Code

Math

// Math.h
#pragma once

int max(int x, int y);
int factorial(int n);
int sum(int n);

// Math.cpp
int max(int x, int y)
{
    return x > y ? x : y;
}

int factorial(int n)
{
    return n <= 1 ? 1 : n * factorial(n - 1);
}

int sum(int n)
{
    return n <= 0 ? 0 : n + sum(n - 1);
}

Weapon

// Weapon.h
#pragma once

class Weapon
{
public:
    Weapon();
    void fire();

private:
    int m_bullets{ 23 };
};

// Weapon.cpp
#include "Weapon.h"

Weapon::Weapon()
{

}

void Weapon::fire()
{
    if (m_bullets <= 0) return;
    --m_bullets;
}

Character

// Character.h
#pragma once

class Weapon;

class Character
{
public:
    Character(const char* name);
    ~Character();

    void f1(Weapon*);
    void f2(Weapon&); 
    void f3(const Weapon&);
    Weapon f4();
    void f5(Weapon);

private:
    const char* m_name;
    Weapon* m_weapon;
};

// Character.cpp
#include "Character.h"
#include "Weapon.h"
#include <iostream>

Character::Character(const char* name) : m_name(name)
{
    m_weapon = new Weapon();
    m_weapon->fire();

    std::cout << sizeof(Weapon) << std::endl;
    std::cout << sizeof(Weapon*) << std::endl;
    std::cin.get();
}

Character::~Character()
{
    delete m_weapon;
    m_weapon = nullptr;
}

void Character::f1(Weapon*)
{
}

void Character::f2(Weapon&)
{
}

void Character::f3(const Weapon&)
{
}

Weapon Character::f4()
{
    return Weapon();
}

void Character::f5(Weapon)
{
}

Main

// main.cpp
#include "Character.h"

int main() {
    const Character character("Nux");
    // int counter = 10;
    // counter %= 0;
    return 0;
}