C++（11）：友元、嵌套类

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友元（Friend）

友元函数（Friend Function）

Hiding data inside a class and letting only class member functions have direct access to private data is a very important OOP concept.
But C++ also provides another of function to access members in class friend functions
– friend functions are not member functions but can still access class private members
– defined outside of class scope

Reasons to have friend functions:
– to access private members of two or more different classes
– for I/O or operator functions

作为I/O或者运算符的重载，我在前面的文章总结过了。

Review：C++运算符重载

友元函数的特性
– placed inside the class definition and preceded by the friend keyword
– defined outside the class as a normal, nonmember function
– called like a normal non-member function.
If a function is a friend of two or more different classes, each class must contain the friend function prototype within its body.
A friend function cannot be inherited but can be a member of one class.

注意。

1. 类内声明（要加friend修饰词），类外定义（不能加friend修饰词）

2. 不能被继承。

3. 友元函数不是成员函数！使用的时候，一定不能写 obj1.myFriendFun1();！因为不是成员函数！点不出来的！

友元类（Friend Classes）

An entire class can be a friend of another class
– can be used when all member functions of one class should access the data of another class
– require prototypes to be placed within each class individually

An entire class can be designed as friend
– all its member functions automatically granted a friendship by the class
– but “class B is a friend of class A” does not imply “class A is a friend of class B”

//in CPoint.hclass CPoint {friend class CLine; // 我（CPoint）把你（CLine）当朋友，你就可以取到我的东西int x, y;void Offset(int diff) {x += diff; y += diff;}public:CPoint() { x=0; y=0; }CPoint(int a, int b) { x=a; y=b; }void set(int a, int b) {x=a; y=b;} void Print() {cout << x << “ ” << y << endl;}};

//in CLine.hclass CLine {CPoint p1, p2;public:CLine(int x, int y, int w, int z) {p1.x = x; p1.y = y; //access privatep2.x = w; p2.y = z; }void Print()() {//call public Print in CPointcout << “Point 1:”; p1.Print();cout << “Point 2:”; p2.Print(); }void Display() {offset(p1,100); //call friend funcp2.Offset(200); //call private funcPrint(p1, p2); }};

友元类，好像现在看起来，发明者提供这种写法，好像也只是为了让代码的写作更加简单……

e.g. A为了访问B中的成员变量，就用set和get不就好了吗……

嵌套类（Nested Class）

感觉有点像内部类。我在做Android开发的时候，还遇到过匿名内部类……

e.g. xxxButton.setOnClickListener(new xxxListener() {

@override

void xxxOnClick(Event e) {

xxx;

}

});

这种…也不知道记错没有……感觉好像是这样…………

Nested class cannot access any private member of the outer class by default.

嵌套类默认情况下，不能访问外部的类的私有成员。但，可以用friend解决。

e.g. 在外部类A中声明嵌套类B是它的朋友，friend class B;

Inner class is not visible globally.

A nested class CTwo in COne can have the same kind of members as a nonnested class.

链表的定义，一种递归的定义。

class Chain { //outer classpublic:class Item { //all members are privatefriend class Chain;Item(int val = 0); //constructorItem *next; //point to its own classint val;};//...private:Item *chain;Item *at_end;};

注意，在List中，这个Item * next不是自己指向自己，而是说，可以指向这个相同class的实例！

（当然，也可以自己指向自己…）

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