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std::is_constant_evaluated

Defined in header <type_traits>
constexpr bool is_constant_evaluated() noexcept;
(since C++20)

Detects whether the function call occurs within a constant-evaluated context. Returns true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated; otherwise returns false.

The following expressions (including conversions to the destination type) are manifestly constant-evaluated:

To test the last two conditions, compilers may first perform a trial constant evaluation of the initializers. It is not recommended to depend on the result in this case.

int y;
const int a = std::is_constant_evaluated() ? y : 1;
// Trial constant evaluation fails. The constant evaluation is discarded.
// Variable a is dynamically initialized with 1
 
const int b = std::is_constant_evaluated() ? 2 : y;
// Constant evaluation with std::is_constant_evaluation() == true succeeds.
// Variable b is statically initialized with 2

Parameters

(none).

Return value

true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated; otherwise false.

Notes

When directly used as the condition of static_assert declaration or constexpr if statement, std::is_constant_evaluated() always returns true.

Example

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