C++ interface vs D and com

Adam Sansier via Digitalmars-d-learn digitalmars-d-learn at puremagic.com
Tue Jul 12 23:44:36 PDT 2016


On Wednesday, 13 July 2016 at 03:38:03 UTC, Mike Parker wrote:
> On Wednesday, 13 July 2016 at 02:49:54 UTC, Adam Sansier wrote:
>> On Wednesday, 13 July 2016 at 02:34:14 UTC, Mike Parker wrote:
>
>>> What happens when you declare an interface that extends from 
>>> IUnknown (and not extern(C++)), then cast the pointer 
>>> returned from the COM API? It should just work without 
>>> needing to muck around with the vtable.
>>
>> That was what I tried first, It didn't work. I don't know what 
>> the problem though. I either get an access violation or the 
>> functions don't do anything.
>
> Perhaps you forgot to call CoInitialize{Ex}?

Nope...

>
>>
>> I think it's more complex because without extern(C++) the 
>> vtable is in a different place than expected(it's offset by 
>> 1), so simple casting does not work.
>>
>> "A COM interface differs from a regular interface in that 
>> there is no object.Interface entry in vtbl[0]; the entries 
>> vtbl[0..$] are all the virtual function pointers, in the order 
>> that they were declared. This matches the COM object layout 
>> used by Windows.
>>
>> A C++ interface differs from a regular interface in that it 
>> matches the layout of a C++ class using single inheritance on 
>> the target machine. "
>
> You don't need extern(C++) for COM interfaces. There are 
> several declared in the Windows bindings that each inherit from 
> IUnknown and there's no extern(C++) in sight (they existed long 
> before C++ support did). Here's a working example using one of 
> them, IShellLinkW, declared in core.sys.windows.shlobj.
>
> ```
> import core.sys.windows.windows,
>        core.sys.windows.shlobj,
>        core.sys.windows.com;
>
> pragma(lib, "Ole32");
>
> void main()
> {
>     IShellLinkW iface;
>     auto shellLinkCLSID = CLSID_ShellLink;
>     auto shellLinkIID = IID_IShellLinkW;
>
>     CoInitialize(null);
>     scope(exit)CoUninitialize();
>
>     auto hr = CoCreateInstance(
>         &shellLinkCLSID,
>         null,
>         CLSCTX_INPROC_SERVER,
>         &shellLinkIID,
>         cast(void**)&iface
>     );
>     if(SUCCEEDED(hr)) {
>         import std.stdio : writeln;
>         writeln("Got it!");
>         iface.Release();
>     }
>     else throw new Exception("Failed to create IShellLink 
> instance");
> }
> ```
>
> There's a minor annoyance here in that the IID constants are 
> all declared in the Windows bindings as manifest constants, 
> which is normally the smart thing to do with constants. 
> However, they're intended to be used as lvalues with the COM 
> API, so I had to save them off in local variables in order to 
> take their addresses. You can do whatever you want with your 
> own declarations, of course.

You don't have to beleive me, but if I don't mark the methods 
extern(C++), then only 0 arg methods work.

In fact, Release does not work unless I mark it extern (C++).

So, while you may think it should work one way, and maybe it does 
for you in some case, it doesn't for me and has given me quite an 
amount of grief.

Regardless of what you think, I can prove that the code won't 
work when it is marked extern(Windows) and works when it is 
marked extern (C++)... so what you should be asking yourself is 
why it is doing that rather than assuming I'm making it up or 
doing something wrong.









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