2.6 Introduction to classes  (Page 5/9)

You define and declare constructor functions the same way you define other functions, although you do not include a return type since constructor functions do not return values.

Example:

class Payroll{

public:

Payroll( ){ // constructor function

dFedTax = 0.28;

dStateTax = 0.05;

};

private:

double dFedTax;

double dStateTax;

}

You also include just a function prototype in the interface file for the constructor function and then create the function definition in the implementation file.

Payroll::Payroll( ){ // constructor function

dFedTax = 0.28;

dStateTax = 0.05;

};

Example

#include<iostream.h>

#include<iomanip.h>

// class declaration section

class Date

{

private:

int month;

int day;

int year;

public:

Date(int = 7, int = 4, int = 2001); // constructor with default values

};

// implementation section

Date::Date(int mm, int dd, int yyyy) // constructor

{

month = mm;

day = dd;

year = yyyy;

cout<<"Created a new data object with data values "

<<month<<", "<<day<<", "<<year<<endl;

}

int main()

{

Date a; // declare an object

Date b; // declare an object

Date c(4,1,2002); // declare an object

return 0;

}

The output of the above program:

Created a new data object with data values 7, 4, 2001

Created a new data object with data values 7, 4, 2001

Created a new data object with data values 4,1, 2002

Default constructor refers to any constructor that does not require any parameters when it is called.

In the above example, the prototype Date(int = 7, int = 4, int = 2001) is valid for a default constructor. Here, each argument has been given a default value. Then an object can be declared as type Date without supplying any further arguments.

Although any legitimate C++ statement can be used within a constructor function, such as the cout statement used in above example, it is best to keep constructors simple and use them only for initialization purposes.

Structures vesus classes

Similarities between Structures and Classes

1. Both can be used to model objects with different attributes represented as data members (also called fields or instance varibles). They can thus be used to process non-homogeneous data sets.
2. They have essentially the same syntax.

Differences between Structures and Classes

1. Member of a structure by default are public. Member of a class by default are private unless explicitly declared to be public
2. A structure consists of only data elements while a class consists of not only data elements but also functions (operations) which are operated on the data elements.

Dynamic memory allocation with operators new and delete

The new and delete operators provides a nice means of performing dynamic memory allocation (for any built-in or user-defined type). Consider the following code

TypeName *typeNamPtr;

typeNamePtr = new TypeName;

The new operator automatically creates an object of the proper size, calls the constructor for the object and returns a pointer of the correct type.

To destroy the object and free the space for this object in C++ you must use the delete operator as follows:

delete typeNamePtr;

For built-in data types, we also can use the new and delete operators.