What Does the Double Colon '::' Mean in C Namespace Resolution?

Understanding the Meaning of Prepended Double Colon "::" in C

In C , you may encounter instances where a double colon "::" is prepended to a class name, as seen in the code snippet:

::Configuration * tmpCo = m_configurationDB;

This prepended double colon plays a crucial role in namespace resolution and ensures global access.

Global Namespace Resolution

When declaring a class or referencing a type, C searches for that name within the current namespace. However, double colons allow you to explicitly specify that the search should be performed from the global namespace. This is useful when dealing with multiple namespaces that might have conflicting class names.

For example, consider the following code:

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(...)
    }
}

In this situation, the function "blah" is within the "MyApp" namespace. Without the double colons, "Configuration::doStuff(...)" would resolve to "MyApp::Configuration::doStuff(...)." However, by prepending double colons, you instruct the compiler to look for "Configuration" in the global namespace, resolving it to "class 1" instead of "class 2."

Traversal to Global Namespace

This usage of double colons is particularly helpful because it allows you to traverse up to the global namespace, ensuring that you can access types even if they are obscured by other namespaces.

In summary, prepending a double colon to a class name indicates that resolution should start from the global namespace, overriding any local namespace definitions. This allows for unambiguous type referencing and facilitates navigation through complex namespaces.

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