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

The :: operator is called the scope-resolution operator and does just that, it resolves scope. So, by prefixing a type-name with this, it tells your compiler to look in the global namespace for the type.

Example:

int count = 0;  int main(void) {   int count = 0;   ::count = 1;  // set global count to 1   count = 2;    // set local count to 2   return 0; }