I have come across a typedef in some code which is this:
typedef void (NE_API *NeWindowProcCallback)(void* hWnd, NEuint uMsgId, NEuint wParam, NEuint64 lParam);
however, I am unfamiliar with this syntax. can anyone explain this?
Also, if I jump to the declaration of NE_API, I find this:
# define NE_API __stdcall
I thought this may be relevant to the answer, and an explanation of this also would be very appreciated. Thank you.
That is the typedef of a function pointer for which the function which returns void, has __stdcall calling convention, and accepts four parameters. So you can assign the address of any function which has this signature to a variable of type NeWindowProcCallback. This variable can then be passed as a parameter to other functions which expect a callback function. See this for more details on how function pointers can be used.
You can see what __stdcall is here: What is __stdcall? As for the typedef - this is the way to typedef a function. Now the type NeWindowProcCallback will be a pointer to a function taking arguments of type (void* hWnd, NEuint uMsgId, NEuint wParam, NEuint64 lParam).
So you can do:
void foo(void* hWnd, NEuint uMsgId, NEuint wParam, NEuint64 lParam);
int main() {
NeWindowProcCallback my_func = foo;
....
// use the pointer
}
NeWindowProcCallback is type for a function pointer. It takes the parameters void* hWnd, NEuint uMsgId, NEuint wParam, NEuint64 lParam, and returns void.
__stdcall is the calling convention, which determines how arguments should be passed in the generated calling assembly code.
the typedef defines NeWindowProcCallback as a type that stores a pointer to a function that returns void and takes (void* hWnd, NEuint uMsgId, NEuint wParam, NEuint64 lParam) as parameters
