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:
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..Code:#define DECLARE_HANDLE(name) struct name##__ { int unused; }; typedef struct name##__ *name
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.Code:DECLARE_HANDLE(HINSTANCE); will expand to: struct HINSTANCE__ { int unused; }; typedef struct HINSTANCE__ *HINSTANCE;
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.
So of the two statements...
...the first declares the dummy type "HINSTANCE__" and the second declares "HINSTANCE" (without underscores) as a pointer to a variable of that type?Code:struct HINSTANCE__ { int unused; }; typedef struct HINSTANCE__ *HINSTANCE;
In VC6 this is also okay to the preproc:
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 iCode:struct HINSTANCE__ { int unused; }; typedef HINSTANCE__* HINSTANCE;
Last edited by willkoh; 04-06-2005 at 10:55 AM.
> 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.
If at first you don't succeed, try writing your phone number on the exam paper.
>> ...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.
