs::graph
>::type
>::vertex_descriptor
>
, boost::mpl::true\_
, boost::mpl::false\_
>
>
{
};
};
struct color_map_predicate
{
template <typename T, typename Args>
struct apply
: boost::mpl::if_<
boost::is_same<
typename boost::property_traits<T>::key_type
, typename boost::graph_traits<
typename boost::parameter::value_type<
Args
, graphs::graph
>::type
>::vertex_descriptor
>
, boost::mpl::true\_
, boost::mpl::false\_
>
{
};
};
Likewise, computing the default value for the color_map parameter is no
trivial matter, so it's best to factor the computation out to a separate
function template.
.. parsed-literal::
template <typename Size, typename IndexMap>
boost::iterator_property_map<
std::vector<boost::default_color_type>::iterator
, IndexMap
, boost::default_color_type
, boost::default_color_type&
>&
default_color_map(Size num_vertices, IndexMap const& index_map)
{
static std::vector<boost::default_color_type> colors(num_vertices);
static boost::iterator_property_map<
std::vector<boost::default_color_type>::iterator
, IndexMap
, boost::default_color_type
, boost::default_color_type&
> m(colors.begin(), index_map);
return m;
}
The signature encloses each predicate metafunction in parentheses preceded by an asterix, as follows:
.. parsed-literal::
BOOST_PARAMETER_FUNCTION((void), depth_first_search, graphs,
(required
(graph, \*(**graph_predicate**))
)
(optional
(visitor
, \* // not easily checkable
, boost::dfs_visitor<>()
)
(root_vertex
, (**vertex_descriptor_predicate**)
, \*vertices(graph).first
)
(index_map
, \*(**index_map_predicate**)
, get(boost::vertex_index, graph)
)
(color_map
, \*(**color_map_predicate**)
, default_color_map(num_vertices(graph), index_map)
)
)
)
.. @example.prepend(''' #include <boost/parameter.hpp> #include <boost/graph/adjacency_list.hpp> #include <boost/graph/depth_first_search.hpp> #include <boost/graph/graph_traits.hpp> #include <boost/property_map/property_map.hpp> #include <boost/mpl/and.hpp> #include <boost/type_traits/is_convertible.hpp> #include <boost/type_traits/is_integral.hpp> #include <boost/type_traits/is_same.hpp> #include <vector> #include <utility>
BOOST_PARAMETER_NAME((_graph, graphs) graph)
BOOST_PARAMETER_NAME((_visitor, graphs) visitor)
BOOST_PARAMETER_NAME((_root_vertex, graphs) in(root_vertex))
BOOST_PARAMETER_NAME((_index_map, graphs) in(index_map))
BOOST_PARAMETER_NAME((_color_map, graphs) in_out(color_map))
''')
.. @example.append(''' { }
#include <boost/core/lightweight_test.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <utility>
int main()
{
typedef boost::adjacency_list<
boost::vecS, boost::vecS, boost::directedS
> G;
enum {u, v, w, x, y, z, N};
typedef std::pair<int, int> E;
E edges[] = {
E(u, v), E(u, x), E(x, v), E(y, x),
E(v, y), E(w, y), E(w,z), E(z, z)
};
G g(edges, edges + sizeof(edges) / sizeof(E), N);
depth_first_search(g);
depth_first_search(g, _root_vertex = static_cast<int>(x));
return boost::report_errors();
}
''')
.. @test('run')
It usually isn't necessary to so completely encode the type requirements on
arguments to generic functions. However, doing so is worth the effort: your
code will be more self-documenting and will often provide a better user
experience. You'll also have an easier transition to the C++20 standard with
language support for constraints and concepts__.
__ ConceptsTS_
More on Type Requirements .........................
Encoding type requirements onto a function's parameters is essential for
enabling the function to have deduced parameter interface. Let's revisit the
new_window example for a moment:
.. parsed-literal::
window\* w = new_window(
movable_=false
, "alert box"
);
window\* w = new_window(
"alert box"
, movable_=false
);
.. @ignore()
The goal this time is to be able to invoke the new_window function without
specifying the keywords. For each parameter that has a required type, we can
enclose that type in parentheses, then replace the * element of the
parameter signature:
.. parsed-literal::
BOOST_PARAMETER_NAME((name\_, keywords) name)
BOOST_PARAMETER_NAME((movable\_, keywords) movable)
BOOST_PARAMETER_FUNCTION((window\*), new_window, keywords,
(deduced
(required
(name, *(char const\*)*)
(movable, *(bool)*)
)
)
)
{
// ...
}
.. @ignore()
The following statements will now work and are equivalent to each other as well as the previous statements:
.. parsed-literal::
window\* w = new_window(false, "alert box");
window\* w = new_window("alert box", false);
.. @ignore()
To further illustrate deduced parameter support, consider the example of the |def|_ function from Boost.Python_. Its signature is roughly as follows:
.. parsed-literal::
template <
typename Function
, typename KeywordExpression
, typename CallPolicies
>
void def(
// Required parameters
char const\* name, Function func
// Optional, deduced parameters
, char const\* docstring = ""
, KeywordExpression keywords = no_keywords()
, CallPolicies policies = default_call_policies()
);
.. @ignore()
Try not to be too distracted by the use of the term “keywords” in this example: although it means something analogous in Boost.Python to what it means in the Parameter library, for the purposes of this exercise you can think of it as being completely different.
