Best Practices: constexpr vs consteval

When deciding between constexpr and consteval, there are several best practices to keep in mind:

Use constexpr for Flexibility

If you want a function that can be evaluated at compile-time but also want the option to use it at runtime, constexpr is the way to go. It gives you the best of both worlds.

1#include <iostream>
2
3constexpr int square(int x) {
4  return x * x;
5}
6
7int main() {
8  constexpr int compile_time_result = square(5);
9  int runtime_input;
10  std::cin >> runtime_input;
11  int runtime_result = square(runtime_input);
12}

Use consteval for Compile Time Guarantees

If you want to ensure that a function is always evaluated at compile-time and never at runtime, use consteval. This is useful for metaprogramming or when you want to enforce compile-time computation.

1consteval int factorial(int n) {
2  if (n <= 1) return 1;
3  return n * factorial(n - 1);
4}
5
6int main() {
7  // This is fine
8  constexpr int result = factorial(5);
9
10  // This will cause a compilation error
11  int x{5};
12  int error = factorial(x);  
13}

1error: 'factorial': call to immediate function is not a constant expression
2note: see usage of 'x'

Performance Considerations

Both constexpr and consteval can lead to performance improvements by moving computations to compile-time.

However, consteval guarantees compile-time evaluation, which can be beneficial for complex calculations that you always want to occur at compile-time.

Code Clarity

Use consteval when you want to make it clear to other developers that a function is intended for compile-time use only. This can help prevent misuse and make your intentions more explicit.

Gradual Adoption

If you're working on an existing codebase, start with constexpr and gradually move to consteval where appropriate. This allows for a smoother transition and maintains backwards compatibility.

Remember, the choice between constexpr and consteval often depends on your specific use case and requirements. Always consider the flexibility, performance, and clarity needs of your project when making this decision.