In C++, constexpr is a feature that allows the evaluation of expressions at compile time. It was introduced in C++11 and has been extended in later versions of the language.

Introduction to constexpr

In C++11, constexpr was limited to a small subset of operations, and the expressions had to be simple enough to be evaluated at compile time. For example, you could declare a constant value using constexpr, but it was limited to basic arithmetic and logical operations.

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

C++14 and extended constexpr

With the introduction of C++14, the constexpr keyword was extended to allow more complex operations, including loops and conditional statements.

constexpr int fibonacci(int n) {
    if (n <= 1) {
        return n;
    } else {
        return fibonacci(n - 1) + fibonacci(n - 2);
    }
}

In this example, the fibonacci function can be evaluated at compile time, as long as the input value is known at compile time as well.

C++20 and consteval

C++20 introduced consteval, which is even more powerful than the extended constexpr. It guarantees that the function will be evaluated at compile time and can be used for a broader range of computations.

consteval int factorial(int n) {
    int result = 1;
    for (int i = 1; i <= n; ++i) {
        result *= i;
    }
    return result;
}

In this example, the factorial function is guaranteed to be evaluated at compile time, providing a more flexible and powerful way to perform computations during compilation.

Conclusion

constexpr in C++ has evolved significantly since its introduction in C++11. With the extensions introduced in C++14 and the addition of consteval in C++20, the language now provides more flexibility and power for compile-time computation, enabling more complex operations to be performed at compile time.

References:

• C++ reference: Constexpr specifier
• Modern C++ features: Constexpr – a mighty beast with superpowers