When calling def, only two arguments are required. The association
between any additional arguments and their parameters can be determined by the
types of the arguments actually passed, so the caller is neither required to
remember argument positions or explicitly specify parameter names for those
arguments. To generate this interface using BOOST_PARAMETER_FUNCTION, we
need only enclose the deduced parameters in a (deduced …) clause, as
follows:
.. parsed-literal::
char const*& blank_char_ptr()
{
static char const* larr = "";
return larr;
}
BOOST_PARAMETER_FUNCTION(
(bool), def, tag,
(required (name, (char const\*)) (func,\*) ) // nondeduced
**(deduced**
(optional
(docstring, (char const\*), "")
(keywords
// see [#is_keyword_expression]_
, \*(is_keyword_expression<boost::mpl::_>)
, no_keywords()
)
(policies
, \*(
boost::mpl::eval_if<
boost::is_convertible<boost::mpl::_,char const\*>
, boost::mpl::false\_
, boost::mpl::if_<
// see [#is_keyword_expression]_
is_keyword_expression<boost::mpl::_>
, boost::mpl::false\_
, boost::mpl::true\_
>
>
)
, default_call_policies()
)
)
**)**
)
{
*…*
}
.. @example.replace_emphasis('return true;')
.. @example.prepend(''' #include <boost/parameter.hpp>
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(func)
BOOST_PARAMETER_NAME(docstring)
BOOST_PARAMETER_NAME(keywords)
BOOST_PARAMETER_NAME(policies)
struct default_call_policies
{
};
struct no_keywords
{
};
struct keywords
{
};
template <typename T>
struct is_keyword_expression
: boost::mpl::false_
{
};
template <>
struct is_keyword_expression<keywords>
: boost::mpl::true_
{
};
default_call_policies some_policies;
void f()
{
}
#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/type_traits/is_convertible.hpp>
''')
.. Admonition:: Syntax Note
A ``(deduced …)`` clause always contains a ``(required …)`` and/or an
``(optional …)`` subclause, and must follow any ``(required …)`` or
``(optional …)`` clauses indicating nondeduced parameters at the outer
level.
With the declaration above, the following two calls are equivalent:
.. parsed-literal::
char const\* f_name = "f";
def(
f_name
, &f
, **some_policies**
, **"Documentation for f"**
);
def(
f_name
, &f
, **"Documentation for f"**
, **some_policies**
);
.. @example.prepend(''' int main() { ''')
If the user wants to pass a policies argument that was also, for some
reason, convertible to char const*, she can always specify the parameter
name explicitly, as follows:
.. parsed-literal::
def(
f_name
, &f
, **_policies = some_policies**
, "Documentation for f"
);
.. @example.append('}') .. @test('compile', howmany='all')
The |deduced_cpp|_ and |deduced_dependent_predicate|_ test programs demonstrate additional usage of deduced parameter support.
.. _Boost.Python: ../../../python/doc/index.html
.. |def| replace:: def
.. _def: ../../../python/doc/v2/def.html
.. |deduced_cpp| replace:: deduced.cpp
.. _deduced_cpp: ../../test/deduced.cpp
.. |deduced_dependent_predicate| replace:: deduced_dependent_predicate.cpp
.. _deduced_dependent_predicate: ../../test/deduced_dependent_predicate.cpp
For some algorithms, the return type depends on at least one of the argument
types. However, there is one gotcha to avoid when encoding this return type
while using BOOST_PARAMETER_FUNCTION or other code generation macros. As
an example, given the following definitions::
BOOST_PARAMETER_NAME(x)
BOOST_PARAMETER_NAME(y)
BOOST_PARAMETER_NAME(z)
.. @ignore()
Let our algorithm simply return one of its arguments. If we naïvely implement
its return type in terms of parameter::value_type::
BOOST_PARAMETER_FUNCTION(
(typename parameter::value_type<Args,tag::y>::type), return_y, tag,
(deduced
(required
(x, (std::map<char const*,std::string>))
(y, (char const*))
)
(optional
(z, (int), 4)
)
)
)
{
return y;
}
.. @ignore()
Then using return_y in any manner other than with positional arguments
will result in a compiler error::
std::map<char const*,std::string> k2s;
assert("foo" == return_y(2, k2s, "foo")); // error!
.. @ignore()
The problem is that even though y is a required parameter,
BOOST_PARAMETER_FUNCTION will generate certain overloads for which the
argument pack type Args does not actually contain the keyword tag type
tag::y. The solution is to use SFINAE to preclude generation of those
overloads. Since parameter::value_type is a metafunction, our tool for
the job is lazy_enable_if::
BOOST_PARAMETER_FUNCTION(
(
// Whenever using 'enable_if', 'disable_if', and so on,
// do not add the 'typename' keyword in front.
boost::lazy_enable_if<
typename mpl::has_key<Args,tag::y>::type
, parameter::value_type<Args,tag::y>
>
// Whenever using 'enable_if', 'disable_if', and so on,
// do not add '::type' here.
), return_y, tag,
(deduced
(required
(x, (std::map<char const*,std::string>))
(y, (char const*))
)
(optional
(z, (int), 4)
)
)
)
{
return y;
}
.. @ignore()
For a working demonstration, see |preprocessor_deduced_cpp|_.
.. |preprocessor_deduced_cpp| replace:: preprocessor_deduced.cpp .. _preprocessor_deduced_cpp: ../../test/preprocessor_deduced.cpp
The BOOST_PARAMETER_MEMBER_FUNCTION and
BOOST_PARAMETER_CONST_MEMBER_FUNCTION macros accept exactly the same
arguments as BOOST_PARAMETER_FUNCTION, but are designed to be used within
the body of a class::
BOOST_PARAMETER_NAME(arg1)
BOOST_PARAMETER_NAME(arg2)
struct callable2
{
BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
(void), call, tag, (required (arg1,(int))(arg2,(int)))
)
{
std::cout << arg1 << ", " << arg2;
std::cout << std::endl;
}
};
#include <boost/core/lightweight_test.hpp>
int main()
{
callable2 c2;
callable2 const& c2_const = c2;
c2_const.call(1, 2);
return boost::report_errors();
}
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> using namespace boost::parameter; ''')
.. @test('run')
These macros don't directly allow a function's interface to be separated from its implementation, but you can always forward arguments on to a separate implementation function::
struct callable2
{
BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
(void), call, tag, (required (arg1,(int))(arg2,(int)))
)
{
call_impl(arg1, arg2);
}
private:
void call_impl(int, int); // implemented elsewhere.
