Template issues in C++ - c++

Question

The issue I'm having here is class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Sadrzaj const &). I have searched far and wide on the Internet for a solution but could not find it. The issue occurs when I try to include Data.h in the main source file, the same happens with Content.h. Down below are the two header files for reference. Any help whatsoever is greatly appreciated. :-)
#pragma once
#include <iostream>
class Content
{
friend std::ostream& operator<<(std::ostream &str, const Content&);
public:
virtual void print(std::ostream&) const = 0;
};
std::ostream& operator<<(std::ostream &str, const Content& temp)
{
temp.print(str);
return str;
}
#pragma once
#include "Content.h"
template <class T>
class Data: public Content
{
public:
Data(const T&);
void print(std::ostream&) const override;
private:
T data;
};
template <class T>
Data<T>::Data(const T& data) : data(data) {}
template <class T>
void Data<T>::print(std::ostream& str) const { str << data; }

Answer 1

Okay, so the problem was that the << operator was defined in the "Content.h" header file. Simply by creating a new source file "Content.cpp" and defining it there, the problem was solved. If anyone knows why this solved the error, I would appreciate it very much if they would explain it to me, so that I don't make the same mistake again. :-)