C++ namespace inclusion in .cc files

Question

I have declared a class X in X.h as follows:

namespace Foo {
 class X{
....
};

}

In X.cc I would like to define the constructors, methods for X.

Do I need to enclose all my definitions inside namespace Foo {...} or prefix X as Foo::X:: for every method ?

It seems that sometimes I can just say (using namespace Foo) and not mention it again, i.e. just define methods as X::X() {...}

What is the correct approach here ?

Answer 1

Any of the three approaches you suggest will work. Given:

namespace N {
    struct S {
        int F();
    };
}

You can put the definition in a namespace block:

namespace N {
    int S::f() { return 42; }
}

You can qualify the member name with the namespace name:

int N::S::f() { return 42; }

Or you can use a using directive (I'd not recommend this, though):

using namespace N;
int S::f() { return 42; }

Generally, I'd recommend against using a using directive. As for the other two (using a namespace block or qualifying the names), I don't think it really matters. I do both in my code.

Answer 2

No, you don't need to use namespace Foo {...} while defining your class. All your approaches are valid, but personally I'd prefer the following:

// In FOO.h
namespace FOO
{
 class A
 {
 public:
  A();
 };
}

And the implementation:

// In FOO.cpp
#include "Foo.h"
FOO::A::A()
{
 cout<<endl;
}