Pointless 'MIDL_INTERFACE' Macro in winapi?

After browsing some old c开发者_如何学编程ode, 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?

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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.

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This is because MinGW does not support COM (or rather, supports it extremely poorly). MIDL_INTERFACE is used when defining a COM component, and it is generated by the IDL compiler, which generates COM type libraries and class definitions for you.

On MSVC, this macro typically expands to more complicated initialization and annotations to expose the given C++ class to COM.

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If I had to guess, it's for one of two use cases:

1. It's possible that there's an external tool that parses the files looking for declarations like these. The idea is that by having the macro evaluate to something harmless, the code itself compiles just fine, but the external tool can still look at the source code and extract information out of it.

2. Another option might be that the code uses something like the X Macro Trick to selectively redefine what this preprocessor directive means so that some other piece of the code can interpret the data in some other way. Depending on where the #define is this may or may not be possible, but it seems reasonable that this might be the use case. This is essentially a special-case of the first option.