Re: Callback Trouble

"Alf P. Steinbach" <>
Sat, 17 Nov 2007 13:05:02 +0100
* Nashirak:

I am trying to implement callbacks in C++. I am modeling my callbacks
after some of the stuff that was done in wxWidgets (if anyone is
familiar with that). The syntax is as follows:

// Header

Please don't hide namespace declarations etc. in macros.

typedef void (PI_PluginAPI::*KMKeyboardMethod)(const KeyboardEvent

The less you use raw pointers, the better.

Pointers to member functions are especially bad (in general).

Try something like

   struct KeyboardEventHandler
       virtual void onEvent( KeyboardEvent const& ) = 0;

class KM_Keymap : public PI_PluginAPI

Why use prefixes instead of namespaces?



// More functions follow ....

bool AddKeymapEntryMethod(const char *top_entry, const char
*name,const char *desc, PI_PluginAPI *obj, KMKeyboardMethod func, void

As mentioned raw pointers are bad, member function pointers especially
bad, and void* pointers, used above, are simply unacceptable in high
level code (if you must interface to C then that's another matter, but
the above doesn't).


// Private stuff here

Then I have a calling class. It structure is as follows:

// Header
class My_Plugin : virtual public NGMX::PI_PluginAPI

// More stuff ...

void ShowStuffCB(const NGMX::KeyboardEvent *event);

// Private stuff

To connect the callback I make this call in a method inside of the
My_Plugin (My_Plugin class uses the NGMX namespace up top) class:

// In source
m_keymap->AddKeymapEntryMethod("BASE","Show Stuff","Turn on and off

C style casts are bad.

Now the error I am getting is as follows (in Visual Studio 2003):
error C2664: 'NGMX::KM_Keymap::AddKeymapEntryMethod' : cannot convert
parameter 5 from 'void (__thiscall My_Plugin::* )(void)' to
Types pointed to are unrelated; conversion requires reinterpret_cast,
C-style cast or function-style cast

The error message is not related to any code shown.

Reproduce the error in a smallest possible complete program and post the
code if that effort in itself doesn't help you.

See the FAQ for help about how to post a question about code that
doesn't work as expected.

So it appears that it doesn't like the &My_Plugin::ShowStuffCB
parameter. My_Plugin inherits from PI_PluginAPI so I don't know why
the cast doesn't work. I am not sure what I am doing wrong. Anyone see
something glaring that needs to be fixed? Thanks in advance for you

See above.

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
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A: Top-posting.
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