c++ advance template

8/12/2019 c++ Advance Template

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C++ ADVANCE TEMPLATE


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C++ Friend Functions

In this C++ tutorials, you will learn about friend functions, need for friend

function, how to define and use friend function and few important points

regarding friend function, explained with example.

Need for Friend Function:

As discussed in the earlier sections on access specifiers, when a data is

declared as private inside a class, then it is not accessible from outside theclass. A function that is not a member or an external class will not be able to

access the private data. A programmer may have a situation where he or she

would need to access private data from non-member functions and external

classes. For handling such cases, the concept of Friend functions is a useful

tool.


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What is a Friend Function?

A friend function is used for accessing the non-public members of a class. A

class can allow non-member functions and other classes to access its own

private data, by making them friends. Thus, a friend function is an ordinary

function or a member of another class.

How to define and use Friend Function in C++:

The friend function is written as any other normal function, except the function

declaration of these functions is preceded with the keyword friend. The friend

function must have the class to which it is declared as friend passed to it inargument.


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Some important points to note while using friend functions in C++:

The keyword friend is placed only in the function declaration of the friend

function and not in the function definition.

.

It is possible to declare a function as friend in any number of classes..

When a class is declared as a friend, the friend class has access to the private

data of the class that made this a friend.

.

A friend function, even though it is not a member function, would have the

rights to access the private members of the class..

It is possible to declare the friend function as either private or public.

.

The function can be invoked without the use of an object. The friend function

has its argument as objects, seen in example below.


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#include

class exforsys

{private:

int a,b;

public:

void test(){

a=100;

b=200;

}friend int compute(exforsys

e1)//Friend Function Declaration with keyword friend and with the object of class

exforsys to which it is friend passed to it

};


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int compute(exforsys e1)

{

//Friend Function Definition which

has access to private datareturn int(e1.a+e2.b)-5;

}

main(){

exforsys e;

e.test();

cout


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The result is:295

The function compute() is a non-member function of the class exforsys. In order

to make this function have access to the private data a and b of class exforsys , it

is created as a friend function for the class exforsys. As a first step, the function

compute() is declared as friend in the class exforsys as:

friend int compute (exforsys e1)

The keyword friend is placed before the function. The function definition

is written as a normal function and thus, the function has access to the

private data a and b of the class exforsys. It is declared as friend insidethe class, the private data values a and b are added, 5 is subtracted from

the result, giving 295 as the result. This is returned by the function and

thus the output is displayed as shown above.


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C++ Pure Virtual Function and Virtual Base Class

What is Pure Virtual Function:

Pure Virtual Function is a Virtual function with no body.

Declaration of Pure Virtual Function:

Since pure virtual function has no body, the programmer must addthe notation =0 for declaration of the pure virtual function in the

base class.

General Syntax of Pure Virtual Function takes the form

class classname //This denotes the base class of C++ virtual function{

public:

virtual void virtualfunctioname() = 0 //This denotes the pure virtual function in

C++

};


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To understand the declaration and usage of Pure Virtual Function, refer to

this example:

class Exforsys

{

public:

virtual void example()=0; //Denotes pure virtual

Function Definition

};

class Exf1:public Exforsys

{

public:

void example(){

cout


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{

public:

void example(){

coutexample();

}


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Since the above example has no body, the pure virtual function example()

is declared with notation =0 in the base class Exforsys. The two derived

class named Exf1 and Exf2 are derived from the base class Exforsys. The

pure virtual function example() takes up new definition. In the main function,

a list of pointers is defined to the base class.

Two objects named e1 and e2 are defined for derived classes Exf1 and

Exf2. The address of the objects e1 and e2 are stored in the array

pointers which are then used for accessing the pure virtual function

example() belonging to both the derived class EXf1 and EXf2 and thus,the output is as in the above example.

The programmer must clearly understand the concept of pure virtual

functions having no body in the base class and the notation =0 isindependent of value assignment. The notation =0 simply indicates the

Virtual function is a pure virtual function as it has no body.


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Some programmers might want to remove this pure virtual function from the

base class as it has no body but this would result in an error. Without the

declaration of the pure virtual function in the base class, accessing statements

of the pure virtual function such as, arra[0]->example() and arra[1]->example()

would result in an error. The pointers should point to the base class Exforsys.Special care must be taken not to remove the statement of declaration of the

pure virtual function in the base class.


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In the above example, there are two derived classes Exf1 and Exf2 from the base

class Exforsys. As shown in the above diagram, the Training class is derived from

both of the derived classes Exf1 and Exf2. In this scenario, if a user has a member

function in the class Training where the user wants to access the data or member

functions of the class Exforsys it would result in error if it is performed like this:


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class Exforsys

{

protected:

int x;};


{ };


{ };

class Training:public Exf1,public

Exf2

{public:

int example()

{

return x;

}};


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F E l


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class Exforsys

{

protected:

int x;

;

class Exf1:virtual public Exforsys

{ };

class Exf2:virtual public Exforsys

{ };

class Training:public Exf1,public

Exf2{

public:

int example()

{

return x;

}

For Example:


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UnionsA unionis a user-defined data or class type that, at any given time, contains

only one object from its list of members (although that object can be an arrayor a class type).

union [tag] { member-list } [declarators];

[union] tag declarators;


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Parameters

tag

Thetype name given to the union.member-list

List of the types of data the union can contain. See Remarks.

declarators

Declarator list specifying the names of the union.

Declaring a Union

Begin the declaration of a union with the union keyword, and enclose

the member list in curly braces:

#i l d tdi h


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#include

int main()

{

union data

{char a;

int x;

float f;} myData;

int mode = 1;

myData.a = 'A';printf("Here is the Data:\n%c\n%i\n%.3f\n", myData.a, myData.x, myData.f

);

myData.x = 42;

mode = 2;

printf("Here is the Data:\n%c\n%i\n%.3f\n", myData.a, myData.x, myData.f

);

myData.f = 101.357;

mode = 3;

printf("Here is the Data:\n%c\n%i\n%.3f\n", myData.a, myData.x, myData.f);


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if( mode == 1 )

printf("The char is being used\n");

else if( mode == 2 )

printf("The int is being used\n");else if( mode == 3 )

printf("The float is being used\n");

return 0;

Summary

Union allows same storage to be referenced in

different ways

Only one way is valid at any given time


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Usage

access individual bytes of larger type

variable format input records (coded records)

sharing an area to save storage usageunions not used nearly as much as structures


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enum in C++

enum(enumeration) is a user-defined type consisting of a set of

enumerators( enumerator --- named integer constant)


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nue_acomany{Audi, BMW, Cadillac, Ford, Jaguar,

Lexus Maybach, RollsRoyce, Saab

};


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e_acompany my_car_brand;

my_car_brand = RollsRoyce;//... if(my_car_brand == Ford)

cout


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