Summary - constexpr

Combine constexpr, consteval and constinit to write a lookup table.

Summary - constexpr

In this group you saw the compile-time tools: constexpr variable, constexpr function, if constexpr, consteval and constinit. All produce values at compile time; 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(4); produces the constant f = 24. This is the same as the recursive template but without template syntax; the code is readable and can also be called at run time.

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(4); 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(4);
    std::cout << f;
    return 0;
}

Test yourself

Which keyword makes a function run only at compile time?
```
constexpr int a(int);
consteval int b(int);
constinit int c = 0;
```
- consteval
- constinit
- constexpr
- const
Show answer

consteval — consteval (immediate function) 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

constexpr

Flexible compile/run
consteval

Compile-time only
constinit

Static initialization
if constexpr

Compile-time branch

Next lesson: C++ memory model

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