};
.. @example.prepend(''' #include <boost/parameter.hpp>
BOOST_PARAMETER_NAME(arg1)
BOOST_PARAMETER_NAME(arg2)
using namespace boost::parameter;
''')
.. @test('compile')
To expose a static member function, simply insert the keyword “static”
before the function name:
.. parsed-literal::
BOOST_PARAMETER_NAME(arg1)
struct somebody
{
BOOST_PARAMETER_MEMBER_FUNCTION(
(void), **static** f, tag, (optional (arg1,(int),0))
)
{
std::cout << arg1 << std::endl;
}
};
#include <boost/core/lightweight_test.hpp>
int main()
{
somebody::f();
somebody::f(4);
return boost::report_errors();
}
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> using namespace boost::parameter; ''')
.. @test('run')
The BOOST_PARAMETER_FUNCTION_CALL_OPERATOR and
BOOST_PARAMETER_CONST_FUNCTION_CALL_OPERATOR macros accept the same
arguments as the BOOST_PARAMETER_MEMBER_FUNCTION and
BOOST_PARAMETER_CONST_MEMBER_FUNCTION macros except for the function name,
because these macros allow instances of the enclosing classes to be treated as
function objects::
BOOST_PARAMETER_NAME(first_arg)
BOOST_PARAMETER_NAME(second_arg)
struct callable2
{
BOOST_PARAMETER_CONST_FUNCTION_CALL_OPERATOR(
(void), tag, (required (first_arg,(int))(second_arg,(int)))
)
{
std::cout << first_arg << ", ";
std::cout << second_arg << std::endl;
}
};
#include <boost/core/lightweight_test.hpp>
int main()
{
callable2 c2;
callable2 const& c2_const = c2;
c2_const(1, 2);
return boost::report_errors();
}
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> using namespace boost::parameter; ''')
.. @test('run')
The lack of a “delegating constructor” feature in C++ (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1986.pdf) limits somewhat the quality of interface this library can provide for defining parameter-enabled constructors. The usual workaround for a lack of constructor delegation applies: one must factor the common logic into one or more base classes.
Let's build a parameter-enabled constructor that simply prints its arguments. The first step is to write a base class whose constructor accepts a single argument known as an |ArgumentPack|_: a bundle of references to the actual arguments, tagged with their keywords. The values of the actual arguments are extracted from the |ArgumentPack| by indexing it with keyword objects::
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)
struct myclass_impl
{
template <typename ArgumentPack>
myclass_impl(ArgumentPack const& args)
{
std::cout << "name = " << args[_name];
std::cout << "; index = " << args[_index | 42];
std::cout << std::endl;
}
};
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> ''')
Note that the bitwise or (“\ |\ ”) operator has a special meaning when
applied to keyword objects that are passed to an |ArgumentPack|\ 's indexing
operator: it is used to indicate a default value. In this case if there is no
index parameter in the |ArgumentPack|, 42 will be used instead.
Now we are ready to write the parameter-enabled constructor interface::
struct myclass : myclass_impl
{
BOOST_PARAMETER_CONSTRUCTOR(
myclass, (myclass_impl), tag
, (required (name,*)) (optional (index,*))
) // no semicolon
};
Since we have supplied a default value for index but not for name,
only name is required. We can exercise our new interface as follows::
myclass x("bob", 3); // positional
myclass y(_index = 12, _name = "sally"); // named
myclass z("june"); // positional/defaulted
.. @example.wrap(''' #include <boost/core/lightweight_test.hpp>
int main() {
''', ' return boost::report_errors(); }')
.. @test('run', howmany='all')
For more on |ArgumentPack| manipulation, see the Advanced Topics_ section.
In this section we'll use Boost.Parameter to build Boost.Python_
's class__ template, whose “signature” is:
.. parsed-literal::
template <
ValueType, BaseList = bases<>
, HeldType = ValueType, Copyable = void
>
class class\_;
.. @ignore()
Only the first argument, ValueType, is required.
.. class: http://www.boost.org/libs/python/doc/v2/class.html#class_-spec
First, we'll build an interface that allows users to pass arguments positionally or by name:
.. parsed-literal::
struct B
{
virtual ~B() = 0;
};
struct D : B
{
~D();
};
class_<
**class_type<B>**
, **copyable<boost::noncopyable>**
> …;
class_<
**D**
, **held_type<std::auto_ptr<D> >**
, **base_list<bases<B> >**
> …;
.. @ignore()
The first step is to define keywords for each template parameter::
namespace boost { namespace python {
BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)
}}
.. @example.prepend('#include <boost/parameter.hpp>') .. @test('compile')
The declaration of the class_type keyword you see here is equivalent to::
namespace boost { namespace python {
namespace tag {
struct class_type; // keyword tag type
}
template <typename T>
struct class_type
: parameter::template_keyword<tag::class_type,T>
{
};
}}
.. @example.prepend('#include <boost/parameter.hpp>') .. @test('compile')
It defines a keyword tag type named tag::class_type and a
parameter passing template named class_type.
