c++ - Unresolved template argument -

i testing template codes today and, find out interesting not find reason explain why happen. request consider , enlighten me knowledge. time.

this code block working without problem.

template<class titem> class printablequeue : public queue<titem> {     public:         friend ostream& operator<<(ostream& os, const printablequeue<titem>& queue) {             copy(queue.c.begin(), queue.c.end(), ostream_iterator<titem>(os, " "));             return os;         } }; int main(int argc, const char* argv[])     printablequeue<int> queue;     queue.push(1);     queue.push(2);      cout << queue; } 

however, when put definition of friend function outside of class, not work.

template<class titem> class printablequeue : public queue<titem> {     public:         friend ostream& operator<<(ostream& os, const printablequeue<titem>& queue); }; ostream& operator<<(ostream& os, const printablequeue<titem>& queue) {     copy(queue.c.begin(), queue.c.end(), ostream_iterator<titem>(os, " "));     return os; } 

the errors got below.

'titem' : undeclared identifier 'printablequeue' : 'titem' not valid template type argument parameter 'titem' 

my question is, why compiler can not resolve titem ?

you need make function function template.

first change decleration of operator<< in class , include titem signature

friend ostream& operator<< <titem>(ostream& os, const printablequeue<titem>& queue); 

than change function definition template function

template <typename titem> ostream& operator<<(ostream& os, const printablequeue<titem>& queue) {     copy(queue.c.begin(), queue.c.end(), ostream_iterator<titem>(os, " "));     return os; }