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<chapter id="ole">
<title>COM/OLE in Wine</title>
<sect1 id="com-writing">
<title>Writing OLE Components for Wine</title>
<para>
This section describes how to create your own natively
compiled COM/OLE components.
</para>
<sect2>
<title>Macros to define a COM interface</title>
<para>
The goal of the following set of definitions is to provide a
way to use the same header file definitions to provide both
a C interface and a C++ object oriented interface to COM
interfaces. The type of interface is selected automatically
depending on the language but it is always possible to get
the C interface in C++ by defining CINTERFACE.
</para>
<para>
It is based on the following assumptions:
</para>
<itemizedlist>
<listitem>
<para>
all COM interfaces derive from IUnknown, this should not
be a problem.
</para>
</listitem>
<listitem>
<para>
the header file only defines the interface, the actual
fields are defined separately in the C file implementing
the interface.
</para>
</listitem>
</itemizedlist>
<para>
The natural approach to this problem would be to make sure
we get a C++ class and virtual methods in C++ and a
structure with a table of pointer to functions in C.
Unfortunately the layout of the virtual table is compiler
specific, the layout of g++ virtual tables is not the same
as that of an egcs virtual table which is not the same as
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that generated by Visual C++. There are work arounds to make
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the virtual tables compatible via padding but unfortunately
the one which is imposed to the Wine emulator by the Windows
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binaries, i.e. the Visual C++ one, is the most compact of
all.
</para>
<para>
So the solution I finally adopted does not use virtual
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tables. Instead I use in-line non virtual methods that
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dereference the method pointer themselves and perform the
call.
</para>
<para>
Let's take Direct3D as an example:
</para>
<programlisting>#define ICOM_INTERFACE IDirect3D
#define IDirect3D_METHODS \
ICOM_METHOD1(HRESULT,Initialize, REFIID,) \
ICOM_METHOD2(HRESULT,EnumDevices, LPD3DENUMDEVICESCALLBACK,, LPVOID,) \
ICOM_METHOD2(HRESULT,CreateLight, LPDIRECT3DLIGHT*,, IUnknown*,) \
ICOM_METHOD2(HRESULT,CreateMaterial,LPDIRECT3DMATERIAL*,, IUnknown*,) \
ICOM_METHOD2(HRESULT,CreateViewport,LPDIRECT3DVIEWPORT*,, IUnknown*,) \
ICOM_METHOD2(HRESULT,FindDevice, LPD3DFINDDEVICESEARCH,, LPD3DFINDDEVICERESULT,)
#define IDirect3D_IMETHODS \
IUnknown_IMETHODS \
IDirect3D_METHODS
ICOM_DEFINE(IDirect3D,IUnknown)
#undef ICOM_INTERFACE
#ifdef ICOM_CINTERFACE
// *** IUnknown methods *** //
#define IDirect3D_QueryInterface(p,a,b) ICOM_CALL2(QueryInterface,p,a,b)
#define IDirect3D_AddRef(p) ICOM_CALL (AddRef,p)
#define IDirect3D_Release(p) ICOM_CALL (Release,p)
// *** IDirect3D methods *** //
#define IDirect3D_Initialize(p,a) ICOM_CALL1(Initialize,p,a)
#define IDirect3D_EnumDevices(p,a,b) ICOM_CALL2(EnumDevice,p,a,b)
#define IDirect3D_CreateLight(p,a,b) ICOM_CALL2(CreateLight,p,a,b)
#define IDirect3D_CreateMaterial(p,a,b) ICOM_CALL2(CreateMaterial,p,a,b)
#define IDirect3D_CreateViewport(p,a,b) ICOM_CALL2(CreateViewport,p,a,b)
#define IDirect3D_FindDevice(p,a,b) ICOM_CALL2(FindDevice,p,a,b)
#endif</programlisting>
<para>
Comments:
</para>
<para>
The ICOM_INTERFACE macro is used in the ICOM_METHOD macros
to define the type of the 'this' pointer. Defining this
macro here saves us the trouble of having to repeat the
interface name everywhere. Note however that because of the
way macros work, a macro like ICOM_METHOD1 cannot use
'ICOM_INTERFACE##_VTABLE' because this would give
'ICOM_INTERFACE_VTABLE' and not 'IDirect3D_VTABLE'.