The next step is to define the skeleton of our class template, which has three
optional parameters. Because the user may pass arguments in any order, we
don't know the actual identities of these parameters, so it would be premature
to use descriptive names or write out the actual default values for any of
them. Instead, we'll give them generic names and use the special type
boost::parameter::void_ as a default:
.. parsed-literal::
namespace boost { namespace python {
template <
typename A0
, typename A1 = boost::parameter::void\_
, typename A2 = boost::parameter::void\_
, typename A3 = boost::parameter::void\_
>
struct class\_
{
*…*
};
}}
.. @example.prepend('#include <boost/parameter.hpp>') .. @example.replace_emphasis('') .. @test('compile')
Next, we need to build a type, known as a |ParameterSpec|, describing the
“signature” of boost::python::class_. A |ParameterSpec| enumerates the
required and optional parameters in their positional order, along with any
type requirements (note that it does not specify defaults -- those will be
dealt with separately)::
namespace boost { namespace python {
using boost::mpl::_;
typedef parameter::parameters<
required<tag::class_type, boost::is_class<_> >
, parameter::optional<tag::base_list, mpl::is_sequence<_> >
, parameter::optional<tag::held_type>
, parameter::optional<tag::copyable>
> class_signature;
}}
.. @example.prepend(''' #include <boost/parameter.hpp> #include <boost/mpl/is_sequence.hpp> #include <boost/noncopyable.hpp> #include <boost/type_traits/is_class.hpp> #include <memory>
using namespace boost::parameter;
namespace boost { namespace python {
BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)
template <typename B = int>
struct bases
{
};
}}
''')
.. |ParameterSpec| replace:: :concept:ParameterSpec
.. _ParameterSpec: reference.html#parameterspec
.. _binding_intro:
Next, within the body of class_ , we use the |ParameterSpec|\ 's
nested ::bind< … > template to bundle the actual arguments into an
|ArgumentPack|_ type, and then use the library's value_type< … >
metafunction to extract “logical parameters”. value_type< … > is
a lot like binding< … >, but no reference is added to the actual
argument type. Note that defaults are specified by passing it an
optional third argument::
namespace boost { namespace python {
template <
typename A0
, typename A1 = boost::parameter::void_
, typename A2 = boost::parameter::void_
, typename A3 = boost::parameter::void_
>
struct class_
{
// Create ArgumentPack
typedef typename class_signature::template bind<
A0, A1, A2, A3
>::type args;
// Extract first logical parameter.
typedef typename parameter::value_type<
args, tag::class_type
>::type class_type;
typedef typename parameter::value_type<
args, tag::base_list, bases<>
>::type base_list;
typedef typename parameter::value_type<
args, tag::held_type, class_type
>::type held_type;
typedef typename parameter::value_type<
args, tag::copyable, void
>::type copyable;
};
}}
.. |ArgumentPack| replace:: :concept:ArgumentPack
.. _ArgumentPack: reference.html#argumentpack
.. compound::
Revisiting our original examples, ::
typedef boost::python::class_<
class_type<B>, copyable<boost::noncopyable>
> c1;
typedef boost::python::class_<
D
, held_type<std::auto_ptr<D> >
, base_list<bases<B> >
> c2;
.. @example.prepend('''
using boost::python::class_type;
using boost::python::copyable;
using boost::python::held_type;
using boost::python::base_list;
using boost::python::bases;
struct B
{
};
struct D
{
};
''')
we can now examine the intended parameters::
BOOST_MPL_ASSERT((boost::is_same<c1::class_type, B>));
BOOST_MPL_ASSERT((boost::is_same<c1::base_list, bases<> >));
BOOST_MPL_ASSERT((boost::is_same<c1::held_type, B>));
BOOST_MPL_ASSERT((
boost::is_same<c1::copyable, boost::noncopyable>
));
BOOST_MPL_ASSERT((boost::is_same<c2::class_type, D>));
BOOST_MPL_ASSERT((boost::is_same<c2::base_list, bases<B> >));
BOOST_MPL_ASSERT((
boost::is_same<c2::held_type, std::auto_ptr<D> >
));
BOOST_MPL_ASSERT((boost::is_same<c2::copyable, void>));
.. @test('compile', howmany='all')
To apply a deduced parameter interface here, we need only make the type
requirements a bit tighter so the held_type and copyable parameters
can be crisply distinguished from the others. Boost.Python_ does this by
requiring that base_list be a specialization of its bases< … >
template (as opposed to being any old MPL sequence) and by requiring that
copyable, if explicitly supplied, be boost::noncopyable. One easy way
of identifying specializations of bases< … > is to derive them all from
the same class, as an implementation detail:
.. parsed-literal::
namespace boost { namespace python {
namespace detail {
struct bases_base
{
};
}
template <
typename A0 = void, typename A1 = void, typename A2 = void *…*
>
struct bases **: detail::bases_base**
{
};
}}
.. @example.replace_emphasis('') .. @example.prepend(''' #include <boost/parameter.hpp> #include <boost/mpl/is_sequence.hpp> #include <boost/noncopyable.hpp> #include <memory>
using namespace boost::parameter;
using boost::mpl::_;
namespace boost { namespace python {
BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)
}}
''')
Now we can rewrite our signature to make all three optional parameters deducible::
typedef parameter::parameters<
required<tag::class_type, is_class<_> >
, parameter::optional<
deduced<tag::base_list>
, is_base_and_derived<detail::bases_base,_>
>
, parameter::optional<
deduced<tag::held_type>
, mpl::not_<
mpl::or_<
is_base_and_derived<detail::bases_base,_>
, is_same<noncopyable,_>
>
>
>
, parameter::optional<
deduced<tag::copyable>
, is_same<noncopyable,_>
>
> class_signature;
.. @example.prepend(''' #include <boost/type_traits/is_class.hpp> namespace boost { namespace python { ''')
.. @example.append(''' template < typename A0 , typename A1 = boost::parameter::void_ , typename A2 = boost::parameter::void_ , typename A3 = boost::parameter::void_ > struct class_ { // Create ArgumentPack typedef typename class_signature::bind< A0, A1, A2, A3 >::type args;
// Extract first logical parameter.
