class classname;

[hasanfaraz]

I have seen in several codes in which it is written like class classname; inside a namespace. What does it means declaring a classname?

[webJose]

I guess you refer to:

namespace MyNS
{
    class MyClass;
}

If yes, that's a forward class declaration used to break the need for circular references in header files.

[hasanfaraz]

thanks webJose!!