std::is_constant_evaluated

constexpr bool is_constant_evaluated() noexcept;

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.

To determine whether initializers of following variables are manifestly constant-evaluated, compilers may first perform a trial constant evaluation:

• variables with reference type or const-qualified integral or enumeration type;
• static and thread local variables.

It is not recommended to depend on the result in this case.

int y = 0;
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_evaluated() == 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.

Possible implementation

// This implementation requires C++23 if consteval.
constexpr bool is_constant_evaluated() noexcept
{
    if consteval
    {
        return true;
    }
    else 
    {
        return false;
    }
}

Notes

When directly used as the condition of static_assert declaration or