Use of "using namespace" when declaring that namespace

Question

Suppose I have the following files:

// SomeClass.h
namespace Example
{
    class SomeClass
    {
        ...
        SomeClass someFunction();
        ...
    };

}

// SomeClass.cpp
Example::SomeClass Example::SomeClass::SomeFunction()
{
    ...
}

Would there be any consequences to add "using namespace Example;" before the namespace in SomeClass.h to eliminate the need of adding the "Example::" scope operator to things in the Someclass.cpp file? Even if there are no conesequences, would this be considered bad coding practice?

The change would be as follows:

// SomeClass.h
using namespace Example;

namespace Example
{
    class SomeClass
    {
        ...
        SomeClass someFunction();
        ...
    };

}

// SomeClass.cpp
SomeClass SomeClass::SomeFunction()
{
    ...
}

I use that all the time in .cpp files. I haven't seen any downsides yet. However, I haven't used them in .h files. Keep in mind that you can't use using namesapce Example; before the namespace is introduced. — R Sahu
– R Sahu, Commented Nov 12, 2014 at 5:13
As R Sahu said, do not throw a using namespace in your header. It pollutes the global namespace of every file which includes it (not fun). — Ed Swangren
– Ed Swangren, Commented Nov 12, 2014 at 5:23

Galik · Accepted Answer · 2015-01-04 21:54:12Z

1

No, please don't put using namespace ...; in the global area. You can just do this:

SomeClass.h

// using namespace Example; // never here please

namespace Example
{
    using namespace OtherExample; // this is okay (not global)

    class SomeClass
    {
        ...
        SomeClass someFunction();
        ...
    };

}

SomeClass.cpp

namespace Example // same as in .h
{
    using namespace OtherExample; // this is okay (not global)

    SomeClass SomeClass::SomeFunction()
    {
        ...
    }
}

And I would also suggest with potentially huge namespaces like std:: to never use using namespace std; even within your own namespaces because they simply drag in too many common symbol names.

edited Jan 4, 2015 at 21:54

answered Nov 12, 2014 at 5:29

Galik

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