C++ Comments

A comment is text the compiler ignores. Comments explain the code to people: why something is done, what a function expects, or a warning for the next person who changes it.

Single-line comments

// starts a comment that runs to the end of the line.

#include <iostream>

int main() {
    // Greet the user
    std::cout << "Hello\n";   // also ignored
    return 0;
}

Hello

Multi-line comments

/* starts a comment that ends at */, possibly lines later. Such comments cannot be nested.

#include <iostream>

int main() {
    /* This program prints one line.
       The second line is switched off. */
    std::cout << "Line one\n";
    /* std::cout << "Line two\n"; */
    return 0;
}

Line one

Comments that help

A good comment explains why, not what: the code already says what. Compare:

total = total + 1;   // add one to total                 (not useful)
total = total + 1;   // the header row counts as a line too (useful)

Functions are often introduced by a short comment saying what they do, what they need and what they return:

// Returns the average of the values; values must not be empty.
double average(const std::vector<double>& values);

Many projects write such comments in a fixed style (for example with /// or /** ... */) so that tools like Doxygen can turn them into documentation pages.

Keep comments short and up to date. A comment that no longer matches the code misleads the reader; when you change code, change its comment too. Often the best "comment" is a clear name: int studentCount needs no explanation, int n does.

Switching code off

While testing, it is common to "comment out" a line to see what happens without it. That is fine during work, but delete such lines before you share the code; version control keeps the old versions for you.

Practice exercises

1. Add a comment above each line of the Hello World program.
2. Switch a line off with //, run the program, then switch it back on.
3. Rewrite a comment that only repeats the code so it explains a reason instead.