2.6 Introduction to classes  (Page 7/9)

public:

int iNumShares;

double dPurchasePricePerShare;

double dCurrentPricePerShare;

};

double totalValue(Stocks* pCurStock){

double dTotalValue;

dTotalValue = pCurStock->dCurrentPricePerShar*pCurStock->iNumShares;

return dTotalValue;

}

int main( ){

//allocated on the stack with a pointer to the stack object

Stocks stockPick;

Stocks* pStackStock =&stockPick;

pStackStock->iNumShares = 500;

pStackStock->dPurchasePricePerShare = 10.785;

pStackStock->dCurrentPricePerShare = 6.5;

cout<<totalValue(pStackStock)<<endl;

//allocated on the heap

Stocks* pHeapStock = new Stocks;

pHeapStock->iNumShares = 200;

pHeapStock->dPurchasePricePerShare = 32.5;

pHeapStock->dCurrentPricePerShare = 48.25;

cout<<totalValue(pHeapStock)<<endl;

return 0;

}

The output of the above program:

3250

9650

In the above program, the new operator in the statement:

Stocks* pHeapStock = new Stocks;

invokes the constructor of the Stocks class to create a Stocks object on the heap and returns a pointer which is assigned to the pointer variable pHeapStock.

Note:

1. The totalValue() function is not a function member of the Stocks class. Rather, it is a function that is available to the entire program.

2. When declaring and using pointers and references to class objects, follow the same rules as you would when declaring and using pointers and references to structures. You can use the indirect member selection operator (->) to access class members through a pointer to an object either on stack or on the heap.

As we will see, using new and delete offers other benefits as well. In particular, new invokes the constructor and delete invokes the class’destructor.

Pointers as class members

A class can contain any C++ data type. Thus, the inclusion of a pointer variable in a class should not seem surprising.

In some cases, pointers as class members are advantageous. For example, assume that in the class Book we need to store a book title. Rather than using a fixed length character array as a data member to hold each book title, we could include a pointer member to a character array and then allocate the correct size array for each book title as it is needed.

Example

#include<iostream.h>

#include<string.h>

// class declaration

class Book

{

private:

char *title; // a pointer to a book title

public:

Book(char * = NULL); // constructor with a default value

void showtitle(); // display the title

};

// class implementation

Book::Book(char *strng)

{

title = new char[strlen(strng)+1]; // allocate memory

strcpy(title,strng); // store the string

}

void Book::showtitle()

{

cout<<title<<endl;

return;

}

int main()

{

Book book1("DOS Primer"); // create 1st title

Book book2("A Brief History of Western Civilization"); // 2nd title

book1.showtitle(); // display book1's title

book2.showtitle(); // display book2's title

return 0;

}

The output of the above program:

DOS Primer

A Brief History of Western Civilization

The body of the Book() constructor contains two statements. The first statement performs two taks: First, the statement allocates enough storage for the length of the name parameter plus one to accommodate the end-of-string null character, ‘\n’. Next, the address of the first allocated character position is assigned to the pointer variable title.