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The Copy Constructor

 

Copying an Object

After creating an object and assigning appropriate values to its member variables, you can perform any regular operation on it. We have already learned:

How to assign Example
A value to a variable int a = 250;
The value of one variable to another NbrOfBoys = NbrOfGirls;
A value to an objectís member Video.Category = L 'H'

Assigning a variable to another is equivalent to making a copy of that variable. As you assign a variable to another, you can assign one object to another. Both objects must be recognizably equivalent types to the compiler. Here is an example:

int main()
{
    CFlower ^ flower = gcnew CFlower(L"Lilies", L"Pink", L"Bouquet", 45.65);
    Show(flower);
    Console::WriteLine(L"");

    CFlower ^ inspiration;

    inspiration = flower;
    Show(inspiration);

    Console::WriteLine(L"");
    return 0;
}

This would produce:

Flower Type:  Lilies
Flower Color: Pink
Arrangement:  Bouquet
Price:        $45.65

Flower Type:  Lilies
Flower Color: Pink
Arrangement:  Bouquet
Price:        $45.65

Press any key to continue . . .

Notice that both orders display the same thing.

Using a Copy Constructor

Besides the default constructor, the compiler creates another method called the copy constructor. This constructor is used for operations such as copying an object into another. Remember, we have seen that a variable could be initialized using the = symbol or the parentheses. When you have two instances of a class such as:

CFlower ^ Inspiration, ^ Elegance;

you can assign one object to another like this:

Elegance = Inspiration;

This operation indeed assigns a copy of the Inspiration flower to the Elegance object. Behind the scenes, this transaction is handled by the copy constructor. Like the default constructor, the compiler automatically creates a copy constructor when an object is instantiated. Like the default constructor, you can explicitly create a copy constructor; it has a different syntax although it also holds the same name as the class. The syntax of the copy constructor.

ClassName(ObjectName ^ & Name);
Copy Constructor

The copy constructor takes one argument, which is the same as the class itself. When a copy is made, it holds and carries the building construction of the object. This object is specified as the argument. As a copy whose value still resides with the object, this argument should be passed as a reference and it should not be modified. It is only used to pass a copy of the object to the other objects that need it. Therefore, the argument should not be modified. As a result, it should be declared as a constant. The syntax of the copy constructor becomes:

ClassName(const ClassName^ & Name);

To copy one object to another, first create a copy constructor:

public ref class CFlower
{
public:
    String ^ Type;
    String ^ Color;
    String ^ Arrangement;
    double   UnitPrice;

    CFlower() : Type(L"Roses"), Color(L"Red"),
	        Arrangement(L"Basket"), UnitPrice(35.95)
    {
    }

    CFlower(String ^ type);
    CFlower(String ^ type, String ^ color,
		    String ^ argn, double price);
    CFlower(const CFlower ^ &copier);
};

To implement the copy constructor, at a minimum, assign a member variable of the copy constructor to the equivalent member of the object:

CFlower::CFlower(const CFlower ^ &copier)
{
    Type = copier->Type;
    Color = copier->Color;
    Arrangement = copier->Arrangement;
    UnitPrice = copier->UnitPrice;
}

In some cases, instead of a one-to-one assignment of member variables from one object to another, you can change how the target object would receive the values.

 

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