.. _Constraints:
Constrained APIs
================
Starting from C++20, most of |full_name| APIs are constrained to
enforce `named requirements <https://spec.oneapi.com/versions/latest/elements/oneTBB/source/named_requirements.html>`_ 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.
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