Question

Possible Duplicate:
Using std Namespace

I was just wondering if there was some reason to include std:: in some operations, like std::sort() for example. Is it because of possible overloading?

Answer 1

Apart from the usually well known reasons of polluting the current namespace with unnecessary symbol names and readability there is a subtle another reasoning.

Consider the example of std::swap,it is an standard library algorithm to swap two values. With Koenig algorithm/ADL one would have to be cautious while using this algorithm because:

std::swap(obj1,obj2);    

may not show the same behavior as:

using std::swap;
swap(obj1, obj2);

With ADL, which version of swap function gets called would depend on the namespace of the arguments passed to it.
If there exists an namespace A and if A::obj1, A::obj2 & A::swap() exist then the second example will result in a call to A::swap() which might not be what the user wanted.

Further, if for some reason both:
A::swap(A::MyClass&, A::MyClass&) and std::swap(A::MyClass&, A::MyClass&) are defined, then the first example will call std::swap(A::MyClass&, A::MyClass&) but the second will not compile because swap(obj1, obj2) would be ambiguous.