Operator << overloading - c++

Question

I'm working on my project and I want to overload operator << which should print out my object on console.
Object is called "config" and in its template class it has 4 arrays called attribute1, attribute2. attribute3 and attribute4.
So far I've tried this :
#include "stdafx.h"
#include <string>
#include <iostream>
using namespace std;
template<typename T> class config
{
T *attribute1, *attribute2, *attribute3, *attribute4;
string attribName1, attribName2, attribName3, attribName4;
public:
config(void)
{
attribute1 = new T[3];
attribute2 = new T[3];
attribute3 = new T[3];
attribute4 = new T[3];
}
~config(void)//destruktor
{
delete [] attribute1, attribute2, attribute3, attribute4;
}
//operatory
friend ostream& operator<<(ostream &out, const config<T> &c);
};//class ends
template <typename T> ostream& operator<<(ostream &out, const config<T> &c)
{
for(int i=0;i<3;i++)
{
out<<c.attribute1[i]<<c.attribute2[i]<<c.attribute3[i]<<c.attribute2[i];
}
return out;
}
Whenever I try to compile it, it gives some kind of weird error, but I know the problem is in the operator.
This is the error it gives :
Error 1 error LNK2001: 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 config<double> const &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#ABV?$config#N###Z) D:\Docs\Visual Studio 2010\Projects\project_v2.obj project_v2
and:
Error 2 error LNK1120: 1 unresolved externals D:\Docs\Visual Studio 2010\Projects\project_v2.exe project_v2
And line or column isnt specified.

Answer 1

Here's one way to solve the problem.
Declare the operator<< function first, before config is defined. In order to declare the function, you have to forward declare the class template.
template <typename T> class config;
template <typename T> std::ostream& operator<<(std::ostream &out,
const config<T> &c);
In the class, make the function a friend by using T as the template parameter.
friend ostream& operator<< <T>(ostream &out, const config &c);
// Notice this ^^^^
// This makes sure that operator<< <int> is not a friend of
// config<double>. Only operator<< <double> is a friend of
// config<double>
Here's a working program with those changes:
#include <string>
#include <iostream>
template <typename T> class config;
template <typename T> std::ostream& operator<<(std::ostream &out,
const config<T> &c);
template <typename T> class config
{
T *attribute1, *attribute2, *attribute3, *attribute4;
std::string attribName1, attribName2, attribName3, attribName4;
public:
config(void)
{
attribute1 = new T[3];
attribute2 = new T[3];
attribute3 = new T[3];
attribute4 = new T[3];
}
~config(void)//destructor
{
delete [] attribute1;
delete [] attribute2;
delete [] attribute3;
delete [] attribute4;
}
//operator
friend std::ostream& operator<< <T>(std::ostream &out,
const config &c);
};
template <typename T> std::ostream& operator<<(std::ostream &out,
const config<T> &c)
{
for(int i=0;i<3;i++)
{
out << c.attribute1[i] << " "
<< c.attribute2[i] << " "
<< c.attribute3[i] << " "
<< c.attribute2[i] << std::endl;
}
return out;
}
int main()
{
config<double> x;
std::cout << x << std::endl;
}
Output:
0 0 0 0
0 0 0 0
0 0 0 0