Difference between _declspec and __declspec?

Question

I sometimes see keywords starting with two underscores and other times just one. Is there any difference?

Answer 1

I believe that _declspec is older name of the same Microsoft specific keyword __declspec. From a C++ Standard point of view, two underscores are more correct than a single underscore for an extension like this. That's according to 17.4.3.1.2/1:

Certain sets of names and function signatures are always reserved to the implementation:

• Each name that contains a double underscore (_ _) or begins with an underscore followed by an upper- case letter (2.11) is reserved to the implementation for any use.
• Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.