What Does the Double Colon (::) Mean in C ?

What Lies Behind the Prepended Double Colon (::)?

When encountering code that utilizes the prepended double colon (::), such as the line ": :Configuration * tmpCo = m_configurationDB;", it's easy to feel bewildered. Let's delve into the meaning behind this enigmatic syntax.

The double colon :: is used for two primary purposes:

1. Global Namespace Resolution:

   • By prepending :: to a class name, you can ensure that resolution occurs from the global namespace, regardless of the current namespace.
   • This becomes crucial when multiple classes with the same name exist across different namespaces. With global namespace resolution, you can unambiguously refer to the intended class.

2. Avoiding Namespace Clobbering:

   • Namespaces allow you to organize code into logical groups. However, a new definition within a namespace can potentially override an existing one in a parent or global namespace.
   • By using ::, you can access an identifier at the global level, avoiding this potential conflict and ensuring that the correct definition is referenced.

To illustrate this concept further, consider the following example:

class Configuration; // Global namespace, Class A
namespace MyApp
{
    class Configuration; // Local namespace, Class B
    void blah()
    {
        // Resolve to Class B (local)
        Configuration::doStuff(...)

        // Resolve to Class A (global)
        ::Configuration::doStuff(...)
    }
}

As you can see, using :: in ": :Configuration * tmpCo = m_configurationDB;" ensures that the resolution of the class Configuration starts from the global namespace. This prevents any potential ambiguities that might arise due to name conflicts within other namespaces.

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