1. The
concept of friend : through friends, the encapsulation of the class is broken, and all members in the class can be accessed
Category: friend function, friend class
2. Friend function
concept:
-
The friend function is an ordinary function, not belonging to the class, but the friend relationship needs to be indicated in the class
-
Friend function can access all members in the class, but the class cannot access the internal data of the friend function
format
- When a friend function can be declared in a class, the keyword friend needs to be added
- Friend functions can be declared outside the class, and defined outside the class. Need to add the keyword friend
class Cperson
{
private:
int age;
public:
friend void setPersonAge(Cperson& p,int age);
};
void setPersonAge(Cperson& p,int age) // 函数在类外声明和定义
{
p.age=age;
}
int main()
{
Cperson person;
setPersonAge(person,18);
}
class Cperson
{
private:
int age;
public:
friend void setPersonAge(Cperson& p,int age) // 友元函数在类内定义
{
p.age=age;
}
};
void setPersonAge(Cperson& p,int age)；
int main()
{
Cperson person;
setPersonAge(person,18);
}
Three, Tomomoto
-
The friend class is not a member of the class, and does not have this pointer
-
When a class A becomes a friend class of another class B, class A can access all members of class B
There are two forms of friend classes:
making the whole class a friend,
making a certain part of the function in the class a friend
Make the whole class friends
class Cb; // 声明类
class Ca
{
private:
int num;
public:
friend class Cb; // 使整个类成为Ca的友元，则Cb所有的成员都可以访问Ca的所有成员
};
class Cb
{
public:
void setCaNum(Ca& a);
void Func();
};
Make a certain part of the function in the class friends
class Cb; // 声明类
class Ca
{
private:
int num;
public:
friend void Cb::setCaNum(Ca& a);// 只有Cb的setCaNum成为Ca的友元
};
class Cb
{
public:
void setCaNum(Ca& a);
void Func();
};
The friendship relationship cannot be inherited.
4. The characteristics of friendship
-
Unidirectional: Friends can access classes, but classes cannot access friends
-
Non-transitivity: A is a friend of B, C is a friend of A, but C is not a friend of B
-
No inheritance: There is an introduction in the above friend class.
5. The relationship between the declaration of the friend and the scope -
When using a friend, consider the declaration and scope of its friend, please see the following code
class X
{
public:
friend void f(){
} // 友元函数在类内定义
X() {
f(); } // 错误，f()函数在下面才声明，此处检测不到
void g();
void h();
};
void X::g(){
return f(); } // 错误，此时f()函数还没有被声明
void f(); // 声明函数
void X::h(){
return f(); } // 正确，检测到f()函数被声明