A friend function of a class is defined outside that class' scope but it has the right to access all private and protected members of the class. Even though the prototypes for friend functions appear in the class definition, friends are not member functions.
A friend can be a function, function template, or member function, or a class or class template, in which case the entire class and all of its members are friends.
To declare a function as a friend of a class, precede the function prototype in the class definition with keyword friend as follows −
class Box { double width; public: double length; friend void printWidth[ Box box ]; void setWidth[ double wid ]; };
To declare all member functions of class ClassTwo as friends of class ClassOne, place a following declaration in the definition of class ClassOne −
friend class ClassTwo;
Consider the following program −
#include using namespace std; class Box { double width; public: friend void printWidth[ Box box ]; void setWidth[ double wid ]; }; // Member function definition void Box::setWidth[ double wid ] { width = wid; } // Note: printWidth[] is not a member function of any class. void printWidth[ Box box ] { /* Because printWidth[] is a friend of Box, it can directly access any member of this class */ cout edges_.insert[to]; } auto begin[] const { return vertices_.cbegin[]; } auto end[] const { return vertices_.cend[]; } private: std::unordered_set vertices_; };
Encapsulation[edit]
A proper use of friend classes increases encapsulation, because it allows to extend the private access of a data-structure to its parts --- which the data-structure owns --- without allowing private access to any other external class. This way the data-structure stays protected against accidental attempts at breaking the invariants of the data-structure from outside.
It is important to notice that a class cannot give itself access to another class's private part; that would break encapsulation. Rather, a class gives access to its own private parts to another class --- by declaring that class as a friend. In the graph example, Graph cannot declare itself a friend Vertex. Rather, Vertex declares Graph a friend, and so provides Graph an access to its private fields.
The fact that a class chooses its own friends means that friendship is not symmetric in general. In the graph example, Vertex cannot access private fields of Graph, although Graph can access private fields of Vertex.
Alternatives[edit]
A similar, but not equivalent, language feature is given by C#'s internal keyword, which allows classes inside the same assembly to access the private parts of other classes. This corresponds to marking each class a friend of another in the same assembly; friend classes are more fine-grained.
Programming languages which lack support for friend classes, or a similar language feature, will have to implement workarounds to achieve a safe part-based interface to a data-structure. Examples of such workarounds are:
- Make the parts' fields public. This solution decreases encapsulation by making it possible to violate invariants of the data-structure from outside.
- Move all mutable structural data away from the part to the data-structure, and introduce indirection back from each part to its data-structure. This solution changes the organization of the data structure, and increases memory consumption in cases where there would otherwise be no need for this information.
Properties[edit]
- Friendships are not symmetric – if class
A
is a friend of classB
, classB
is not automatically a friend of classA
. - Friendships are not transitive – if class
A
is a friend of classB
, and classB
is a friend of classC
, classA
is not automatically a friend of classC
. - Friendships are not
inherited – if class
Base
is a friend of classX
, subclassDerived
is not automatically a friend of classX
; and if classX
is a friend of classBase
, classX
is not automatically a friend of subclassDerived
. However, if classY
is a friend of subclassDerived
, classY
will also have access to protected portions of classBase
, just as subclassDerived
does.
See also[edit]
- Friend function
References[edit]
- ^ "9 More on C++".
External links[edit]
- //publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fcplr043.htm
- //www.cplusplus.com/doc/tutorial/inheritance/