std::is_constant_evaluated - cppreference.com (original) (raw)

| | | | | -------------------------------------------------- | | ------------- | | 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

Contents

• 1 Parameters
• 2 Return value
• 3 Possible implementation
• 4 Notes
• 5 Example
• 6 See also

[edit] Parameters

(none)

[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; } else { 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.

[edit] Example

#include #include #include   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:

[edit] See also