Friend

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Friend Function 介紹

我們不能從該Class的外部access該Class的private members。
但是，如果將non member function聲明為class的friend，我們就可以使其access該class的private member。

這是通過在Class中包含此外部function的聲明並在其前面加上關鍵字friend來實現的。

Syntax :

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friend return_type function_name(parameters);

Friend Function(Pass by Reference)

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class MyClass {
 public:
  MyClass() {
   regVar = 0;
  }
 private:
  int regVar;
    
  //Allow someFunc() to access private member of class MyClass
  friend void someFunc(MyClass &obj); //Pass by reference
  //Note that when passing an object to the function,
  //we need to pass it by reference
  //using the & operator.
};

//The function someFunc() is defined as a regular function outside the class. 
//It takes an object of type MyClass as its parameter, 
//and is able to access the private data members of that object.
void someFunc(MyClass &obj) {
  obj.regVar = 42;
  cout << obj.regVar;
}

int main() {
  MyClass obj;
  someFunc(obj);
}

//Outputs 42

請注意，將Object傳遞給function時，我們需要使用＆運算符通過reference將其傳遞(Pass by reference)。因此，我們可以使用實際對象，而不是對象的copy(副本)。 如果不用＆的話，我們是Pass by value(通過值傳遞)，創建對象的副本。

Friend Function(Pass by value)

如果用 Pass by value呢？

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#include <iostream>
using namespace std;
class XYZ {
private:
   int num=100;
   char ch='Z';
public:
   friend void disp(XYZ obj); //Pass by value
};
//Global Function
void disp(XYZ obj){
   cout<<obj.num<<endl; 
   cout<<obj.ch<<endl;
}
int main() {
   XYZ obj;
   disp(obj);
   return 0;

   //output:
   //100
   //Z
}

friend的用法就是在兩個不同Class之間進行的操作，它們訪問兩者的private member。

而且您可以在任意多個Class中聲明一個friend function。

實例：

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#include <iostream>
using namespace std;
// forward declaration
class B;
class A {
    private:
      int numA;
    public:
      A(): numA(12) { }
      // friend function declaration
      friend int add(A, B);
};
class B {
    private:
       int numB;
    public:
       B(): numB(1) { }
       // friend function declaration
       friend int add(A , B);
};
// Function add() is the friend function of classes A and B
// that accesses the member variables numA and numB
int add(A objectA, B objectB)
{
   return (objectA.numA + objectB.numB);
}
int main()
{
    A objectA;
    B objectB;
    cout<<"Sum: "<< add(objectA, objectB);
    return 0;

    //Sum: 13
}

Friend Class

您可以定義一個friend class,讓這個friend class access其他class的private member。當一個class成為friend class時，該class的所有member functions都將成為friend functions。

在此示例中，我們有兩個Class - XYZ和ABC。 XYZ Class具有兩個private member ch和num，該類將ABC聲明為friend class。 這意味著ABC可以訪問XYZ的private member ，在ABC class的function disp()access private member num和ch的示例中已經證明了這一點。

在此示例中，我們是用Pass by value。

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#include <iostream>
using namespace std;
class XYZ {
private:
   char ch='A';
   int num = 11;
public:
   /* This statement would make class ABC
    * a friend class of XYZ, this means that
    * ABC can access the private and protected
    * members of XYZ class. 
    */
   friend class ABC;
};
class ABC {
public:
   void disp(XYZ obj){
      cout<<obj.ch<<endl;
      cout<<obj.num<<endl;
   }
};
int main() {
   ABC obj;
   XYZ obj2;
   obj.disp(obj2);
   return 0;

   //output :
   //A
   //11
}

This

C++中的每個Object都可以通過稱為this pointer來訪問其自己的address(地址)。
在member function中，this指 refer to the invoking object。

friend function沒有this pointer，因為friends不是class的member。

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class MyClass {
 public:
  MyClass(int a) : var(a)
  { }
  void printInfo() {
   //All three alternatives will produce the same result.
   cout << var<<endl;
   cout << this->var<<endl; //this is a pointer to the object, so the arrow selection operator is used to select the member variable.
   cout << (*this).var<<endl; 
  }
 private:
  int var;
};

int main() {
  MyClass obj(42);
  obj.printInfo();
}

/* Outputs
42
42
42
*/

注意，只有member function具有this指針。

this關鍵字在operator overloading中有著重要的作用(其實不太重要)。

Reference

C++ friend Function and friend Classes
Friend Class and Friend Functions in C++