Question

I am trying to overload the stream operator <<, for a class Foo which has already a toString() function returning a string, with the following code:

std::ostream &operator<<( std::ostream &flux, Foo const& foo )
{
    flux <<  foo.toString(); 
    return flux;
}

In order to use it in a main.cppfile

My question is: Where to put that piece of code?

• If I place it in the main.cpp, before its usage, it works well, but i may want to use it in other files.

• If I place it in foo.cpp, I get a ‘no such function’ error:

  src/main.cpp:77: error: no match for ‘operator<<’ in ‘std::cout << foo’
  

  which make sense since the code is not included to the main.cpp file

• If I place it in the foo.hclass header, outside class declaration, I get a ‘multiple definition’ error:

  foo.o: In function `operator<<(std::basic_ostream<char, std::char_traits<char> >&, Foo const&)':
  foo.cpp:(.text+0x0): multiple definition of `operator<<(std::basic_ostream<char, std::char_traits<char> >&, Matrix const&)'
  bar.o:bar.cpp:(.text+0x0): first defined here
  

  The foo.h header is indeed included in different classes/files, but there is a ifdef guard, so I don’t understand this.

So How should I do?

Answer 1

There are multiple options:

Declare it in the header, after Foo, and define it in Foo.cpp.

//foo.h
class Foo
{};
std::ostream &operator<<( std::ostream &s, Foo const& foo );

//foo.cpp
#include "foo.h"
std::ostream &operator<<( std::ostream &s, Foo const& foo )
{
    return s;
}

Define it as a friend inside the class definition.

//Foo.h
class Foo
{
   friend std::ostream &operator<<( std::ostream &s, Foo const& foo )
   {
      return s;
   }
};

Define it in the header, outside the class definition, and mark it as inline to prevent the multiple definition.

//Foo.h
class Foo
{
};

inline std::ostream &operator<<( std::ostream &s, Foo const& foo )
{
   return s;
}