I'm wondering a bit about the namespace and using in C++ basically I would like to know the differences and figure out how to use it in the best way.
As I see it there are (at least) three ways to resolve a class name and I am not sure how to choose among them:
using namespace <namespace>using <namespace>::<what_to_use><namespace>::<what_to_use> <use_it>I would like to know the advantages especially if there are performance involved in one or the other way, if it's just syntactical and a matter of preference or if there are other things I haven't considered regarding this.
First is an using namespace directive, it brings all the symbol names from the specified namespace in your current namespace, irrespective of whether you need/use them. Certainly undesirable.
Second is using namespace declaration. It only brings the specified symbol name in your current namespace. Advantage is you don't have to type the fully qualified name everytime.
Third is an fully qualified names of the symbol. Disadvantage is that you have to type the fully qualified name everywhere you use the symbol.
Clearly, Second & Third are the more suitable ones. There is no performance difference in either of them. The only difference is the amount of characters you type in. Simply, choose either depending on what your coding standard specifies.
EDIT:
As @Jerry points out, using declaration in combination with ADL(Argument dependent lookup) can lead to undesirable effects.
You can find a detailed explanation in one of my answers:
Detailed explanation on how Koenig lookup works with namespaces and why its a good thing?
under the section,
Why the criticism of Koenig Algorithm?