Question

template <> 
class test<int> {
    int y; 
public:     
    test(int k) : y(k) {}     
    friend ofstream& operator<< <test<int>> (ofstream& os, const test<int>& t); 
};  
template<> 
ofstream& operator<< <test<int> > (ofstream& os, const test<int>& t) 
{
    os << t.y;
    return os;
}  

The code above is specialized template class of test in a int version. I am trying to overload ofstream operator<< function. But it shows error message;

C2027: use of undefined type ‘std::basic_ofstream<_Elem,_Traits>’

Besides, the same method works on a ordinary function (not ofstream operator<< but the function that I make) Is there anyway to us operator<< function of ofstream in a specialized template class ?

Answer 1

You need to include

 #include <iostream>

At the time of instantiation of the function template. Perhaps you only included

 #include <iosfwd>

Besides, you shouldn’t be defining (static) friend as a template: https://ideone.com/1HRlZ

#include <iostream>

template <typename> class test;

template <> 
class test<int> {
    int y; 
public:     
    test(int k) : y(k) {}     
    friend std::ostream& operator<<(std::ostream& os, const test& t); 
};  

std::ostream& operator<< (std::ostream& os, const test<int>& t) 
{
    return os << t.y;
}  

int main()
{
    test<int> a(42);
    std::cout << a << std::endl;
}

Note that it isn’t a good idea to have ‘using namespace std’ in your header file, which is why I removed it from the sample. (It might cause conflicts for users of your header file when they include your header)