类模板 友元

3类友元   1>非模板友元   2>约束模板友元(外部声明模板)   3>非约束模板友元(内部声明模板)
 
1>非模板友元

这类友元不是成员函数，所以可能需要用到的实例化 都要有定义(函数重载)
/////////////////////////////////////////////////////////
//
// frnd2tmp.cpp -- template class with non-template friends
//
/////////////////////////////////////////////////////////
#include <iostream>
using std::cout;
using std::endl;

template <typename T>
class HasFriend
{
private:
    T item;
    static int ct;
public:
    HasFriend(const T & i) : item(i) {ct++;}
    ~HasFriend()  {ct--; }
    friend void counts();
    friend void reports(HasFriend<T> &); // template parameter
};

// each specialization has its own static data member
template <typename T>
int HasFriend<T>::ct = 0;

// non-template friend to all HasFriend<T> classes
void counts()
{
    cout << "int count: " << HasFriend<int>::ct << "; ";
    cout << "double count: " << HasFriend<double>::ct << endl;
}

// non-template friend to the HasFriend<int> class
void reports(HasFriend<int> & hf)                  //重载
{
    cout <<"HasFriend<int>: " << hf.item << endl;
}

// non-template friend to the HasFriend<double> class
void reports(HasFriend<double> & hf)              //重载
{
    cout <<"HasFriend<double>: " << hf.item << endl;
}

int main()
{
    cout << "No objects declared: ";
    counts();
    HasFriend<int> hfi1(10);
    cout << "After hfi1 declared: ";
    counts();
    HasFriend<int> hfi2(20);
    cout << "After hfi2 declared: ";
    counts();
    HasFriend<double> hfdb(10.5);
    cout << "After hfdb declared: ";
    counts();
    reports(hfi1);
    reports(hfi2);
    reports(hfdb);
 
    return 0;
}

 

 

 

 

 

 

 

2>约束模板友元(外部声明模板)

1.类定义前先对友元声明
2.在类中再次声明  (即是为模板函数具体化)   (与模板类被实例化的数据类型一致)

/////////////////////////////////////////////////////////
//
// tmp2tmp.cpp -- template friends to a template class
//
////////////////////////////////////////////////////////
#include <iostream>
using std::cout;
using std::endl;

// template prototypes
template <typename T> void counts();
template <typename T> void report(T &);

// template class
template <typename TT>
class HasFriendT
{
private:
    TT item;
    static int ct;
public:
    HasFriendT(const TT & i) : item(i) {ct++;}
    ~HasFriendT() { ct--; }
    friend void counts<TT>();
    friend void report<HasFriendT<TT> >(HasFriendT<TT> &);
};

template <typename T>
int HasFriendT<T>::ct = 0;

// template friend functions definitions
template <typename T>
void counts()
{
    cout << "template size: " << sizeof(HasFriendT<T>) << "; ";
    cout << "template counts(): " << HasFriendT<T>::ct << endl;
}

template <typename T>
void report(T & hf)
{
    cout << hf.item << endl;
}

int main()
{
    counts<int>();
    HasFriendT<int> hfi1(10);
    HasFriendT<int> hfi2(20);
    HasFriendT<double> hfdb(10.5);
    report(hfi1);  // generate report(HasFriendT<int> &)
    report(hfi2);  // generate report(HasFriendT<int> &)
    report(hfdb);  // generate report(HasFriendT<double> &)
    cout << "counts<int>() output:\n";
    counts<int>();
    cout << "counts<double>() output:\n";
    counts<double>();
   
   
    return 0;
}

 

 

 

 

 

 

 

3>非约束模板友元(内部声明模板)  (根据传入函数参数来确定实例化)

// manyfrnd.cpp -- unbound template friend to a template class
#include <iostream>
using std::cout;
using std::endl;

template <typename T>
class ManyFriend
{
private:
    T item;
public:
    ManyFriend(const T & i) : item(i) {}
    template <typename C, typename D> friend void show2(C &, D &);
};

template <typename C, typename D> void show2(C & c, D & d)
{
    cout << c.item << ", " << d.item << endl;
}

int main()
{
    ManyFriend<int> hfi1(10);
    ManyFriend<int> hfi2(20);
    ManyFriend<double> hfdb(10.5);
    cout << "hfi1, hfi2: ";
    show2(hfi1, hfi2);
    cout << "hfdb, hfi2: ";
    show2(hfdb, hfi2);

    return 0;
}