Constructors

Setting every member by hand after creating an object is tiring, and easy to forget. A constructor is a special method that runs automatically when an object is created. It has the same name as the class and no return type:

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

class Dog {
public:
    string name;
    int age;

    Dog(string n, int a) {
        name = n;
        age = a;
        cout << "A dog called " << name << " was created\n";
    }
};

int main() {
    Dog a("Rex", 3);
    Dog b("Bella", 5);
    cout << b.name << " is " << b.age << "\n";
    return 0;
}

Output

A dog called Rex was created
A dog called Bella was created
Bella is 5

The member initializer list

C++ has a neater way to set members in a constructor: a colon after the brackets, then each member with its starting value. This is the style most C++ programmers use:

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

class Book {
public:
    string title;
    int pages;

    Book(string t, int p) : title(t), pages(p) {}
};

int main() {
    Book b("Dune", 412);
    cout << b.title << ": " << b.pages << " pages\n";
    return 0;
}

Output

Dune: 412 pages

Several constructors

Like functions, constructors can be overloaded. A constructor with no parameters is called the default constructor:

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

class Player {
public:
    string name;
    int lives;

    Player() : name("Unknown"), lives(3) {}
    Player(string n) : name(n), lives(3) {}
    Player(string n, int l) : name(n), lives(l) {}
};

int main() {
    Player a;
    Player b("Mario");
    Player c("Luigi", 5);
    cout << a.name << " " << a.lives << "\n";
    cout << b.name << " " << b.lives << "\n";
    cout << c.name << " " << c.lives << "\n";
    return 0;
}

Output

Unknown 3
Mario 3
Luigi 5

Remember: If you write any constructor yourself, C++ stops giving you the free default one. Add ClassName() {} yourself if you still need to create objects without arguments.