What is the meaning of this C++ macro?

Question

I can't figure out what this macro means:

#define DECLARE_HANDLE(n) typedef struct n##__{int i;}*n

DECLARE_HANDLE(HWND);

I have learned from the C program that:

"##" means connect the parameter.

so the macro equals:

typedef struct HWND__{int i;}*HWND

Is this right?

If it is right, what is the meaning of that sentence?

==================

Code from a game Bombermaaan (for Windows and Linux),
link http://sourceforge.net/p/bombermaaan/code/HEAD/tree/trunk/src/Bombermaaan/winreplace.h,
line No 90.

Answer 1

Your assumption is correct.

You can check this with the following simple test code.

DECLARE_HANDLE(HWND);
struct HWND__ s;
HWND p = (HWND) malloc(sizeof(struct HWND__));
s.i = 20;
p->i = 100;
cout << "i valueis " << s.i<<" and " << p->i <<endl;

Answer 2

DECLARE_HANDLE(HWND); indeed expands to

typedef struct HWND__{int i;}*HWND;

Now, you're asking what it means? Just split the typedef into two steps:

struct HWND__
{
    int i;
};

typedef HWND__* HWND;  // C++ only; for both C++ and C: typedef struct HWND__* HWND;

Thus, it defines a struct HWND__ containing an int (named i) and declares a type alias HWND as pointer to HWND__.