Using Constructors

C++ provides two ways to initialize an object by using a constructor. The first is to call the constructor explicitly:

Stock food = Stock("World Cabbage", 250, 1.25);

This sets the company member of the food object to the string "World Cabbage", the shares member to 250, and so on.

The second way is to call the constructor implicitly:

Stock garment("Furry Mason", 50, 2.5);

This more compact form is equivalent to the following explicit call:

Stock garment = Stock("Furry Mason", 50, 2.5));

C++ uses a class constructor whenever you create an object of that class, even when you use new for dynamic memory allocation. Here’s how to use the constructor with new:

Stock *pstock = new Stock("Electroshock Games", 18, 19.0);

This statement creates a Stock object, initializes it to the values provided by the arguments, and assigns the address of the object to the pstock pointer. In this case, the object doesn’t have a name, but you can use the pointer to manage the object. We’ll discuss pointers to objects further in Chapter 11.

Constructors are used differently from the other class methods. Normally, you use an object to invoke a method:

stock1.show();  // stock1 object invokes show() method

However, you can’t use an object to invoke a constructor because until the constructor finishes its work of making the object, there is no object. Rather than being invoked by an object, the constructor is used to create the object.