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.

While testing whether initializers of following variables are constant initializers, compilers may first perform a trial constant evaluation of the initializers:

• variables with reference type or const-qualified integral or enumeration type
  • if the initializers are constant expressions, the variables are usable in constant expressions
• static and thread local variables
  • if when all subexpressions of the initializers (including constructor calls and implicit conversions) are constant expressions, static initialization is performed, which can be asserted by constinit

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.

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 by a compiler extension.

Example

#include <type_traits>
#include <cmath>
#include <iostream>
 
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, p = x > 0 ? b : 1.0 / b;
        auto u = unsigned(x > 0 ? x : -x);
        while (u != 0) {
            if (u & 1) r *= p;
            u /= 2;
            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)
}

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See also