Step 2 of 5

Pass by reference

References shine as function parameters. A reference parameter is bound to the caller's variable, so the function can change it, without any of C's pointer syntax:

#include <iostream>
#include <string>

void add_suffix(std::string& s, int n) {
    s += "#" + std::to_string(n);
}

void split_minutes(int total, int& minutes, int& seconds) {
    minutes = total / 60;
    seconds = total % 60;
}

int main() {
    std::string name = "player";
    add_suffix(name, 7);
    int m, s;
    split_minutes(135, m, s);
    std::cout << name << " " << m << ":" << s << "\n";
}

player#7 2:15

Compared with C

In C you'd write void add_suffix(char *s, ...), call it as add_suffix(&name, 7), and use *s inside. With references:

• The parameter is declared std::string& s.
• The call site passes the variable plainly: add_suffix(name, 7).
• Inside, s is used like a normal variable.

split_minutes shows out parameters with references: the function fills in minutes and seconds for the caller.

The downside

At the call site, add_suffix(name, 7) doesn't show that name might change. Good names (and const& for parameters that don't change, next step) make intent clear. Many teams also prefer returning values (possibly a struct) over out parameters when practical.

std::to_string(n) converts a number to a std::string.

Your turn: write void swap_values(int& a, int& b).

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