.. _fg_ctad:

Class Template Argument Deduction for Flow Graph Nodes

Starting from C++17, many Flow Graph nodes support Class Template Argument Deduction (CTAD), which allows the compiler to deduce class template parameters from constructor arguments. This eliminates the need to explicitly specify input and output types for Flow Graph nodes.

.. contents:: :local: :depth: 1

Deduction from Body Type

Flow Graph functional nodes can deduce template parameters from the signature of any callable object supported by std::invoke passed as the body.

For example, a function_node body that takes int and returns double results in deducing the node type as function_node<int, double>:

.. code:: cpp

function_node f(g, unlimited, [](int input) -> double { return ...; });
// f is deduced as function_node<int, double>

For nodes supporting different node policies, the object of the policy can be passed as an additional constructor argument to allow the deduction:

.. code:: cpp

function_node f(g, unlimited, [](int input) -> double { return ...; }, rejecting{});
// f is deduced as function_node<int, double, rejecting>

The following nodes support CTAD from the body type:

• function_node - deduces node's input and output types from the body
• continue_node - deduces output type from the body's return type (void maps to continue_msg)
• input_node - deduces output type from the body's return type
• sequencer_node - deduces message type from the sequencer body's input type
• join_node with key_matching policy - deduces output tuple and key-matching policy from the provided bodies

multifunction_node and async_node do not support CTAD because their body arguments depend on the complete node type through the output_ports_type parameter.

Example

Without CTAD, the template parameters must be specified explicitly:

.. literalinclude:: ./examples/fg_ctad.cpp :language: c++ :start-after: /begin_fg_ctad_no_ctad_example/ :end-before: /end_fg_ctad_no_ctad_example/

With CTAD, the compiler deduces the types from the constructor arguments:

.. literalinclude:: ./examples/fg_ctad.cpp :language: c++ :start-after: /begin_fg_ctad_with_ctad_example/ :end-before: /end_fg_ctad_with_ctad_example/

Deduction from Predecessors and Successors

.. note:: To enable this feature, define the TBB_PREVIEW_FLOW_GRAPH_FEATURES macro to 1.

When using the follows and precedes helper functions to construct nodes, non-functional nodes can deduce their input and output types from their predecessors and successors.

The following additional nodes support CTAD through follows/precedes:

• broadcast_node
• buffer_node, queue_node, priority_queue_node
• overwrite_node, write_once_node
• limiter_node
• join_node with queueing or reserving policy
• indexer_node
• split_node

CTAD for functional nodes also works with follows and precedes, but deduces the node's input and output types from the provided body, as described in the section above.

Example

.. literalinclude:: ./examples/fg_ctad.cpp :language: c++ :start-after: /begin_fg_ctad_preview_example/ :end-before: /end_fg_ctad_preview_example/