CODEWITHSUNDEEP
c++namespaces
links77
links77

Reputation: 1454

Creating a C++ namespace in header and source (cpp)

Is there any difference between wrapping both header and cpp file contents in a namespace or wrapping just the header contents and then doing using namespace in the cpp file?

By difference I mean any sort performance penalty or slightly different semantics that can cause problems or anything I need to be aware of.

Example:

// header
namespace X
{
  class Foo
  {
  public:
    void TheFunc();
  };
}

// cpp
namespace X
{
  void Foo::TheFunc()
  {
    return;
  }
}

VS

// header
namespace X
{
  class Foo
  {
  public:
    void TheFunc();
  };
}

// cpp
using namespace X;
{
  void Foo::TheFunc()
  {
    return;
  }
}

If there is no difference what is the preferred form and why?

Upvotes: 103

Views: 122918

Answers (9)

orbit
orbit

Reputation: 182

Or you can do the following:

// asdf.h
namespace X
{
  class Foo
  {
  public:
    void TheFunc();
  };
}

Then

// asdf.cpp
#include "asdf.h"

void X::Foo::TheFunc()
{
  return;
}

Upvotes: 3

user11381830
user11381830

Reputation:

If you're attempting to use variables from one to the other, then I'd recommend externalizing them, then initializing them in the source file like so: 

// [.hh]
namespace example
{
   extern int a, b, c;
}
// [.cc]
// Include your header, then init the vars:
namespace example
{
   int a, b, c;
}
// Then in the function below, you can init them as what you want: 
void reference
{
    example::a = 0;
}

Upvotes: 3

Kjetil Hvalstrand
Kjetil Hvalstrand

Reputation: 61

I think right thing to do here is to use namespace for scoping.

namespace catagory
{
    enum status
    {
      none,
      active,
      paused
    }
};

void func()
{
    catagory::status status;
    status = category::active;
}

Upvotes: 0

Roee Gavirel
Roee Gavirel

Reputation: 19473

The difference in "namespace X" to "using namespace X" is in the first one any new declarations will be under the name space while in the second one it won't.

In your example there are no new declaration - so no difference hence no preferred way.

Upvotes: 60

vprajan
vprajan

Reputation: 1147

Namespace is just a way to mangle function signature so that they will not conflict. Some prefer the first way and other prefer the second version. Both versions do not have any effect on compile time performance. Note that namespaces are just a compile time entity.

The only problem that arises with using namespace is when we have same nested namespace names (i.e) X::X::Foo. Doing that creates more confusion with or without using keyword.

Upvotes: 44

AlexTheo
AlexTheo

Reputation: 4184

In case if you do wrap only the .h content you have to write using namespace ... in cpp file otherwise you every time working on the valid namespace. Normally you wrap both .cpp and .h files otherwise you are in risk to use objects from another namespace which may generate a lot of problems.

Upvotes: 1

Michael Krelin - hacker
Michael Krelin - hacker

Reputation: 143299

There's no performance penalties, since the resulting could would be the same, but putting your Foo into namespace implicitly introduces ambiguity in case you have Foos in different namespaces. You can get your code fubar, indeed. I'd recommend avoiding using using for this purpose.

And you have a stray { after using namespace ;-)

Upvotes: 9

bert-jan
bert-jan

Reputation: 968

The Foo::TheFunc() is not in the correct namespacein the VS-case. Use 'void X::Foo::TheFunc() {}' to implement the function in the correct namespace (X).

Upvotes: 1

holgac
holgac

Reputation: 1539

If the second one compiles as well, there should be no differences. Namespaces are processed in compile-time and should not affect the runtime actions.

But for design issues, second is horrible. Even if it compiles (not sure), it makes no sense at all.

Upvotes: 1

Related Questions