typedef typename parameter::value_type<
args, tag::class_type
>::type class_type;
typedef typename parameter::value_type<
args, tag::base_list, bases<>
>::type base_list;
typedef typename parameter::value_type<
args, tag::held_type, class_type
>::type held_type;
typedef typename parameter::value_type<
args, tag::copyable, void
>::type copyable;
};
}}
''')
It may seem like we've added a great deal of complexity, but the benefits to our users are greater. Our original examples can now be written without explicit parameter names:
.. parsed-literal::
typedef boost::python::class_<**B**, **boost::noncopyable**> c1;
typedef boost::python::class_<
**D**, **std::auto_ptr<D>**, **bases<B>**
> c2;
.. @example.prepend(''' struct B { };
struct D
{
};
using boost::python::bases;
''')
.. @example.append(''' BOOST_MPL_ASSERT((boost::is_same<c1::class_type, B>)); BOOST_MPL_ASSERT((boost::is_same<c1::base_list, bases<> >)); BOOST_MPL_ASSERT((boost::is_same<c1::held_type, B>)); BOOST_MPL_ASSERT(( boost::is_same<c1::copyable, boost::noncopyable> ));
BOOST_MPL_ASSERT((boost::is_same<c2::class_type, D>));
BOOST_MPL_ASSERT((boost::is_same<c2::base_list, bases<B> >));
BOOST_MPL_ASSERT((
boost::is_same<c2::held_type, std::auto_ptr<D> >
));
BOOST_MPL_ASSERT((boost::is_same<c2::copyable, void>));
''')
.. @test('compile', howmany='all')
At this point, you should have a good grasp of the basics. In this section we'll cover some more esoteric uses of the library.
If you don't like the leading-underscore naming convention used to refer to
keyword objects, or you need the name tag for something other than the
keyword type namespace, there's another way to use BOOST_PARAMETER_NAME:
.. parsed-literal::
BOOST_PARAMETER_NAME(
**(**
*object-name*
**,** *tag-namespace*
**)** *parameter-name*
)
.. @ignore()
Here is a usage example:
.. parsed-literal::
BOOST_PARAMETER_NAME(
(
**pass_foo**, **keywords**
) **foo**
)
BOOST_PARAMETER_FUNCTION(
(int), f,
**keywords**, (required (**foo**, \*))
)
{
return **foo** + 1;
}
int x = f(**pass_foo** = 41);
.. @example.prepend('#include <boost/parameter.hpp>') .. @example.append(''' int main() { return 0; } ''') .. @test('run')
Before you use this more verbose form, however, please read the section on
best practices for keyword object naming__.
__ Keyword Naming_
We've already seen |ArgumentPack|\ s when we looked at
parameter-enabled constructors_ and class templates__. As you
might have guessed, |ArgumentPack|\ s actually lie at the heart of
everything this library does; in this section we'll examine ways to
build and manipulate them more effectively.
__ binding_intro_
The simplest |ArgumentPack| is the result of assigning into a keyword object::
BOOST_PARAMETER_NAME(index)
template <typename ArgumentPack>
int print_index(ArgumentPack const& args)
{
std::cout << "index = " << args[_index];
std::cout << std::endl;
return 0;
}
int x = print_index(_index = 3); // prints "index = 3"
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> ''')
Also, |ArgumentPack|\ s can be composed using the comma operator. The extra
parentheses below are used to prevent the compiler from seeing two separate
arguments to print_name_and_index::
BOOST_PARAMETER_NAME(name)
template <typename ArgumentPack>
int print_name_and_index(ArgumentPack const& args)
{
std::cout << "name = " << args[_name];
std::cout << "; ";
return print_index(args);
}
int y = print_name_and_index((_index = 3, _name = "jones"));
The |compose_cpp|_ test program shows more examples of this feature.
