Fold Expressions and Constexpr

Yes, fold expressions can be used in constexpr functions. In fact, fold expressions are particularly useful in constexpr contexts because they allow you to perform compile-time computations on parameter packs.

When a fold expression is used in a constexpr function and all the arguments passed to the function are constant expressions, the fold expression is evaluated at compile-time, and the result is also a constant expression.

Here's an example that demonstrates the use of fold expressions in a constexpr function:

1#include <iostream>
2
3template <typename... Types>
4constexpr auto Sum(Types... Args) {
5  return (... + Args);  
6}
7
8int main() {
9  constexpr int Result1 = Sum(1, 2, 3, 4, 5);
10  static_assert(Result1 == 15,
11    "Compile-time assertion failed");
12
13  int x = 10;
14  int Result2 = Sum(x, 20, 30);
15  std::cout << Result2;
16}

160

In this example, the Sum() function is declared as constexpr, indicating that it can be evaluated at compile-time when its arguments are constant expressions.

The fold expression (... + Args) is used to calculate the sum of the elements in the parameter pack Args.

In the main function:

• Result1 is initialized with a call to Sum() with constant integer arguments. Since all the arguments are constant expressions, the fold expression is evaluated at compile-time, and Result1 is a constant expression. The static_assert statement verifies that the compile-time result is equal to 15.
• Result2 is initialized with a call to Sum with a mix of constant and non-constant arguments. In this case, the fold expression is evaluated at runtime, and Result2 is not a constant expression. The result is computed at runtime and printed to the console.

Fold expressions in constexpr functions offer several benefits:

• They enable compile-time evaluation of parameter packs, allowing for more efficient and optimized code.
• They can be used to perform compile-time assertions and validations on parameter packs.
• They can be combined with other constexpr features, such as if constexpr, to create more flexible and powerful compile-time computations.

However, it's important to note that for a fold expression in a constexpr function to be evaluated at compile-time, all the arguments passed to the function must be constant expressions. If any of the arguments are not constant expressions, the fold expression will be evaluated at runtime.

By leveraging fold expressions in constexpr functions, you can write more expressive and efficient code that takes advantage of compile-time computations and validations.