</para>
<para>
ICOM_METHODS defines the methods specific to this
interface. It is then aggregated with the inherited methods
to form ICOM_IMETHODS.
</para>
<para>
ICOM_IMETHODS defines the list of methods that are
inheritable from this interface. It must be written manually
(rather than using a macro to generate the equivalent code)
to avoid macro recursion (which compilers don't like).
</para>
<para>
The ICOM_DEFINE finally declares all the structures
necessary for the interface. We have to explicitly use the
interface name for macro expansion reasons again. Inherited
methods are inherited in C by using the IDirect3D_METHODS
macro and the parent's Xxx_IMETHODS macro. In C++ we need
only use the IDirect3D_METHODS since method inheritance is
taken care of by the language.
</para>
<para>
In C++ the ICOM_METHOD macros generate a function prototype
and a call to a function pointer method. This means using
once 't1 p1, t2 p2, ...' and once 'p1, p2' without the
types. The only way I found to handle this is to have one
ICOM_METHOD macro per number of parameters and to have it
take only the type information (with const if necessary) as
parameters. The 'undef ICOM_INTERFACE' is here to remind
you that using ICOM_INTERFACE in the following macros will
not work. This time it's because the ICOM_CALL macro
expansion is done only once the 'IDirect3D_Xxx' macro is
expanded. And by that time ICOM_INTERFACE will be long gone
anyway.
</para>
<para>
You may have noticed the double commas after each parameter
type. This allows you to put the name of that parameter
which I think is good for documentation. It is not required
and since I did not know what to put there for this example
(I could only find doc about IDirect3D2), I left them blank.
</para>
<para>
Finally the set of 'IDirect3D_Xxx' macros is a standard set
of macros defined to ease access to the interface methods in
C. Unfortunately I don't see any way to avoid having to
duplicate the inherited method definitions there. This time
I could have used a trick to use only one macro whatever the
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number of parameters but I preferred to have it work the same
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way as above.
</para>
<para>
You probably have noticed that we don't define the fields we
need to actually implement this interface: reference count,
pointer to other resources and miscellaneous fields. That's
because these interfaces are just that: interfaces. They may
be implemented more than once, in different contexts and
sometimes not even in Wine. Thus it would not make sense to
impose that the interface contains some specific fields.
</para>
</sect2>
<sect2>
<title>Bindings in C</title>
<para>
In C this gives:
</para>
<programlisting>typedef struct IDirect3DVtbl IDirect3DVtbl;
struct IDirect3D {
IDirect3DVtbl* lpVtbl;
};
struct IDirect3DVtbl {
HRESULT (*fnQueryInterface)(IDirect3D* me, REFIID riid, LPVOID* ppvObj);
ULONG (*fnAddRef)(IDirect3D* me);
ULONG (*fnRelease)(IDirect3D* me);
HRESULT (*fnInitialize)(IDirect3D* me, REFIID a);
HRESULT (*fnEnumDevices)(IDirect3D* me, LPD3DENUMDEVICESCALLBACK a, LPVOID b);
HRESULT (*fnCreateLight)(IDirect3D* me, LPDIRECT3DLIGHT* a, IUnknown* b);
HRESULT (*fnCreateMaterial)(IDirect3D* me, LPDIRECT3DMATERIAL* a, IUnknown* b);
HRESULT (*fnCreateViewport)(IDirect3D* me, LPDIRECT3DVIEWPORT* a, IUnknown* b);
HRESULT (*fnFindDevice)(IDirect3D* me, LPD3DFINDDEVICESEARCH a, LPD3DFINDDEVICERESULT b);
};
#ifdef ICOM_CINTERFACE
// *** IUnknown methods *** //
#define IDirect3D_QueryInterface(p,a,b) (p)->lpVtbl->fnQueryInterface(p,a,b)
