UWP/WinRT Headers are Fun (not)

As established before, <windows.h> is a bit of a mess that has accumulated over 30+ years. Symbols in global namespace, preprocessor macros, ugh:

#include <windows.h>
// my code
void* GetObject(...);
// welp, GetObject is actually GetObjectW now

So naturally, someone at Microsoft decided it’s time to make a “v2” API set for programming Windows, without any of the horrors of the past, using more modern approaches, and so on. And so in 2012 Windows Runtime was born.

No more preprocessor hijacking identifiers! No more global namespaces! …hmm. or is it? Try this (tested with Windows SDK 10.0.16299.0, VS2017):

class Plane;
#include <windows.ui.core.h>

What does the compiler say?

Windows.Foundation.Numerics.h(490): error C2371: 'ABI::Windows::Foundation::Numerics::Plane': redefinition; different basic types
Windows.Foundation.Numerics.h(317): note: see declaration of 'ABI::Windows::Foundation::Numerics::Plane'

Compiling with /W3 tells more detail on why that happens:

Windows.Foundation.Numerics.h(317): warning C4099: 'Plane': type name first seen using 'class' now seen using 'struct'
test.cpp(2): note: see declaration of 'Plane'

Lo and behold, turns out that Windows.Foundation.Numerics.h (which is included by a lot of WinRT headers) has this:

namespace ABI {
    namespace Windows {
        namespace Foundation {
            namespace Numerics {                
                typedef struct Plane Plane;

The code tried to be namespace-aware, but typedef struct Plane Plane apparently is not. Why does it have that thing in the first place? No idea!

But this means you can’t forward-declare classes/structs, that match WinRT structs/classes (even inside namespaces!), before including WinRT headers. Your own forward-declarations have to come after WinRT header inclusion.

Since that pattern in headers essentially creates code like this, which is a compile error on all C++ compilers:

class Plane;
namespace Test
    typedef struct Plane Plane;
    struct Plane { int a; };
// clang 5.0.0:
// warning: struct 'Plane' was previously declared as a class [-Wmismatched-tags]
// error: definition of type 'Plane' conflicts with typedef of the same name
// vs2017:
// warning C4099: 'Plane': type name first seen using 'class' now seen using 'struct'
// error C2371: 'Test::Plane': redefinition; different basic types
// gcc 7.2:
// error: using typedef-name 'Test::Plane' after 'struct'

“Great” job, WinRT headers. At least in WinAPI times I could undo most of the damage with some #undef