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?

[dont't use either](http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice). — juanchopanza, Jun 21 '13 at 16:46

score 6 · Accepted Answer · answered Jun 21 '13 at 16:44

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 '13 at 16:44

so it is not the same as `std` we regularly mean? – taocp Jun 21 '13 at 16:44
@taocp it is the same, since `std` is in the global namespace. – juanchopanza Jun 21 '13 at 16:45
@taocp: It is the same. `::std` and `std` are equivelant in this context. – John Dibling Jun 21 '13 at 16:45
@taocp It is, since `std` is in the global scope, using `std` or `::std` is the same. – Some programmer dude Jun 21 '13 at 16:45
Thank you @juanchopanza,John Dibling, Joachim Pileborg – taocp Jun 21 '13 at 16:46

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