How can friend function be implemented?

Answer 1

All class member functions have the following qualities:

1. Private access to the class members.

2. Scoped to the class.

3. Has an implicit 'this' pointer.

Static member functions only have the first two qualities while friend functions only have the first quality.

Friend functions must be defined outside of the class in which they are declared a friend -- only the function prototype is required in the friend declaration.

Answer 2

If a non-member function requires access to a class representation where a public interface would be undesirable, you have to declare that function a friend of the class. Only the class itself can grant friend access.

All non-static member functions have the following three properties:

1. Private access to the class representation.

2. Scoped to the class.

3. Must be invoked on an object of the class (has a this pointer).

Static member functions only have the first two properties while friend functions only have the first property. All other functions have none of these properties.

Answer 3

Friend functions extend the class interface outside of the class. Normally, classes are self-contained entities combining data and the methods that operate upon that data. Only class members have access to the private members of the class. However, sometimes it is desirable to allow external functions to gain access to the private members of a class. This should only be done when a function cannot be considered a member of the class, but where the public interface is insufficient for the function to operate.

Friend functions are best utilised when two or more classes must work closely with one another. In a one-way friendship, one or more of the dominant class' methods will be declared as friend functions of the subservient class. In a two-way friendship, the entire dominant class will be declared a friend of the subservient class while one or more of the subservient class' methods are declared friend functions of the dominant class.

External functions that are not members of a class but that require private access to a class can also be declared friends of the class. External functions are often employed where two or more classes work quite closely together but have no friendship with each other. The external functions then act as a link between these otherwise separate classes.

Since friends are highly privileged, you should never grant friendship to a class or function that you have no control over. Allowing a third party to gain friend access to your class could easily undermine your class' objectives. But while many will claim friendship undermines encapsulation this is, frankly, errant nonsense. When you are in total control of the class and all its friends, then only you -- the programmer -- can undermine the encapsulation. Friends are effectively members of the class and they must adhere to the exact same rules that internal members of the class must adhere to. And since you wrote the rules in the first place, the onus is upon you to stick by them: the compiler cannot help you on this regard.

That said, friendship should only be granted when it is absolutely necessary to do so. That is, if the public interface is insufficient, then you can either create a public interface to suit the function (which is not always desirable), or grant friend access to the private interface.

Answer 4

A friend function is a function that is granted privileged access to the private members of the class in which it is declared a friend. In other words, it has the same access rights as a class member function.

Friends are used to extend a class interface. However, there should always be a good reason for declaring a friend. If a function does not require privileged access to private members of a class, then it has no business being a friend. When people say friends undermine encapsulation, this is only true when friend access is inappropriate. There are always alternatives to granting a friend access to private data, but when used appropriately, friends can only enhance encapsulation, never undermine it.

Answer 5

Friend functions are implemented just like any other function. The only real difference is that they have private access to the classes in which they are declared friends.

We can think of an ordinary class member function as having three basic properties:

1. It has access to the private parts of the class.

2. It is scoped to the class.

3. It is scoped to an instance of the class (has a this pointer).

A static member function only has the first two properties while a friend function only has the first property.

Answer 6

The only use of a friend function when overloading an operator (or indeed with regard to any external function) is when no interface is available that would allow the operator (or function) to perform the operation, and where enabling a suitable interface would result in undesirable exposure that would undermine the encapsulation of the class.

It is important that the friend be treated just as if it were an actual member of the class. That is, just as members of the class must enforce encapsulation from within the class, friend operators and functions must enforce encapsulation from outwith the class. This generally means that the programmer directly responsible for the class implementation must also be responsible for implementing the friend. Allowing third parties to gain control over the friend implementation will undermine encapsulation, in which case it would far less harmful to simply provide a suitable interface and prohibit the friend access altogether.

Answer 7

A friend function is a function that cannot be declared a member of a class but which requires private access to that class. For example, a function that operates upon two different classes cannot be a member of both classes, but if the function requires private access to both classes then it has to be a friend to at least one of them.

To fully appreciate friend functions, consider that a non-static member function has the following three properties:

1. Has private access to the class.
2. Is scoped to the class.
3. Must be invoked against an object of the class (has a 'this' pointer).

Static member functions have the first two properties only while friend functions have the first property only. All other non-member functions have none of these properties.

Answer 8

It is a function with keyword friend. it is a non member function of a class that as the access to the private and protected members of a class.hope that helps regard SID

Answer 9

A friend function is a function that has access to all the private and/or protected members of the class in which it is declared.