A friend is permitted full access to private and protected members. 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.
Use the friend
specifier to
declare a friend in the class granting friendship. Note that friendship
is given, not taken. In other words, if class A contains the declaration
friend
class
B;
, class B can access the private members of A, but A has no
special privileges to access B (unless B declares A as a friend).
By convention, the friend
specifier is usually first, although it can appear in any order with
other function and type specifiers. The friend
declaration can appear anywhere in the
class; the access level is not relevant.
You cannot use a storage class specifier in a friend declaration. Instead,
you should declare the function before the class definition (with the
storage class, but without the friend
specifier), then redeclare the function in the class definition (with
the friend
specifier and without the
storage class). The function retains its original linkage. If the
friend
declaration is the first
declaration of a function, the function gets external linkage. (See
Chapter 2 for more information
about storage classes and linkage.) For example:
class demo; static void func(demo& d); class demo { friend void func(demo&); ...
Friendship is not transitive—that is, the friend of my friend is
not my friend (unless I declare so in a separate friend
declaration)—nor is a nested class a
friend just because the outer class is a friend. (See the next section,
Section 6.7, for more
information.)
Friendship is not inherited. If a base class is a friend, derived classes do not get any special privileges.
You cannot define a class in a friend
declaration, but you can define a
function, provided the class granting friendship is not local to a
block. The function body is in the class scope, which affects name
lookup (see Chapter 2). The friend
function is automatically inline. Usually, friend functions are
declared, not defined, in the class.
A declaration or definition of a friend function does not make that function a member of the class and does not introduce the name into the class scope. Example 6-26 shows several different kinds of friends.
Example 6-26. Friend functions and classes
#include <iterator> // Simple container for singly-linked lists template<typename T> class slist { // Private type for a link (node) in the list template<typename U> struct link { link* next; U value; }; typedef link<T> link_type; // Base class for iterator and const_iterator. Keeps track of current node, and // previous node to support erase( ). class iter_base : public std::iterator<std::forward_iterator_tag, T> { protected:friend class slist;
// So slist can construct iterators iter_base(slist::link_type* prv, slist::link_type* node); slist::link_type* node_; slist::link_type* prev_; }; public: typedef T value_type; typedef std::size_t size_type; class iterator : public iter_base { // Members omitted for bevity . . . private:friend class slist;
// So slist can call constructor iterator(slist::link_type* prev, slist::link_type* node) : iter_base(prev, node) {} };friend class iter_base;
// So iter_base can use link_typefriend class iterator;
// So iterator can use link_typetemplate<typename U> friend void swap(slist<U>& a, slist<U>& b);
iterator begin( ) { return iterator(0, head_); } iterator end( ) { return iterator(0, 0); } private: link_type* head_; size_type count_; }; template<typename T> slist<T>::iter_base::iter_base(slist::link_type* prev, slist::link_type* node) : prev_(prev), node_(node) {} // Swap two lists in constant time by swapping members. template<typename T> void swap(slist<T>& a, slist<T>& b) { typename slist<T>::link_type* tmp_head = a.head_; typename slist<T>::size_type tmp_count = a.count_; a.head_ = b.head_; a.count_ = b.count_; b.head_ = tmp_head; b.count_ = tmp_count; }