std::is_constant_evaluated

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

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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.

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 constexpr if statement, std::is_constant_evaluated() always returns true.

Because if consteval is absent in C++20, std::is_constant_evaluated is typically implemented using a compiler extension.

Feature-test macro	Value	Std	Feature
__cpp_lib_is_constant_evaluated	201811L	(C++20)	std::is_constant_evaluated

Example

Run this code

#include <cmath>
#include <iostream>
#include <type_traits>
 
constexpr double power(double b, int x)
{
    if (std::is_constant_evaluated() && !(b == 0.0 && x < 0))
    {
        // A constant-evaluation context: Use a constexpr-friendly algorithm.
        if (x == 0)
            return 1.0;
        double r {1.0};
        double p {x > 0 ? b : 1.0 / b};
        for (auto u = unsigned(x > 0 ? x : -x); u != 0; u /= 2)
        {
            if (u & 1)
                r *= p;
            p *= p;
        }
        return r;
    }
    else
    {
        // Let the code generator figure it out.
        return std::pow(b, double(x));
    }
}
 
int main()
{
    // A constant-expression context
    constexpr double kilo = power(10.0, 3);
    int n = 3;
    // Not a constant expression, because n cannot be converted to an rvalue
    // in a constant-expression context
    // Equivalent to std::pow(10.0, double(n))
    double mucho = power(10.0, n);
 
    std::cout << kilo << " " << mucho << "\n"; // (3)
}

Output:

1000 1000

See also

constexpr specifier(C++11)	specifies that the value of a variable or function can be computed at compile time
consteval specifier(C++20)	specifies that a function is an immediate function, that is, every call to the function must be in a constant evaluation
constinit specifier(C++20)	asserts that a variable has static initialization, i.e. zero initialization and constant initialization