Functions

A function packages up a piece of logic so you can reuse it by name, instead of retyping the same code everywhere.

#include <iostream>
#include <string>

int add(int a, int b) {
    return a + b;
}

void greet(const std::string& name) {
    std::cout << "Hello, " << name << "!" << std::endl;
}

int main() {
    std::cout << "add(2, 3) = " << add(2, 3) << std::endl;
    greet("Kodstigen");
    return 0;
}

Key ideas

• A function's declared return type (int, void, ...) must match what it actually returns, void means "returns nothing".
• In add(int a, int b), a and b are parameters, placeholders for whatever values get passed in when the function is called (the actual values, like 2 and 3, are called arguments).
• Passing a parameter as const std::string& passes it by reference instead of copying it, faster for large objects like strings, and const promises the function won't modify it.
• Functions must generally be declared (or defined) before they're used, that's why add and greet appear above main.