Jaskamal Kainth

Constexpr in C++

The constexpr keyword was introduced in C++11 and enhanced in C++14 and C++17. It enables computations to be performed at compile time.

Basic Usage

A constexpr function can be evaluated at compile time if its arguments are compile-time constants:

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

// Used at compile time
constexpr int result = square(5); // Computed during compilation

// Used at runtime
int input;
std::cin >> input;
int dynamic_result = square(input); // Computed at runtime

Constexpr vs Template Metaprogramming

While template metaprogramming also enables compile-time computation, constexpr provides a more readable and maintainable approach:

// Fibonacci with constexpr (C++14 and later)
constexpr int fibonacci(int n) {
    if (n <= 1) return n;
    return fibonacci(n-1) + fibonacci(n-2);
}

// Usage
constexpr int fib10 = fibonacci(10); // Computed at compile time

The advantage of constexpr over template metaprogramming is that it uses familiar function syntax rather than recursive template instantiations, making the code easier to understand and maintain.