Re: static function tables

[Ken <shiva@compuserve.com>]

Todd Hoff wrote:
> 
> Ken wrote:
> >
> > What's the right way to do static tables containing function pointers in
> > C++?
> >
> > I have an embedded C app that parses a command from a host and looks it
> > up in a group of tables. Each table represents an array of related
> > commands. It contains an 8-byte char[] of the command, a parameter
> > count, some flags, and a pointer to the function that handles that
> > command. There are several such tables, linked in based on what feature
> > set a given app will have.
> >
> > When I move this app to C++, what's a reasonable OOP-ish way of handling
> > these tables?
> 
> How about not using function pointers? Maybe use a table of command
> objects instead? You could have a base command class then
> derive more specific commands from these. The table would
> contain pointers to command objects so you could treat all
> the command the same but the behaviour would be that of the
> derived class. Not sure if this fits your app but if it works

> it is better than using function pointers. Function pointers
> should be very rare in a c++ app.

Agreed. Constant function pointers in a struct are usually an indication
that one needs a virtual method. I've used this idiom in an OS/2
application where I have plenty of virtual memory to play with.

The catch in this case is that the objects are static, and this is a
memory-limited embedded system where I don't want to copy a const array
of structures in ROM into equivalent C++ structures in RAM. This is kind
of a general problem in using C++ in a memory-limited environment. There
are cases where it would be nice to initialize static const C++ objects
at compile or link time (on the host) rather than just before main() (on
the target).

I've seen a trick lately (I think in Embedded Systems Programming) where
one exposes a C++ class that defines an internal private struct for the
static const data. Something like

 class public_class
 {
 private:
     struct static_data { ... };
     static const static_data data[]; // stored in ROM
 };

This looks applicable to the situation.

-- 
Ken
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