Tricky #define macro...

This is a discussion on Tricky #define macro... within the Windows Programming forums, part of the Platform Specific Boards category; When looking up HINSTANCE (yes, I know what it means but I wanted to see the var tp) i see ...

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    Tricky #define macro...

    When looking up HINSTANCE (yes, I know what it means but I wanted to see the var tp) i see that its defined by DECLARE_HANDLE, a rather advanced macro:

    Code:
    #define DECLARE_HANDLE(name) struct name##__ { int unused; }; typedef struct name##__ *name
    Could someone take the time to explain what it means? I figured a handle would be just number (prob. Unisgned Int 32) but this is something else... ant it's not a struct with any members either..

  2. #2
    Yes, my avatar is stolen anonytmouse's Avatar
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    Code:
    DECLARE_HANDLE(HINSTANCE); will expand to:
    
    struct HINSTANCE__ { int unused; }; typedef struct HINSTANCE__ *HINSTANCE;
    Windows declares handles as pointers to dummy structures for type safety. This means the compiler can give a warning if you try to assign an HINSTANCE to another type. If it was simply declared as a void*, there would be less type safety.

    You can consider a handle as a numeric value rather than a pointer to anything. For most handle types, the meaning of the number is opaque. However, both a HINSTANCE and a HMODULE are documented as containing the base address of the module they refer to.

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    So of the two statements...

    Code:
    struct HINSTANCE__ { int unused; };
    typedef struct HINSTANCE__ *HINSTANCE;
    ...the first declares the dummy type "HINSTANCE__" and the second declares "HINSTANCE" (without underscores) as a pointer to a variable of that type?

    In VC6 this is also okay to the preproc:

    Code:
    struct HINSTANCE__ { int unused; };
    typedef HINSTANCE__* HINSTANCE;
    I hope I figured it out right. The first looks much trickier, because there is the word "struct" after typedef (we've already made the struct), and no space between the original type (HINSTANCE__*) and it's alias HINSTANCE. Likewise it's also clearer with int* ip than int *ip in my opinion because it follows type variable format as in int i
    Last edited by willkoh; 04-06-2005 at 10:55 AM.

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    and the hat of wrongness Salem's Avatar
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    > struct name##__
    The ## is the pre-processor token pasting operator. The left token is appended with the right token.

    > Likewise it's also clearer with int* ip than int *ip in my opinion
    Until you see this
    int* p, q;
    What type is q ?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  5. #5
    Yes, my avatar is stolen anonytmouse's Avatar
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    >> ...the first declares the dummy type "HINSTANCE__" and the second declares "HINSTANCE" (without underscores) as a pointer to a variable of that type? <<

    Yep.

    >> In VC6 this is also okay to the preproc: <<

    Only in C++. In C, the struct keyword is required.

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