To build an |ArgumentPack| with positional arguments, we can use a
|ParameterSpec|. As introduced described in the section on Class Template Signatures, a |ParameterSpec| describes the positional order of parameters
and any associated type requirements. Just as we can build an |ArgumentPack|
type with its nested ::bind< … > template, we can build an
|ArgumentPack| object by invoking its function call operator:
.. parsed-literal::
parameter::parameters<
required<tag::\ name, is_convertible<_,char const*> >
, optional<tag::\ index, is_convertible<_,int> >
> spec;
char const sam[] = "sam";
int twelve = 12;
int z0 = print_name_and_index(
**spec(** sam, twelve **)**
);
int z1 = print_name_and_index(
**spec(** _index=12, _name="sam" **)**
);
.. @example.prepend(''' namespace parameter = boost::parameter; using parameter::required; using parameter::optional; using boost::is_convertible; using boost::mpl::_; ''')
.. @example.append(''' int main() { return 0; } ''')
.. @test('run', howmany='all')
If we want to know the types of the arguments passed to
print_name_and_index, we have a couple of options. The
simplest and least error-prone approach is to forward them to a
function template and allow it to do type deduction::
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)
template <typename Name, typename Index>
int deduce_arg_types_impl(Name&& name, Index&& index)
{
// we know the types
Name&& n2 = boost::forward<Name>(name);
Index&& i2 = boost::forward<Index>(index);
return index;
}
template <typename ArgumentPack>
int deduce_arg_types(ArgumentPack const& args)
{
return deduce_arg_types_impl(args[_name], args[_index | 42]);
}
.. @example.prepend(''' #include <boost/parameter.hpp> ''')
.. @example.append(''' #include <boost/core/lightweight_test.hpp>
int main()
{
int a1 = deduce_arg_types((_name = "foo"));
int a2 = deduce_arg_types((_name = "foo", _index = 3));
BOOST_TEST_EQ(a1, 42);
BOOST_TEST_EQ(a2, 3);
return boost::report_errors();
}
''')
.. @test('run')
Occasionally one needs to deduce argument types without an extra layer of
function call. For example, suppose we wanted to return twice the value of
the index parameter? In that case we can use the value_type< … >
metafunction introduced earlier__::
BOOST_PARAMETER_NAME(index)
template <typename ArgumentPack>
typename boost::parameter::value_type<ArgumentPack,tag::index,int>::type
twice_index(ArgumentPack const& args)
{
return 2 * args[_index | 42];
}
.. @example.prepend(''' #include <boost/parameter.hpp> ''')
.. @example.append(''' #include <boost/core/lightweight_test.hpp>
int main()
{
int six = twice_index(_index = 3);
BOOST_TEST_EQ(six, 6);
return boost::report_errors();
}
''')
.. @test('run', howmany='all')
Note that if we had used binding< … > rather than value_type< … >, we
would end up returning a reference to the temporary created in the 2 * …
expression.
__ binding_intro_
When a default value is expensive to compute, it would be preferable to avoid
it until we're sure it's absolutely necessary. BOOST_PARAMETER_FUNCTION
takes care of that problem for us, but when using |ArgumentPack|\ s
explicitly, we need a tool other than operator|::
BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)
template <typename ArgumentPack>
std::string f(ArgumentPack const& args)
{
std::string const& s1 = args[_s1];
std::string const& s2 = args[_s2];
typename parameter::binding<
ArgumentPack,tag::s3,std::string
>::type s3 = args[_s3 | (s1 + s2)]; // always constructs s1 + s2
return s3;
}
std::string x = f((
_s1="hello,", _s2=" world", _s3="hi world"
));
.. @example.prepend(''' #include <boost/parameter.hpp> #include <string>
namespace parameter = boost::parameter;
''')
.. @example.append(''' int main() { return 0; } ''')
.. @test('run')
In the example above, the string "hello, world" is constructed despite the
fact that the user passed us a value for s3. To remedy that, we can
compute the default value lazily (that is, only on demand), by using
boost::bind() to create a function object.
.. danielw: I'm leaving the text below in the source, because we might .. want to change back to it after 1.34, and if I remove it now we .. might forget about it.
.. by combining the logical-or (“||”) operator
.. with a function object built by the Boost Lambda_ library: [#bind]_
.. parsed-literal::
typename parameter::binding<
ArgumentPack,tag::s3,std::string
>::type s3 = args[
_s3 **|| boost::bind(
std::plus<std::string>(), boost::ref(s1), boost::ref(s2)
)**
];
.. @example.prepend(''' #include <boost/bind.hpp> #include <boost/ref.hpp> #include <boost/parameter.hpp> #include <string> #include <functional>
namespace parameter = boost::parameter;
BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)
template <typename ArgumentPack>
std::string f(ArgumentPack const& args)
{
std::string const& s1 = args[_s1];
std::string const& s2 = args[_s2];
''')
.. @example.append(''' return s3; }
std::string x = f((_s1="hello,", _s2=" world", _s3="hi world"));
int main()
{
return 0;
}
''')
.. @test('run')
.. .. _Lambda: ../../../lambda/index.html
.. sidebar:: Mnemonics
To remember the difference between ``|`` and ``||``, recall that ``||``
normally uses short-circuit evaluation: its second argument is only
evaluated if its first argument is ``false``. Similarly, in
``color_map[param || f]``, ``f`` is only invoked if no ``color_map``
argument was supplied.
The expression bind(std::plus<std::string>(), ref(s1), ref(s2)) yields a
function object that, when invoked, adds the two strings together. That
function will only be invoked if no s3 argument is supplied by the caller.
.. The expression lambda::var(s1) + lambda::var(s2) yields a
.. function object that, when invoked, adds the two strings
.. together. That function will only be invoked if no s3 argument
.. is supplied by the caller.
By now you should have a fairly good idea of how to use the Parameter library. This section points out a few more-marginal issues that will help you use the library more effectively.
BOOST_PARAMETER_NAME prepends a leading underscore to the names of all our
keyword objects in order to avoid the following usually-silent bug:
.. parsed-literal::
namespace people
{
namespace tag
{
struct name
{
typedef boost::parameter::forward_reference qualifier;
};
struct age
{
typedef boost::parameter::forward_reference qualifier;
};
}
namespace // unnamed
{
boost::parameter::keyword<tag::name>& **name**
= boost::parameter::keyword<tag::name>::instance;
boost::parameter::keyword<tag::age>& **age**
= boost::parameter::keyword<tag::age>::instance;
}
BOOST_PARAMETER_FUNCTION(
(void), g, tag, (optional (name, \*, "bob")(age, \*, 42))
)
{
std::cout << name << ":" << age;
}
void f(int age)
{
:vellipsis:`\
.
.
.
`
g(**age** = 3); // whoops!
}
}
.. @ignore()
Although in the case above, the user was trying to pass the value 3 as the
age parameter to g, what happened instead was that f\ 's age
argument got reassigned the value 3, and was then passed as a positional
argument to g. Since g's first positional parameter is name, the
default value for age is used, and g prints 3:42. Our leading
underscore naming convention makes this problem less likely to occur.
