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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


[edit] Parameters


[edit] 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.

[edit] Possible implementation

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

[edit] 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

[edit] Example

#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;
        // 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)


1000 1000

[edit] See also

constexpr specifier(C++11) specifies that the value of a variable or function can be computed at compile time[edit]
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[edit]
constinit specifier(C++20) asserts that a variable has static initialization, i.e. zero initialization and constant initialization[edit]