C++ using namespace declaration

Question

So for a while I've been using...

using namespace::std;

and recently I realized that's supposed to be

using namespace std;

Can somebody explain to me why what I was doing worked, and how it differs from the correct way of declaring the usage of a certain namespace?

Some programmer dude · Accepted Answer · 2013-06-21 16:44:10Z

6

The first version works because the compiler sees it as

using namespace ::std;
// Notice space^

That is simply telling the compiler to look for the name-space std in the global scope.

The scope operator :: without anything on the left-hand side is the same as the global scope.

answered Jun 21, 2013 at 16:44

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5 Comments

so it is not the same as std we regularly mean?

@taocp it is the same, since std is in the global namespace.

@taocp: It is the same. ::std and std are equivelant in this context.

@taocp It is, since std is in the global scope, using std or ::std is the same.

Thank you @juanchopanza,John Dibling, Joachim Pileborg