# std::is_constant_evaluated

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< cpp‎ | types

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 | Comment
--- | --- | --- | ---
`__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

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