剖析C++编程中friend关键字所修饰的友元函数和友元类

  friend class-name;
friend function-declarator;

class ForwardDeclared; // Class name is known.
class HasFriends
{
 friend int ForwardDeclared::IsAFriend(); // C2039 error expected
};

// friend_functions.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
class Point
{
 friend void ChangePrivate( Point & );
public:
 Point( void ) : m_i(0) {}
 void PrintPrivate( void ){cout << m_i << endl; }

private:
 int m_i;
};

void ChangePrivate ( Point &i ) { i.m_i++; }

int main()
{
 Point sPoint;
 sPoint.PrintPrivate();
 ChangePrivate(sPoint);
 sPoint.PrintPrivate();
// Output: 0
   1
}

// classes_as_friends1.cpp
// compile with: /c
class B;
class A {
public:
 int Func1( B& b );
private:
 int Func2( B& b );
};
class B {
private:
 int _b;
 // A::Func1 is a friend function to class B
 // so A::Func1 has access to all members of B
 friend int A::Func1( B& );
};
int A::Func1( B& b ) { return b._b; } // OK
int A::Func2( B& b ) { return b._b; } // C2248

friend class A;

// classes_as_friends2.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
class YourClass {
friend class YourOtherClass; // Declare a friend class
public:
 YourClass() : topSecret(0){}
 void printMember() { cout << topSecret << endl; }
private:
 int topSecret;
};

class YourOtherClass {
public:
 void change( YourClass& yc, int x ){yc.topSecret = x;}
};

int main() {
 YourClass yc1;
 YourOtherClass yoc1;
 yc1.printMember();
 yoc1.change( yc1, 5 );
 yc1.printMember();
}