#define IDirect3D_AddRef(p) (p)->lpVtbl->fnAddRef(p)
#define IDirect3D_Release(p) (p)->lpVtbl->fnRelease(p)
// *** IDirect3D methods *** //
#define IDirect3D_Initialize(p,a) (p)->lpVtbl->fnInitialize(p,a)
#define IDirect3D_EnumDevices(p,a,b) (p)->lpVtbl->fnEnumDevice(p,a,b)
#define IDirect3D_CreateLight(p,a,b) (p)->lpVtbl->fnCreateLight(p,a,b)
#define IDirect3D_CreateMaterial(p,a,b) (p)->lpVtbl->fnCreateMaterial(p,a,b)
#define IDirect3D_CreateViewport(p,a,b) (p)->lpVtbl->fnCreateViewport(p,a,b)
#define IDirect3D_FindDevice(p,a,b) (p)->lpVtbl->fnFindDevice(p,a,b)
#endif</programlisting>
<para>
Comments:
</para>
<para>
IDirect3D only contains a pointer to the IDirect3D
virtual/jump table. This is the only thing the user needs to
know to use the interface. Of course the structure we will
define to implement this interface will have more fields but
the first one will match this pointer.
</para>
<para>
The code generated by ICOM_DEFINE defines both the structure
representing the interface and the structure for the jump
table. ICOM_DEFINE uses the parent's Xxx_IMETHODS macro to
automatically repeat the prototypes of all the inherited
methods and then uses IDirect3D_METHODS to define the
IDirect3D methods.
</para>
<para>
Each method is declared as a pointer to function field in
the jump table. The implementation will fill this jump table
with appropriate values, probably using a static variable,
and initialize the lpVtbl field to point to this variable.
</para>
<para>
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The IDirect3D_Xxx macros then just dereference the lpVtbl
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pointer and use the function pointer corresponding to the
macro name. This emulates the behavior of a virtual table
and should be just as fast.
</para>
<para>
This C code should be quite compatible with the Windows
headers both for code that uses COM interfaces and for code
implementing a COM interface.
</para>
</sect2>
<sect2>
<title>Bindings in C++</title>
<para>
And in C++ (with gcc's g++):
</para>
<programlisting>typedef struct IDirect3D: public IUnknown {
private: HRESULT (*fnInitialize)(IDirect3D* me, REFIID a);
public: inline HRESULT Initialize(REFIID a) { return ((IDirect3D*)t.lpVtbl)->fnInitialize(this,a); };
private: HRESULT (*fnEnumDevices)(IDirect3D* me, LPD3DENUMDEVICESCALLBACK a, LPVOID b);
public: inline HRESULT EnumDevices(LPD3DENUMDEVICESCALLBACK a, LPVOID b)
{ return ((IDirect3D*)t.lpVtbl)->fnEnumDevices(this,a,b); };
private: HRESULT (*fnCreateLight)(IDirect3D* me, LPDIRECT3DLIGHT* a, IUnknown* b);
public: inline HRESULT CreateLight(LPDIRECT3DLIGHT* a, IUnknown* b)
{ return ((IDirect3D*)t.lpVtbl)->fnCreateLight(this,a,b); };
private: HRESULT (*fnCreateMaterial)(IDirect3D* me, LPDIRECT3DMATERIAL* a, IUnknown* b);
public: inline HRESULT CreateMaterial(LPDIRECT3DMATERIAL* a, IUnknown* b)
{ return ((IDirect3D*)t.lpVtbl)->fnCreateMaterial(this,a,b); };
private: HRESULT (*fnCreateViewport)(IDirect3D* me, LPDIRECT3DVIEWPORT* a, IUnknown* b);
public: inline HRESULT CreateViewport(LPDIRECT3DVIEWPORT* a, IUnknown* b)
{ return ((IDirect3D*)t.lpVtbl)->fnCreateViewport(this,a,b); };
private: HRESULT (*fnFindDevice)(IDirect3D* me, LPD3DFINDDEVICESEARCH a, LPD3DFINDDEVICERESULT b);
public: inline HRESULT FindDevice(LPD3DFINDDEVICESEARCH a, LPD3DFINDDEVICERESULT b)
{ return ((IDirect3D*)t.lpVtbl)->fnFindDevice(this,a,b); };
};</programlisting>
<para>
Comments:
</para>
<para>
In C++ IDirect3D does double duty as both the virtual/jump
table and as the interface definition. The reason for this
