c/c++ 模板与STL小例子系列<二> 模板类与友元函数
比如某个类是个模板类D,有个需求是需要重载D的operator<<函数,这时就需要用到友元。
实现这样的友元需要3个必要步骤
1,在模板类D的实现代码的上面声明友元函数
template<typename> class D;//因为友元函数的参数里使用了D,所以要先在这里声明一下
template<typename T>
ostream& operator<< (ostream&, const D<T> &);
2,在模板类D的实现代码里面声明它是我的友元
//注意operator<<后面有<T>
friend ostream& operator<< <T>(ostream& ,const D<T>&);
3,实现友元函数
template<typename T>
//注意operator<<后面没有<T>
ostream& operator << (ostream& out,const D<T>& d){
out << d.x;
return out;
}
例子代码:
#include <iostream>
using namespace std;
template<typename T>
class Test{
public:
Test(T t) : data(t){}
virtual void show() = 0;
private:
T data;
};
template<typename> class D;
template<typename T>
ostream& operator<< (ostream&, const D<T> &);
template<typename T>
class D : public Test<T>{
//注意有<T>
friend ostream& operator<< <T>(ostream& ,const D<T>&);
public:
//注意不是Test(t1)
D(T t1, T t2) : Test<T>(t1), x(t2){}
void show(){
cout << x << ", " << x << endl;
}
private:
T x;
};
template<typename T>
ostream& operator << (ostream& out,const D<T>& d){
out << d.x;
return out;
}
int main(void){
Test<int> *p = new D<int>(10, 21);
p->show();
D<int> d(10,20);
cout << d << endl;
return 0;
}
模板类继承非模板类,非模板类继承模板类
下面的例子没有什么实际意义,只看语法。
#include <iostream>
using namespace std;
class Foo{
public:
Foo(int a, int b, int c) : x(a), y(b), z(c){}
void show(){
cout << x << "," << y << "," << z << endl;
}
private:
int x, y, z;
};
template <typename T>
class Goo : public Foo{
public:
Goo(T t, int a, int b, int c):Foo(a,b,c), data(t){}
void show(){
cout << data << endl;
cout << "Goo show" << endl;
}
private:
T data;
};
class Hoo : public Goo<int>{
public:
Hoo(int a1,int a2,int a3,int a4,int a5):
Goo(a1,a2,a3,a4),ho(a5){}
void show(){
cout << "Hoo show" << endl;
}
private:
int ho;
};
int main(void){
Hoo hoo(1,2,3,4,5);
hoo.show();
Goo<string> goo("abc",1,2,3);
goo.show();
return 0;
}