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: 
It should be in the `bar` namespace. You must consider [what makes up the interface for the class](http://www.gotw.ca/publications/mill02.htm), and group those together.

"A class describes a set of data along with the functions that operate on that data." Your free function operates on a `Foo`, therefore it is part of `Foo`. It should be grouped with `Foo` in the namespace `bar`.

[Argument-dependent lookup](http://en.wikipedia.org/wiki/Argument-dependent_name_lookup), or ADL, will find the function.

We also know that we should [prefer non-friend non-member functions](http://www.drdobbs.com/184401197). What this means is that, in general, your classes will have their definition and member functions, followed immediately by free functions which operate on the class.
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.)