Summary - Metaprogramming

Combine recursive templates, constexpr and consteval to write a factorial.

Summary - Metaprogramming

In this group you saw three paths to compile-time computation: recursive templates, constexpr functions and consteval. All of them bake constants into the executable; the difference is flexibility versus strictness.

Now you will combine them into a constexpr factorial. This function delivers the recursive template's behavior with a constexpr function; constexpr int factorial(int n) is usable at both compile time and run time.

constexpr factorial

Inside a constexpr function you write the base case if (n <= 1) return 1; and the recursive step return n * factorial(n - 1);. When called with a constant argument the compiler unfolds the chain at compile time.

Example: constexpr int f = factorial(5); produces the constant f = 120. This is the same as the recursive template but without template syntax; the code is readable.

Exercise Write the constexpr factorial

Write constexpr int factorial(int n). Body: if (n <= 1) return 1; return n * factorial(n - 1);. In main define constexpr int f = factorial(5); and write std::cout << f;.

Show example solution

#include <iostream>

constexpr int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

int main() {
    constexpr int f = factorial(5);
    std::cout << f;
    return 0;
}

Test yourself

Which tool runs only at compile time?
```
constexpr int f(int);
consteval int g(int);
template <int N> struct S { static const int value = N; };
```
- template struct
- constexpr
- consteval
- inline
Show answer

consteval — consteval runs only at compile time.

Complete the recursive step

constexpr int factorial(int n) { return n * factorial(n - ____); }

Show answer

constexpr int factorial(int n) { return n * factorial(n - 1); }

Match each tool to its trait

recursive template

Template specialization
constexpr

Flexible compile/run
consteval

Compile-time only
Typelist

Type pack

Next lesson: constexpr variables

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