Including Headers in CPP Files

This is a common source of confusion! Even though the CPP file contains the implementations of our class functions, we still need to include the header. Here's why:

When we define member functions in a CPP file, we need to tell the compiler these functions belong to our class. The compiler needs to know:

• What class these functions belong to
• What member variables exist in the class
• What access specifiers (public, private) apply to each member

Let's see what happens if we don't include the header:

1// Character.cpp
2void Character::TakeDamage(int Damage) {
3  Health -= Damage;
4}

1error: 'Character' has not been declared
2error: 'Health' was not declared in this scope

Here's the correct way:

1// Character.h
2class Character {
3public:
4  void TakeDamage(int Damage);
5private:
6  int Health{100};
7};

1// Character.cpp
2#include "Character.h"
3
4void Character::TakeDamage(int Damage) {
5  Health -= Damage;
6}

Including the header also helps us catch errors. If our implementation doesn't match the declaration in the header, we'll get a compiler error. For example:

1// Character.h
2class Character {
3public:
4  void TakeDamage(int Damage);
5};

1// Character.cpp
2#include "Character.h"
3
4void Character::TakeDamage(float Damage) {
5  Health -= Damage;
6}

1error: parameter type mismatch
2note: previous declaration was 'void Character::TakeDamage(int)'

This helps us maintain consistency between our header and implementation files.