How I can pass the WndProc as a parameter?

[Mike Parker]

On Saturday, 10 September 2022 at 10:39:12 UTC, Injeckt wrote:

To elaborate on why you need the above...

> But I get these bugs:

WndProc is a function, and you can't pass a function as a runtime 
function parameter, only pointers to functions. The first two 
errors tell you exactly what the problem is.

> server.d(29): Error: function `server.WndProc(void* hwnd, uint 
> message, uint wParam, int lParam)` is not callable using 
> argument types `()`

Note the bit that says "not callable using argument types `()`". 
In D, functions that have an empty parameter list can be called 
without an argument list, i.e., `void foo()` can be called as 
`foo`, the compiler rewrites it to `foo()`.

This error by itself tells you what's wrong. The compiler knows 
`WndProc` is a function, so it's trying to call `WndProc()` when 
it sees the `WndProc` in your argument list in the function call 
to `KK.CreateWindowClass` at line 29 of server.d.

> server.d(29): too few arguments, expected `4`, got `0`

And this reinforces that: the `WndProc` function takes 4 
arguments, but none were provided.

When doing Win32 programming in D, it sometimes helps to search 
online for problematic types. Microsoft's Win32 API documentation 
is really good. For functions, that's the only reference you 
need. But for other types and aliases, it sometimes helps to go 
one step further: use the MS docs to find out which header the 
type is defined in, then go the DRuntime source to find the 
corresponding binding.

In this case, searching for `WNDPROC` would turn up this page:

https://docs.microsoft.com/en-us/windows/win32/api/winuser/nc-winuser-wndproc

At the bottom of which we see that it's defined in `winuser.h`. 
So then you can go to the DRuntime sorce directory (it's 
installed with the compiler) and open `core/sys/winuser.d`, in 
which a search for `WNDPROC` eventually leads to this:

`alias LRESULT function(HWND, UINT, WPARAM, LPARAM)   WNDPROC;`

That tells you it's a function pointer, meaning your function 
call needs `&WndProc`, since that's how we get function pointers 
in D.