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: 
The rule is that, when looking for a suitable function overload, both the current namespace and all namespaces of the argument type definitions are considered. This is called Argument Dependent Lookup (ADL).

So when you have this code:

```
  ::std::ostream& os = /* something */;
  const ::bar::Foo& foo = /* something */;
  os << foo;

```

The following namespaces are considered:

* The current namespace
* ::std, because os' type is defined there
* ::bar, because foo's type is defined there

So all three possibilities you named, will work and thus are 'good enough' on first glance.

However....

You are not allowed to define new functions in ::std, so you can't put your overloaded operator in that namespace. (You are allowed to specialize templates in ::std, but that's not what we are doing here)

Secondly, the "current namespace" may change, so if you put your function definition in that namespace, it might not always be found.

So in the end, the best place to put the overloaded operator is in the same namespace as Foo:

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

```
The good practice is to declare the (non-member) operators in the same namespace as the class whose interface they belong to.

For something like `operator+`, this is rather easy: It operates only on Foo objects, so it should go in the same namespace as Foo itself.
For `operator<<` and `operator>>`, you could still chose between namespaces `std` and `bar`. First of all, you are not supposed to add function/operator overloads to namespace `std`. And secondly, the important part of these overloads is not that they work with a stream, but that they read/write a Foo object. So it makes more sense to bundle it with the Foo class.

It should also be noted that the rules of C++ are designed such that overloaded operators that are defined in the same namespace as the class they operate on, will almost always be correctly found, while this will go wrong much more often if the operators are declared in some other, unrelated, namespace.