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is to avoid having to duplicate the method definitions: once
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to have the function pointers in the jump table and once to
have the methods in the interface class. Here one macro can
generate both. This means though that the first pointer,
t.lpVtbl defined in IUnknown, must be interpreted as the
jump table pointer if we interpret the structure as the
interface class, and as the function pointer to the
QueryInterface method, t.fnQueryInterface, if we interpret
the structure as the jump table. Fortunately this gymnastic
is entirely taken care of in the header of IUnknown.
</para>
<para>
Of course in C++ we use inheritance so that we don't have to
duplicate the method definitions.
</para>
<para>
Since IDirect3D does double duty, each ICOM_METHOD macro
defines both a function pointer and a non-virtual inline
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method which dereferences it and calls it. This way this
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method behaves just like a virtual method but does not
create a true C++ virtual table which would break the
structure layout. If you look at the implementation of these
methods you'll notice that they would not work for void
functions. We have to return something and fortunately this
seems to be what all the COM methods do (otherwise we would
need another set of macros).
</para>
<para>
Note how the ICOM_METHOD generates both function prototypes
mixing types and formal parameter names and the method
invocation using only the formal parameter name. This is the
reason why we need different macros to handle different
numbers of parameters.
</para>
<para>
Finally there is no IDirect3D_Xxx macro. These are not
needed in C++ unless the CINTERFACE macro is defined in
which case we would not be here.
</para>
<para>
This C++ code works well for code that just uses COM
interfaces. But it will not work with C++ code implement a
COM interface. That's because such code assumes the
interface methods are declared as virtual C++ methods which
is not the case here.
</para>
</sect2>
<sect2>
<title>Implementing a COM interface.</title>
<para>
This continues the above example. This example assumes that
the implementation is in C.
</para>
<programlisting>typedef struct _IDirect3D {
void* lpVtbl;
// ...
} _IDirect3D;
static ICOM_VTABLE(IDirect3D) d3dvt;
// implement the IDirect3D methods here
int IDirect3D_fnQueryInterface(IDirect3D* me)
{
ICOM_THIS(IDirect3D,me);
// ...
}
// ...
static ICOM_VTABLE(IDirect3D) d3dvt = {
ICOM_MSVTABLE_COMPAT_DummyRTTIVALUE
IDirect3D_fnQueryInterface,
IDirect3D_fnAdd,
IDirect3D_fnAdd2,
IDirect3D_fnInitialize,
IDirect3D_fnSetWidth
};</programlisting>
<para>
Comments:
</para>
<para>
We first define what the interface really contains. This is
the _IDirect3D structure. The first field must of course be
the virtual table pointer. Everything else is free.
</para>
<para>
Then we predeclare our static virtual table variable, we
will need its address in some methods to initialize the
virtual table pointer of the returned interface objects.
</para>
<para>
Then we implement the interface methods. To match what has
been declared in the header file they must take a pointer to
a IDirect3D structure and we must cast it to an _IDirect3D
so that we can manipulate the fields. This is performed by
the ICOM_THIS macro.
</para>
<para>
Finally we initialize the virtual table.
</para>
</sect2>
</sect1>
</chapter>
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