In this particular case, the problem could have been detected if f's age
parameter had been made const, which is always a good idea whenever
possible. Finally, we recommend that you use an enclosing namespace for all
your code, but particularly for names with leading underscores. If we were to
leave out the people namespace above, names in the global namespace
beginning with leading underscores—which are reserved to your C++
compiler—might become irretrievably ambiguous with those in our
unnamed namespace.
In our examples we've always declared keyword objects in (an unnamed namespace within) the same namespace as the Boost.Parameter-enabled functions using those keywords:
.. parsed-literal::
namespace lib {
**BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)**
BOOST_PARAMETER_FUNCTION(
(int), f, tag,
(optional (name,*,"bob")(index,(int),1))
)
{
std::cout << name << ":" << index;
std::cout << std::endl;
return index;
}
}
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> ''') .. @namespace_setup = str(example) .. @ignore()
Users of these functions have a few choices:
Full qualification:
.. parsed-literal::
int x = **lib::**\ f(
**lib::**\ _name = "jill"
, **lib::**\ _index = 1
);
This approach is more verbose than many users would like.
.. @example.prepend(namespace_setup) .. @example.append('int main() { return 0; }') .. @test('run')
Make keyword objects available through using-declarations:
.. parsed-literal::
**using lib::_name;
using lib::_index;**
int x = lib::f(_name = "jill", _index = 1);
This version is much better at the actual call site, but the using-declarations themselves can be verbose and hard to manage.
.. @example.prepend(namespace_setup) .. @example.append('int main() { return 0; }') .. @test('run')
Bring in the entire namespace with a using-directive:
.. parsed-literal::
**using namespace lib;**
int x = **f**\ (_name = "jill", _index = 3);
This option is convenient, but it indiscriminately makes the entire
contents of lib available without qualification.
.. @example.prepend(namespace_setup) .. @example.append('int main() { return 0; }') .. @test('run')
If we add an additional namespace around keyword declarations, though, we can give users more control:
.. parsed-literal::
namespace lib {
**namespace keywords {**
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)
**}**
BOOST_PARAMETER_FUNCTION(
(int), f, **keywords::**\ tag,
(optional (name,*,"bob")(index,(int),1))
)
{
std::cout << name << ":" << index;
std::cout << std::endl;
return index;
}
}
.. @example.prepend(''' #include <boost/parameter.hpp> #include <iostream> ''')
Now users need only a single using-directive to bring in just the names of
all keywords associated with lib:
.. parsed-literal::
**using namespace lib::keywords;**
int y = lib::f(_name = "bob", _index = 2);
.. @example.append('int main() { return 0; }') .. @test('run', howmany='all')
The interface idioms enabled by Boost.Parameter are completely new (to C++), and as such are not served by pre-existing documentation conventions.
.. Note:: This space is empty because we haven't settled on any best practices yet. We'd be very pleased to link to your documentation if you've got a style that you think is worth sharing.
Use the regression test results_ for the latest Boost release of
the Parameter library to see how it fares on your favorite
compiler. Additionally, you may need to be aware of the following
issues and workarounds for particular compilers.
.. _regression test results: http://www.boost.org/regression/release/user/parameter.html
If your compiler supports perfect forwarding, then the Parameter library
will #define the macro BOOST_PARAMETER_HAS_PERFECT_FORWARDING unless
you disable it manually. If your compiler does not provide this support, then
parameter::parameters::operator() will treat rvalue references as lvalue
const references to work around the forwarding problem, so in certain
cases you must wrap |boost_ref|_ or |std_ref|_ around any arguments that will
be bound to out parameters. The |evaluate_category|_ and
|preprocessor_eval_category|_ test programs demonstrate this support.
.. _perfect forwarding: http://www.justsoftwaresolutions.co.uk/cplusplus/rvalue_references_and_perfect_forwarding.html
.. _forwarding problem: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm
.. |boost_ref| replace:: boost::ref
.. _boost_ref: ../../../core/doc/html/core/ref.html
.. |std_ref| replace:: std::ref
.. _std_ref: http://en.cppreference.com/w/cpp/utility/functional/ref
.. |evaluate_category| replace:: evaluate_category.cpp
.. _evaluate_category: ../../test/evaluate_category.cpp
.. |preprocessor_eval_category| replace:: preprocessor_eval_category.cpp
.. _preprocessor_eval_category: ../../test/preprocessor_eval_category.cpp
If your compiler is sufficiently compliant with the C++11 standard, then the
Parameter library will #define the macro BOOST_PARAMETER_CAN_USE_MP11
unless you disable it manually. The |singular_cpp|, |compose_cpp|,
|optional_deduced_sfinae_cpp|, and |deduced_dep_pred_cpp| test programs
demonstrate support for Boost.MP11_.
.. _Boost.MP11: ../../../mp11/doc/html/mp11.html
.. |singular_cpp| replace:: singular.cpp
.. _singular_cpp: ../../test/singular.cpp
.. |optional_deduced_sfinae_cpp| replace:: optional_deduced_sfinae.cpp
.. _optional_deduced_sfinae_cpp: ../../test/optional_deduced_sfinae.cpp
.. |deduced_dep_pred_cpp| replace:: deduced_dependent_predicate.cpp
.. _deduced_dep_pred_cpp: ../../test/deduced_dependent_predicate.cpp
Some older compilers don't support SFINAE. If your compiler meets that
criterion, then Boost headers will #define the preprocessor symbol
BOOST_NO_SFINAE, and parameter-enabled functions won't be removed
from the overload set based on their signatures. The |sfinae_cpp|_ and
|optional_deduced_sfinae|_ test programs demonstrate SFINAE support.
