Question

After browsing some old code, I noticed that some classes are defined in this manner:

MIDL_INTERFACE("XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX")
Classname: public IUnknown {
    /* classmembers ... */
};

However, the macro MIDL_INTERFACE is defined as:

#define MIDL_INTERFACE(x) struct

in C:/MinGW/include/rpcndr.h (somewhere around line 17). The macro itself is rather obviously entirely pointless, so what’s the true purpose of this macro?

Answer 1

In the Windows SDK version that macro expands to

 struct __declspec(uuid(x)) __declspec(novtable)

The first one allows use of the __uuidof keyword which is a nice way to get the guid of an interface from the typename. The second one suppresses the generation of the v-table, one that is never used for an interface. A space optimization.