On Wednesday, 13 July 2016 at 06:44:36 UTC, Adam Sansier wrote:
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.
Ok, this is the thing.
In C++ I can do
auto p = *((size_t**)*ptr) + 4;
typedef size_t(__stdcall *fp)(char*);
auto f = (fp)*p;
res = f(n);
to call the 4th function that accepts a char. I can do that for
all the functions > 2.
If I call 0 through 2 I get errors, that is from the IUnknown
interface.
It seems the interface I'm using is built up of static
functions... functions that don't accept a this. Hence they work
and extern(C++) works for them, which I guess doesn't pass this.
Because of the hacked vtble stuff, the class used is not the
original vtbl which is required for IUnknown.
Hence, they crash because they either get no this or the wrong
this.
e.g.,
// Release the interface
auto p = *((size_t**)*ptr) + 2;
typedef size_t(__stdcall *fp)(void*);
auto f = (fp)*p;
res = f(ptr);
I'm not entirely sure if all this is correct but the C++ code
shows that I don't pass a "this" to the interface functions and
it works(if it's passed in ECX and they simply don't use it or if
they are "static" or whatever is going on)... not sure which.
OTOH, I have to pass something to Release to get it not to crash.
So, any ideas?
void** ptr = null;
auto res = CoCreateInstance(&CLS_ID, cast(IUnknown)null,
CLSCTX_INPROC_SERVER, &CLS_ID, cast(void**)&ptr);
I could check to see if addref and release are working if I could
get the number of references. Is this possible? A quick search
doesn't bring up anything.
To sum up this confusion:
1. Are the working interface functions I'm calling "static"(no
this)? Is that possible? extern(Windows) breaks the code and
extern(C++) works. I can call them with a C++ function pointer
without this and it works. Not sure if this is passed on the
stack or ecx and none of the functions I've called use this so
they don't crash.
2. IUnknown's methods are crashing. This seems to be because this
is not passed or an invalid this is passed. This is probably due
to the way I am hacking the vtbl but it is the way it was done in
the C++ code. I can fix this stuff up without a big problem
though as long as I know what to pass for this and possibly check
to make sure the reference count decreases.
Thanks..
