模板类例子

#include<iostream>
using namespace std;
template <typename T>
class HasFriend
{
private:
    T item;
    static int ct;
public:

    HasFriend(const T & i) :item(i) {ct++;}     ----1,
    friend void counts();
    friend void reports(HasFriend<T> &);
};
template <typename T> int HasFriend<T>::ct=0;   ----2,
void counts()
{
    cout<<"int count: "<<HasFriend<int>::ct<<";";      // undefined reference to `HasFriend<int>::ct'   if no ---2 compiler error
}
void reports(HasFriend<int> &hf)
{
    cout<<"reports"<<endl;
}
int main()
{
    counts();
    HasFriend<int> abc(10);  //../template1.cpp:27:23: note: candidates are:
../template1.cpp:4:7: note: HasFriend<int>::HasFriend()
../template1.cpp:4:7: note:   candidate expects 0 arguments, 1 provided
../template1.cpp:4:7: note: HasFriend<int>::HasFriend(const HasFriend<int>&)
../template1.cpp:4:7: note:   no known conversion for argument 1 from ‘int’ to ‘const HasFriend<int>&’    if no ---1 compiler error

    counts();

}

结果 ：int count: 0;int count: 1;