.. |sfinae_cpp| replace:: sfinae.cpp .. _sfinae_cpp: ../../test/sfinae.cpp .. |optional_deduced_sfinae| replace:: optional_deduced_sfinae.cpp .. _optional_deduced_sfinae: ../../test/optional_deduced_sfinae.cpp
.. |result_of| replace:: result_of
.. _result_of: ../../../utility/utility.htm#result_of
Lazy default computation_ relies on the |result_of| class template to
compute the types of default arguments given the type of the function object
that constructs them. On compilers that don't support |result_of|,
BOOST_NO_RESULT_OF will be #define\ d, and the compiler will expect
the function object to contain a nested type name, result_type, that
indicates its return type when invoked without arguments. To use an ordinary
function as a default generator on those compilers, you'll need to wrap it in
a class that provides result_type as a typedef and invokes the
function via its operator().
..
Can't Declare |ParameterSpec| via typedef
=============================================
In principle you can declare a |ParameterSpec| as a ``typedef`` for a
specialization of ``parameters<…>``, but Microsoft Visual C++ 6.x has been
seen to choke on that usage. The workaround is to use inheritance and
declare your |ParameterSpec| as a class:
.. parsed-literal::
**struct dfs_parameters
:** parameter::parameters<
tag::graph, tag::visitor, tag::root_vertex
, tag::index_map, tag::color_map
>
**{
};**
Default Arguments Unsupported on Nested Templates
=============================================
As of this writing, Borland compilers don't support the use of default
template arguments on member class templates. As a result, you have to
supply ``BOOST_PARAMETER_MAX_ARITY`` arguments to every use of
``parameters<…>::match``. Since the actual defaults used are unspecified,
the workaround is to use |BOOST_PARAMETER_MATCH|_ to declare default
arguments for SFINAE.
.. |BOOST_PARAMETER_MATCH| replace:: ``BOOST_PARAMETER_MATCH``
If you use Microsoft Visual C++ 6.x, you may find that the compiler has trouble finding your keyword objects. This problem has been observed, but only on this one compiler, and it disappeared as the test code evolved, so we suggest you use it only as a last resort rather than as a preventative measure. The solution is to add using-declarations to force the names to be available in the enclosing namespace without qualification::
namespace graphs {
using graphs::graph;
using graphs::visitor;
using graphs::root_vertex;
using graphs::index_map;
using graphs::color_map;
}
.. _python: python.html
Follow this link__ for documentation on how to expose
Boost.Parameter-enabled functions to Python with Boost.Python_.
__ ../../../parameter_python/doc/html/index.html
.. _reference: reference.html
Follow this link__ to the Boost.Parameter reference documentation.
__ reference.html
.. _arguments:
the value actually passed to a function or class template.
.. _parameter:
the name used to refer to an argument within a function or class
template. For example, the value of f's parameter x is given by the
argument 3:
.. parsed-literal::
int f(int x) { return x + 1; }
int y = f(3);
The authors would like to thank all the Boosters who participated in the review of this library and its documentation, most especially our review manager, Doug Gregor.
.. [#old_interface] As of Boost 1.33.0 the Graph library was still using an
older named parameter mechanism__, but there are plans to change it to
use Boost.Parameter (this library) in an upcoming release, while keeping
the old interface available for backward-compatibility.
__ ../../../graph/doc/bgl_named_params.html
.. [#odr] The One Definition Rule says that any given entity in a C++ program must have the same definition in all translation units (object files) that make up a program.
.. [#vertex_descriptor] If you're not familiar with the Boost Graph Library,
don't worry about the meaning of any Graph-library-specific details you
encounter. In this case you could replace all mentions of vertex
descriptor types with int in the text, and your understanding of the
Parameter library wouldn't suffer.
.. [#ConceptsTS] This is a major motivation behind C++20 constraints_.
.. _C++20 constraints: http://en.cppreference.com/w/cpp/language/constraints
.. .. [#bind] The Lambda library is known not to work on some .. less-conformant compilers__. When using one of those you could
.. use Boost.Bind_ to generate the function object::
.. boost::bind(std::plus<std::string>(),s1,s2)
.. [#is_keyword_expression] Here we're assuming there's a predicate
metafunction is_keyword_expression that can be used to identify
models of Boost.Python's KeywordExpression concept.
.. .. __ http://www.boost.org/regression/release/user/lambda.html .. _Boost.Bind: ../../../bind/index.html
.. [#using] You can always give the illusion that the function lives in an outer namespace by applying a using-declaration::
namespace foo_overloads {
// foo declarations here
void foo() { ... }
...
}
using foo_overloads::foo;
This technique for avoiding unintentional argument-dependent lookup is due
to Herb Sutter.
.. [#sfinae] This capability depends on your compiler's support for
SFINAE. SFINAE: S\ ubstitution F\ ailure I\ s N\ ot
A\ n E\ rror. If type substitution during the instantiation of a
function template results in an invalid type, no compilation error is
emitted; instead the overload is removed from the overload set. By
producing an invalid type in the function signature depending on the
result of some condition, we can decide whether or not an overload is
considered during overload resolution. The technique is formalized in the
|enable_if|_ utility. Most recent compilers support SFINAE; on compilers
that don't support it, the Boost config library will #define the
symbol BOOST_NO_SFINAE. See
http://www.semantics.org/once_weakly/w02_SFINAE.pdf for more information
on SFINAE.
.. |enable_if| replace:: enable_if
.. _enable_if: ../../../core/doc/html/core/enable_if.html