Operators >> and <<

[Snaksa]

Hello! I'm trying to redefine the >> and << operators. Everything is working well when I try to do it with ordinary data structures. The problem comes when I try to use a template.

ostream& operator<<(ostream& ostr, Stack<T>& stack);
istream& operator>>(istream& istr, Stack<T>& stack);

The compiler says that I can't define Stack<T>& stack;
Can somebody tell me how can I solve this problem?

Greetings!

[S G H]

Try...

template <typename  T>
ostream& operator<<(ostream& ostr, Stack<T>& stack);
template <typename  T>
istream& operator>>(istream& ostr, Stack<T>& stack);

(Yes, you just needed to add template <typename T> before declaration)

[Snaksa]

Oh, I have forgot to say that I have template<class T> in my code. I changed it to template<typename T> but it still prints the same error:

code.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Stack<int> &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@AAV?$Stack@H@@@Z) referenced in function _main

[Cubbi]

That's a linking error, your code actually compiled. I can only guess that you placed the definition of those operators in some source file instead of the header.

[Snaksa]

Everything is in one .cpp file. The program doesn't even run because of this error...

[Snaksa]

Here is my whole code if you want to see it:

#include<iostream>
#include<iomanip>
using namespace std;

template<class T>
struct Node
	{
		T data;
		Node<T> *next;
	};

template<typename T>
class Stack
	{
	private:
		Node<T> *top;
	public:
		Stack();
		void Push(T);
		void Pop();
		void Print_Stack(ostream&);
		friend istream& operator>>(istream&, Stack<T>&);
		friend ostream& operator<<(ostream&, Stack<T>&);
	};

template<typename T>
istream& operator>>(istream& istr, Stack<T> &stack)
	{
		T data;
		istr >> data;
		stack.Push(data);
		return istr;
	}

template<typename T>
ostream& operator<<(ostream& ostr, Stack<T> &stack)
	{
		stack.Print_Stack(ostr);
		return ostr;
	}

template<typename T>
Stack<T>::Stack()
	{
		top = NULL;
	}

template<typename T>
void Stack<T>::Push(T data)
	{
		Node<T> *temp = new Node<T>;
		temp->data = data;
		temp->next = top;
		top = temp;
	}

template<typename T>
void Stack<T>::Pop()
	{
		Node<T> *temp = top;
		top = top->next;
		delete temp;
	}

template<typename T>
void Stack<T>::Print_Stack(ostream& ostr)
	{
		Node<T> *temp = top;
		while(temp)
			{
				ostr << temp->data << " ";
				Pop();
				temp = top;
			}
		cout << endl;
	}



int main()
	{
		Stack<int> A;
		cin >> A;
		cin >> A;
		cin >> A;

		cout << A;

		system("pause");
		return 0;
	}

[Sadegh2007]

friend istream& operator>>(istream&, Stack<T>&);
friend ostream& operator<<(ostream&, Stack<T>&);

here is your problem

because you define >> or << as friend you must type template for it

[sorry for my english]

like this

template<tType> friend istream& operator>>(istream&, Stack<T>&);
template<tType> friend ostream& operator<<(ostream&, Stack<T>&);

template parameter must have different name with your stack class

[LB]

A better solution is to define them inline:

template<typename T>
class Stack
{
    //...
public:
    //...
    //...

    friend istream &operator>>(istream &is, Stack &s)
    {
        //...
        return is;
    }
    friend ostream &operator<<(ostream &os, Stack &s)
    {
        //...
        return os;
    }
};

[Snaksa]

@L B: A better solution is to define them inline

Thanks a lot! That helped. Can you tell me why defining them outside the class is not working?

[S G H]

Because you're using templates.
As templates must be compiled in a different way, they must go in the header file.
If you want them in the source file, it's also ok if you declare/define them inside the class.

@LB: Why shouldn't it be Stack<T> ? Is that a typo? (@lines 9 and 14)

[Snaksa]

@EssGeEich

I tried both Stack and Stack<T>. Both ways work. :)

And so you are telling me that if the whole code except the main() function was in a header file it would work to define them outside the class? I will try :)

[S G H]

All about what's templated should go into headers.
If it's a function or a nontemplate class, it should have source files.

In fact main is a nontemplated function, and as such, it requires to be in a source file.

[Cubbi]

template friends are a little tricky to get right - the C++ FAQ has an entry on them: http://www.parashift.com/c++-faq/template-friends.html

Do they need to be friends in your case?

[Snaksa]

Thanks for the great link :)
If they are not friends the compiler shows another error message.

[Stewbond]

I actually did this same thing about an hour ago for another guy:
http://www.cplusplus.com/forum/general/110656/2/#msg604391