Forward declare HINSTANCE and friends

Question

Is there a way to forward-declare the HINSTANCE type from the WinAPI without including the full (and big) windows.h header?

For example, if I have a class RenderWindow which owns an HINSTANCE mInstance, i will have to include windows.h in RenderWindow.h. So everything that needs RenderWindow also has to include windows.h.

I tried including windef.h but this seems to need some things from windows.h. :-( If I can't forward declare it, is there at least a portable way to use something like long mInstance in RenderWindow instead of HINSTANCE?

Answer 1

HINSTANCE is declared in WinDef.h as typedef HINSTANCE__* HINSTANCE;

You may write in your headers:

#ifndef _WINDEF_
class HINSTANCE__; // Forward or never
typedef HINSTANCE__* HINSTANCE;
#endif

You will get compilation errors referencing a HINSTANCE when WinDef.h is not included.

Answer 2

You could declare it void* and cast the errors away. This is close to a never-ending battle though, sooner or later you'll get tripped up. Use pre-compiled headers so you don't care about the size of windows.h

stdafx.h:

#define WIN32_LEAN_AND_MEAN
#include <windows.h>

Answer 3

For example, if I have a class RenderWindow which owns an HINSTANCE mInstance, i will have to include windows.h in RenderWindow.h. So everything that needs RenderWindow also has to include windows.h.

Have you looked at the Pimpl idiom? This allows you to hide private members. A side-effect is that you don't have to include their headers in your class' header.

Answer 4

Hey @NoSenseEtAl I guess we are still there.

In 2021, HINSTANCE is defined in <minwindef.h>. Including directly <minwindef.h> gives the error: "No Target Architecture"

To work around the error, do the following (assuming building for x64):

#define _AMD64_
#include <minwindef.h>

int main() {
    HINSTANCE h;
}

Note that the macro _AMD64_ is not documented, it starts with underscore, so not to be defined by user.

And it is defined only by <Windows.h>, so there's no smaller header to include make it defined.

Apparently there's more hope that Windows SDK will work fine with Modules, so can fix the build speed by Modules instead.

Answer 5

The best portable way to deal with handles without header including is reinterpret_casting them to a type with exactly the same size.

Most handles have pointer size¹. So void* or uintptr_t will do. Examples:

• _beginthreadex returns uintptr_t instead of HANDLE to a thread.
• MSVC thread::native_handle returns void*

Be sure to static_assert for type sizes in a place where you see the type.

¹ Few handles don't have pointer size, I can only recall the one returned by AcquireCredentialsHandle.