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The All-in-One Guide to C++20

std::is_constant_evaluated Library in C++20

Explore how std::is_constant_evaluated enables detection of compile-time versus run-time execution within constexpr functions. Understand when and why to use this function to write adaptable C++20 code that optimizes behavior depending on the evaluation context.

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The function std::is_constant_evaluted determines whether the function is executed at compile time or run time. Why do we need this function from the type-traits library? In C++20, we have roughly spoken of three kinds of functions:

  • consteval declared functions run at compile time: consteval int alwaysCompiletime();

  • constexpr declared functions can run at compile time or run time: constexpr int itDepends();

  • usual functions run at run time: int ...