Question: > 
> **Possible Duplicate:**  
> 
> [Where should non-member operator overloads be placed?](https://stackoverflow.com/questions/3623631/where-should-non-member-operator-overloads-be-placed) 
> 
> 
> 

While browsing on SO, I often find questions or answer that involves overloading/defining a `std::ostream& operator<<(std::ostream& os, const Foo& foo)` or a `Foo operator+(const Foo& l, const Foo& r)`.

While I know how and when (not) to write these operators, I'm confused about the `namespace` thing.

If I have the following class:

```
namespace bar
{
  class Foo {};
}

```

In which `namespace` should I write the different operator definitions ?

```
// Should it be this

namespace bar
{
  std::ostream& operator<<(std::ostream& os, const Foo& foo);
}

// Or this ?

namespace std
{
  ostream& operator<<(ostream& os, const bar::Foo& foo);
}

// Or this ?

std::ostream& operator<<(std::ostream& os, const bar::Foo& foo);

```

The same question applies for the `operator+`. So, what is the good practice here and **why** ?

Answer: 
For operator overloading to work correctly, the function must be
in the same namespace as one of its operands. Otherwise, ADL
doesn't find it. This means the namespace of your class for
operators like + and -. Theoretically, you could put operator<<
in either std or the same namespace as your class, but the
standard forbids defining new functions in std, so here too, you
put it in the same namespace as the class.

(And of course, you don't usually implement + or -, but += and
-=, and then derive from a template which provides + and
- automatically.)
The best choice is option 1.
Why? Because when you use a unqualified function name (an an overloaded operator is a function), apart from normal name lookup, Argument-Dependent lookup is applied, that is (informally) all the namespaces where the arguments were declared are searched. 
E.g.

```
namespace N
{
   class X(){};
   void f(X){}
}
int main()
{
    N::X x;
    f(x); //works fine, no need to qualify f like N::f
}

```

The same is with operators.

On the other hand, in case of option 2 the operator still will be found because ostream is in std (same ADL rule). But it is not a good idea to add stuff to std namespace.

And the third option is bad, stylistically - why do it if the first option is sufficient?

So, definitely option 1.

HTH.