.. _Constraints:

Constrained APIs

Starting from C++20, most of |full_name| APIs are constrained to enforce named requirements <https://oneapi-spec.uxlfoundation.org/specifications/oneapi/latest/elements/onetbb/source/named_requirements>_ on template arguments types.

The violations of these requirements are detected at a compile time during the template instantiation.

.. rubric:: Example

.. code:: cpp

// Call for body(oneapi::tbb::blocked_range) is ill-formed
// oneapi::tbb::parallel_for call results in constraint failure
auto body = [](const int& r) { /*...*/ };
oneapi::tbb::parallel_for(oneapi::tbb::blocked_range{1, 10}, body);

// Error example:
// error: no matching function to call to oneapi::tbb::parallel_for
// note: constraints not satisfied
// note: the required expression 'body(range)' is invalid
        body(range);

.. caution::

The code that violates named requirements but compiles successfully until C++20,
may not compile in C++20 mode due to early and strict constraints diagnostics.