Understanding #pragma once

#pragma once prevents a header file from being included multiple times in the same translation unit. This is important because multiple inclusions can cause compilation errors and slow down compilation.

The Problem

Without #pragma once, this situation can happen:

1// Weapon.h
2class Weapon {
3  int Damage{10};
4};

1// Character.h
2#include "Weapon.h"
3
4class Character {
5  Weapon* MainWeapon;
6};

1// Game.cpp
2#include "Weapon.h"
3#include "Character.h"
4
5int main() {
6  Weapon Sword;
7}

The Game.cpp file ends up with two copies of the Weapon class definition:

1. Directly from #include "Weapon.h"
2. Indirectly when Character.h includes Weapon.h

This creates a compiler error:

1error: redefinition of 'class Weapon'

The Solution

Adding #pragma once fixes this:

1// Weapon.h
2#pragma once
3
4class Weapon {
5  int Damage{10};
6};

Now the compiler only includes the file once, no matter how many times it appears in include statements.

The Alternative

Before #pragma once, we used include guards:

1// Weapon.h
2#ifndef WEAPON_H
3#define WEAPON_H
4
5class Weapon {
6  int Damage{10};
7};
8
9#endif

Both approaches work, but #pragma once is:

• Shorter and cleaner
• Less prone to naming conflicts
• Often faster for the compiler

That's why #pragma once is now the preferred choice in modern C++, even though include guards are still common in older code and some cross-platform projects.