Introduced in C++20, the consteval keyword is a new addition that complements the constexpr keyword, allowing you to enforce compile-time evaluation of a function. Unlike constexpr, which may be evaluated at compile-time or runtime depending on how the function is called, a function declared as constevalmust always be evaluated at compile-time.

What is consteval?

consteval stands for constant evaluation, and it enforces that a function can only be called at compile-time. Any attempt to call a consteval function at runtime will result in a compilation error. This ensures that the value or result of the function is always computed during compilation, which can be particularly useful in certain scenarios where compile-time guarantees are critical.

Syntax:

consteval returnType functionName(parameters) {
    // Function body
}

Example:

consteval int square(int x) {
    return x * x;
}

int main() {
    constexpr int result = square(5);  // Compile-time evaluation, works fine
    // int runtime_value = 10;
    // int runtime_result = square(runtime_value);  // Error: cannot call consteval function with runtime values
}

In this example:

• square(5) is a compile-time constant, so the consteval function is evaluated at compile-time without any issues.
• If we try to pass a runtime value (runtime_value) to the square function, it will result in a compilation error because consteval functions cannot be called at runtime.

Key Characteristics of consteval

1. Mandatory Compile-time Execution:
   • A consteval function must always be evaluated at compile-time. If you try to call it with runtime data or in a context where the result isn't known at compile-time, the compiler will generate an error.
2. Function Behavior:
   • Like constexpr, a consteval function can contain complex logic, such as loops, branches, and local variables, but the key difference is that it must be evaluated at compile-time.
   • The function cannot be called with values that are only available at runtime.
3. Guaranteed Performance:
   • Since consteval functions are evaluated at compile-time, they have zero runtime cost. This can lead to more efficient code, especially for scenarios that benefit from compile-time computation, such as compile-time configuration or mathematical operations.
4. Difference from constexpr:
   • While constexpr functions can be evaluated at compile-time (if called with constant expressions), they may also be evaluated at runtime (if called with runtime values). In contrast, consteval functions must be evaluated at compile-time and cannot be called at runtime.

Difference Between consteval and constexpr

The differences between consteval and constexpr can be summarized as follows: