类模板(class template)的友元(friend)和普通类的友元, 有许多区别,
主要原因是类模板包括模板参数(template parameter), 就会导致友元和类模板有许多匹配形式;
主要包括六种形式:
1. 类 – 友元(friend): "模板参数是当前类"的类模板;
2. 类 – 友元: "模板参数任意"的模板类;
3. 模板类 – 友元: "模板参数沟通"的模板类;
4. 模板类 – 友元: "模板参数任意"的模板类
5. 模板类 – 友元: 类;
6. 模板类 – 友元: 当前模板参数(类); (C++11)
详细利用拜见代码及注释,
代码如下:
/*
* cppprimer.cpp
*
* Created on: 2013.11.24
* Author: Caroline
*/
/*eclipse cdt, gcc 4.8.1*/
#include <iostream>
#include <string>
template <typename T> class Pal; //pal伴侣
class C {
friend class Pal<C>; //"以类C实例化"的Pal类, 为C的友元
template <typename T> friend class Pal2; //Pal2类的所有实例化, 都为C的友元
private:
void print() { std::cout << "class C" << std::endl; }
};
template <typename T>
class C2 {
friend class Pal<T>; //"与C2类沟通实例化"的Pal类, 为C2的友元
//Pal2类的所有实例化, 都为C2的友元, 留意模板参数(X)不能沟通
template <typename X> friend class Pal2;
friend class Pal3; //普通友元
friend T; //C++11 模板范例参数友元
private:
void print() { std::cout << "class C2" << std::endl; }
};
template <typename T>
class Pal {
C myC;
C2<T> myC2; //必需为T
//C2<double> myC2; //实例化差异, 无法利用
public:
void printC() {
std::cout << "this is class Pal : ";
myC.print();
}
void printC2() {
std::cout << "this is class Pal : ";
myC2.print();
}
};
template <typename T>
class Pal2 {
C myC;
C2<double> myC2;
public:
void printC() {
std::cout << "this is class Pal2 : ";
myC.print();
}
void printC2() {
std::cout << "this is class Pal2 : ";
myC2.print();
}
};
class Pal3 {
C2<double> myC2;
public:
void printC2() {
std::cout << "this is class Pal3 : ";
myC2.print();
}
};
class Pal4 {
C2<Pal4> myC2; //留意Pal4是C2类的模板参数
public:
void printC2() {
std::cout << "this is class Pal4 : ";
myC2.print();
}
};
int main (void) {
std::cout << "Hello Mystra!" << std::endl;
Pal<C> pc; //Pal类必需实例化为C
pc.printC(); //可以利用
//Pal<int> pci;
//pci.print(); //报错, 会见了私有成员
Pal2<int> pi2; //Pal2类可以随意实例化
pi2.printC();
Pal<int> pi; //有沟通的示例化, 在类声明C2<T>
pi.printC2();
pi2.printC2(); //留意, Pal2类被实例化为<int>, 内部C2类被实例化为<double>;
Pal3 p3;
p3.printC2();
Pal4 p4;
p4.printC2();
return 0;
}
输出:
Hello Mystra! this is class Pal : class C this is class Pal2 : class C this is class Pal : class C2 this is class Pal2 : class C2 this is class Pal3 : class C2 this is class Pal4 : class C2
作者：csdn博客 Spike_King
