ostream operator<< () overload problems

**jaybny** · 06-24-2006

I am using gcc on cygwin to compile the following code;

Code:

template<class C>
ostream & operator<<(ostream & os, const vector<C> & v) {

	os << "Vector of size " << (int)v.size() << " contains :" << endl << " | ";

	for ( typename vector<C>::const_iterator iter = v.begin();
			iter != v.end();
			++iter ) {

		os << *iter << " | ";
	}

	os << endl;

	return os;
}

Code:

#include "utilities.h"
#include	<iostream>
#include	<vector>

int main() {

	std::vector<int> v(10,10);

	std::cout << v << std::endl;

	return 0;
}

when I try to link I get this error:

undefined reference to `std::basic_ostream<char, std::char_traits<char> >& operator<< <int>(std::basic_ostream<char, std::char_traits<char> >&, std::vector<int, std::allocator<int> > const&)'

any ideas? thanks

**whiteflags** · 06-24-2006

It would be pretty difficult to get something like that to work. Create an inserter for your class, and then you'd want to access the object you placed in the vector and insert that. Visually this is something like...

Code:

class C {
   int foo;
public:
   explicit C(int foo_in = 0): foo(foo_in) {  }
   ~C() {  }

   friend std::ostream &operator << (std::ostream &outs, const C &c) {
      return outs << c.foo;
   }
};

std::vector<C> vec;
vec.push_back(C(42));
std::cout << vec[0] << "\n";

**Daved** · 06-24-2006

That code should work fine.

Just make sure that the first bit of code you posted is in the header file. Templated classes and functions must be defined in the header, not the source file (or the source file must be included).

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