std::is_constant_evaluated(3)

💖 std::is_constant_evaluated(3) 💖

okay so bestie, welcome to the std::is_constant_evaluated(3) page from the C++ Standard Libary manual. this one is all about asking the compiler “hey, are we in a constant-evaluated context rn?” and getting a very cute true/false back. periodt. 💅

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✨ NAME ✨

std::is_constant_evaluated — literally std::is_constant_evaluated, no cap. that’s the name. 🫶

🦋 Synopsis 🦋

Defined in header <type_traits>
constexpr bool is_constant_evaluated() noexcept;  (since C++20)

This function detects whether the function call occurs within a constant-evaluated context. It returns true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated; otherwise it 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. fr fr, don’t build your whole vibe on this edge 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) — yes bestie, zero parameters. iconic. 💖

🌸 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. That’s it. That’s the whole vibe. ✨

🧚 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. important! no cap.

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

Feature-test macro info, slay:

• Feature-test macro: __cpp_lib_is_constant_evaluated
• Value: 201811L
• Std: (C++20)
• Feature: 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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made with 💖 and way too much group-chat energy by your fave manual-page bestie. source: http://cppreference.com; version/date: 2024.06.10; page: std::is_constant_evaluated(3). byeeee 🫶✨