What is the meaning of prepended double colon “::”

Question

I found this line of a code in a class which I have to modify:

::Configuration * tmpCo = m_configurationDB;//pointer to current db

and I don't know what exactly means the double colon prepended to the class name. Without that I would read: declaration of tmpCo as a pointer to an object of the class Configuration... but the prepended double colon confuses me.

I also found:

typedef ::config::set ConfigSet;

Answer 1

This ensures that resolution occurs from the global namespace, instead of starting at the namespace you're currently in. For instance, if you had two different classes called Configuration as such:

class Configuration; // class 1, in global namespace
namespace MyApp
{
    class Configuration; // class 2, different from class 1
    function blah()
    {
        // resolves to MyApp::Configuration, class 2
        Configuration::doStuff(...) 
        // resolves to top-level Configuration, class 1
        ::Configuration::doStuff(...)
    }
}

Basically, it allows you to traverse up to the global namespace since your name might get clobbered by a new definition inside another namespace, in this case MyApp.

Answer 2

:: is used to link something ( a variable, a function, a class, a typedef etc...) to a namespace, or to a class.

if there is no left hand side before ::, then it underlines the fact you are using the global namespace.

e.g.:

::doMyGlobalFunction();