C++ Class Constructors

Constructors are special member functions that are called automatically when an object of a class is created. In C++, constructors are used to initialize the data members of an object and to provide default values for class objects. In this article, we will explore the basics of C++ class constructors, including how to declare and define class constructors and how to use them to initialize objects of a class.

Declaring Class Constructors

To declare a class constructor, simply write a constructor function inside the class definition with the same name as the class. For example, consider the following code:

1class Rectangle {
2    int width, height;
3  public:
4    Rectangle() {
5        width = 0;
6        height = 0;
7    }
8};

In this example, we declare a class constructor Rectangle that sets the width and height of a Rectangle object to 0.

Defining Class Constructors

Constructors can be defined either inside or outside the class definition, just like class methods. To define a constructor inside the class definition, simply write the function definition inside the class body. To define a constructor outside the class definition, write the function prototype inside the class definition and the full function definition outside the class definition. For example, consider the following code:

1class Rectangle {
2    int width, height;
3  public:
4    Rectangle();
5};
6
7Rectangle::Rectangle() {
8    width = 0;
9    height = 0;
10}

In this example, we define the Rectangle constructor outside the class definition using the syntax ClassName::ConstructorName().

Using Class Constructors to Initialize Objects

To use a class constructor to initialize an object of a class, simply create an object using the constructor. For example, consider the following code:

1#include <iostream>
2
3class Rectangle {
4    int width, height;
5  public:
6    Rectangle() {
7        width = 0;
8        height = 0;
9    }
10};
11
12int main() {
13    Rectangle rect;
14    std::cout << "Width: " << rect.width << std::endl;
15    std::cout << "Height: " << rect.height << std::endl;
16    return 0;
17}

In this example, we create an object rect of the Rectangle class and use the constructor to initialize its data members. The output of this program will be:

1Width: 0
2Height: 0