The Purpose of if consteval in C++23

This article explains the purpose and functionality of the if consteval statement introduced in C++23. It explores why this language feature was designed to replace std::is_constant_evaluated(), how it resolves common developer pitfalls associated with compile-time checks, and how it provides a cleaner, safer syntax for branching code between compile-time and runtime execution paths.

The Problem with std::is_constant_evaluated()

C++20 introduced std::is_constant_evaluated(), a standard library function intended to allow functions to execute optimized code at runtime while retaining the ability to fall back on constant expression-friendly implementations during compile-time evaluation.

However, std::is_constant_evaluated() led to subtle and pervasive bugs when combined with if constexpr. Developers often wrote:

// INCORRECT (Common C++20 Trap)
if constexpr (std::is_constant_evaluated()) {
    // This block ALWAYS executes!
}

Because std::is_constant_evaluated() is a function that returns a boolean value, evaluating the condition std::is_constant_evaluated() itself within an if constexpr statement is a constant evaluation context. As a result, the condition always evaluated to true, discarding the runtime path entirely.

To work correctly, developers had to use a standard runtime if statement:

// Correct in C++20, but unintuitive
if (std::is_constant_evaluated()) {
    // Compile-time path
} else {
    // Runtime path
}

This syntax was unintuitive because developers naturally reached for if constexpr when writing code intended for compile-time execution.

The Purpose of if consteval

To eliminate this ambiguity, C++23 introduced if consteval as a native core language feature rather than a library utility. Its purpose is to explicitly check whether the current evaluation is manifestly constant-evaluated, directly bridging compile-time and runtime code branching without the risk of misuse.

The statement is written directly into the control flow:

constexpr double compute(double x) {
    if consteval {
        // Executed ONLY during compile-time evaluation
        return compile_time_algorithm(x);
    } else {
        // Executed at runtime
        return runtime_optimized_algorithm(x);
    }
}

Inverted Checks with if !consteval

C++23 also standardizes the inverted form, if !consteval, which is particularly useful for optimizing runtime operations where compile-time fallback is rarely needed:

constexpr void process(int* data, std::size_t size) {
    if !consteval {
        // Directly utilize runtime optimizations such as SIMD or assembly
        vectorized_process(data, size);
        return;
    }

    // Default portable compile-time implementation
    for (std::size_t i = 0; i < size; ++i) {
        data[i] *= 2;
    }
}

Key Advantages

1. Bug Prevention: By turning the check into a dedicated keyword statement, it prevents the invalid combination of if constexpr and std::is_constant_evaluated().
2. Type System Safety: The compound statement following if consteval behaves like an if constexpr block in that expressions within it are guaranteed to be evaluated at compile time.
3. Clarity of Intent: It clarifies code semantics by explicitly indicating that the branch is governed by the compiler's evaluation phase, rather than regular boolean runtime logic.