[program-l] Re: Containing callback routines in C++ classes
- From: "qubit" <lauraeaves@xxxxxxxxx>
- To: <program-l@xxxxxxxxxxxxx>
- Date: Mon, 12 Oct 2009 13:16:52 -0500
Hi Tim -- Some musings and a solution.
you say it works outside the function but not inside.
This appears to be a compile time error, correct?
If so, did you try only declaring the 2 member functions inside the class
declaration and moving the function definitions outside the class after the
class is closed?
Any compiler should save the parsing of the body of an inline member to be
compiled after the class to avoid ambiguities, but I don't know what IDE you
are using.
Also, did you modify the argument type to something that can handle a
pointer to member function? Unlike global functions, these pointers are
stored as structures that point you into the vtable and cannot be cast to a
long -- or even a void*.
Ok I know how you can fix this. You can declare the function as a static
member function instead of a regular function. Static member functions are
functions that have no 'this' argument and are not stored in the vtable at
all -- the only thing that makes them different from global functions is
that they are in the scope of the class -- they are called without a calling
object -- for example,
classname::fun(args)
But I believe that if you have code that passes it an object, that object
will be ignored.
The more I think of it the more I think this is your solution.
HTH
Happy hacking.
--le
----- Original Message -----
From: "Tim Burgess" <tim@xxxxxxxxxxxxx>
To: <program-l@xxxxxxxxxxxxx>
Sent: Monday, October 12, 2009 12:29 PM
Subject: [program-l] Containing callback routines in C++ classes
Hi,
I've a call back routine that works fine in the main module of a Win32
application. However, if I turn it into a public member of a class then
refer to it from within another member function of the same class, it throws
the following error at compile time:
error C2440: 'type cast' : cannot convert from 'void (__stdcall
MCP::*)(struct HMIDIIN__ *,unsigned int,unsigned long,unsigned long,unsigned
long)' to 'unsigned long'
Conversion is a valid standard conversion, which can be performed
implicitly or by use of static_cast, C-style cast or function-style cast
The line of code that fails is:
if (!(err = midiInOpen( &myMIDIInDevice, iPortNumber, (DWORD)midiCallback,
0, CALLBACK_FUNCTION)))
{
The actual fault seems to the the reference to the MIDICallback routine - a
friend of mine has suggested that this code is being referenced before it
exists within the compiler's v-table, but he lost me at that point and
couldn't suggest a work-around that didn't involve taking MIDICallback out
of the class module (something I'm trying to avoid, as there'll be derived
code from this base class later in the project).
Any takers?
Best wishes.
Tim Burgess
Raised Bar Ltd
Phone: +44 (0)1827 719822
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