constexpr functions

Learn to write constexpr functions.

constexpr functions

A constexpr function is a function that can be evaluated at compile time. When called with all constant-expression arguments, the result is computed at compile time. Example: constexpr int square(int x) { return x * x; }.

The call constexpr int s = square(5); produces 25 at compile time. But int n = readInput(); int r = square(n); runs at run time; constexpr here only allows it, it does not force it. Flexibility is the power of constexpr.

C++14 extensions

Since C++14, constexpr functions may contain loops, local variables and if. In C++11 only a single return statement was allowed.

Example: constexpr int factorial(int n) { int r = 1; for (int i = 2; i <= n; ++i) r *= i; return r; }. This body is valid in C++14 and later; when called with a constant argument, the compiler unfolds the loop at compile time.

Exercise Write the constexpr square function

Write constexpr int square(int x) with body return x * x;. In main define constexpr int s = square(7); and write std::cout << s;.

Show example solution

#include <iostream>

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

int main() {
    constexpr int s = square(7);
    std::cout << s;
    return 0;
}

Test yourself

Complete the function body

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

Show answer

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

When is a constexpr function evaluated at compile time?
```
constexpr int square(int x) { return x * x; }
```
- When arguments are constant expressions
- Only if it is inline
- Always
- Only inside templates
Show answer

When arguments are constant expressions — With constant arguments it runs at compile time; with variable arguments at run time.
Match each call to its result

square(5)

returns 25
square(7)

returns 49
square(10)

returns 100
square(0)

returns 0

Next lesson: constexpr if

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