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Member functions

In C, a struct holds data, and separate functions operate on it: void take_damage(struct Player *p, int d). C++ lets you put those functions inside the struct, as member functions (also called methods). Data and the operations on it live together.

#include <iostream>

struct Counter {
    int value = 0;
    int step = 1;

    void increment() { value += step; }
    void reset() { value = 0; }
    bool over(int limit) { return value > limit; }
};

int main() {
    Counter c;
    c.step = 5;
    c.increment();
    c.increment();
    std::cout << c.value << " " << c.over(8) << "\n";
    c.reset();
    std::cout << c.value << "\n";
}

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What's new

• Default member values: int value = 0; gives every new Counter a starting value, so no object starts with garbage.
• Member functions are declared inside the struct. Inside them, members are used directly by name: value += step means "this object's value".
• You call them with a dot: c.increment(). Each object has its own data, and the member function works on the object it was called on.
• In C++ you write just Counter c;; the word struct isn't needed when you use the type.

How it works underneath

A member function is compiled like a normal function with a hidden extra parameter: a pointer to the object, called this. c.increment() is roughly increment(&c), and value inside means this->value. The Watch it run example for classes shows this pointing back at the object.

Your turn: complete the member function reload so it refills ammo to mag_size.

Next: public and private