this, static Members and Separate Definitions

this

Inside a method, this is a pointer to the object the method was called on. It's useful when a parameter has the same name as a member:

#include <iostream>
#include <string>
using namespace std;

class Cat {
private:
    string name;

public:
    void setName(string name) {
        this->name = name;      // this->name is the member, name is the parameter
    }
    string getName() const { return name; }
};

int main() {
    Cat c;
    c.setName("Tom");
    cout << c.getName() << "\n";
    return 0;
}

Output

Tom

Tip: this->name is short for (*this).name. The arrow -> reaches a member through a pointer.

static: one value shared by all objects

A normal member belongs to each object. A static member belongs to the class itself, so all objects share one copy. Here it counts how many objects were created:

#include <iostream>
using namespace std;

class Robot {
public:
    static int count;
    int id;

    Robot() {
        count++;
        id = count;
    }
};

int Robot::count = 0;      // the shared value is created once, outside the class

int main() {
    Robot a, b, c;
    cout << "Robots built: " << Robot::count << "\n";
    cout << "c has id " << c.id << "\n";
    return 0;
}

Output

Robots built: 3
c has id 3

Defining methods outside the class

In bigger programs the class only lists its methods, and their bodies are written below using ClassName::. This keeps the class short and easy to scan:

#include <iostream>
using namespace std;

class Circle {
private:
    double radius;

public:
    Circle(double r);
    double area() const;
};

Circle::Circle(double r) : radius(r) {}

double Circle::area() const {
    return 3.14159 * radius * radius;
}

int main() {
    Circle c(2);
    cout << c.area() << "\n";
    return 0;
}

Output

12.5664

Remember: In real projects the class goes in a header file (Circle.h) and the method bodies in Circle.cpp. You'll see how in the last module.