C++ Comments

Single-line `//`, multi-line `/* ... */`, and the rules of writing comments your future self will thank you for.

What are Comments?

Comments are notes for humans — the compiler ignores them entirely. They explain why, document edge cases, and disable code temporarily.

Two Flavours

cpp
// Single-line comment — everything after `//` to the end of line.

/* Multi-line comment —
   spans multiple lines
   until the closing star-slash */

Where to Put Them

cpp
// Above a function, explain what it does
int add(int a, int b) {
    return a + b;   // inline comment for non-obvious lines
}

/* Block comment for whole-file or section explanation
 * (the leading `*` on each line is just convention). */

Doxygen — Auto-generated API Docs

Tools like Doxygen parse a special comment style to generate HTML docs:

cpp
/**
 * Adds two integers.
 * @param a first number
 * @param b second number
 * @return the sum
 */
int add(int a, int b) { return a + b; }

Good vs Bad Comments

Bad — explains the what	Good — explains the why
`i++; // increment i`	`i++; // skip the header row`
`if (x > 0)`	`// guard against the underflow we saw in production on 2024-03-12`
`/* TODO: fix this */` (no context)	`// TODO(arav): replace once gateway v2 ships in Q3`

Disabling Code

Comments are great for temporarily disabling code:

cpp
// debugLog("entered loop");
runProductionPath();

But don't ship commented-out code — git history is the right place for it.

Common Mistakes

Nesting /* */ — /* /* nested */ */ is invalid; the first */ closes. Use // for nestable comments.
Outdated comments — code changes, comment doesn't. Worse than no comment.
Restating obvious code — i++ // adds 1 to i. Skip it.
Walls of /* * */ — for short comments, // is cleaner.
Missing context in TODO — // TODO with no name/date is impossible to action.

Interview Questions

Practice Exercises

Document a function — Add a Doxygen-style comment to int max(int, int). Hint: @param, @return.
Toggle code — Comment out a line of code, run, uncomment, run. Hint: // is fastest.
TODO triage — Pick any open-source repo, search for TODO — note how few have owners. Hint: lesson learned.
Review your last project — Find one "what" comment and rewrite it as a "why". Hint: explain intent.

💡 Think Like a Programmer: Code reads top-to-bottom; comments explain top-to-side. If your code needs a paragraph to explain it, refactor the code first — the best comment is a clearer name.

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# program

Program

C++

// Calculate the area of a circle.
// Uses M_PI from <cmath> (more precise than the literal 3.14).
#include <iostream>
#include <cmath>
using namespace std;
int main() {
    double r = 5.0;
    /* Area formula: A = π × r² */
    double area = M_PI * r * r;
    cout << area << "\n";
    return 0;
}

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