Re: [avr-gcc-list] C++ on AVR 90S8515

[Joerg Wunsch]

As Stumpf Michael wrote:

> > Why depends the use of C++ classes on available memory?
> 
> Because implementation of classes takes some effort :
> 
>   - for every class there has to be a table of member functions (the so
> called vtbl)

Only if there are virtual methods in this class.

>   - every member function gets an implicit argument, the 'this' pointer

OK, that occupies a couple of registers.  However, if you implement
similar problems in C, you most likely won't avoid passing something
similar as a pointer to an object as well.

>   - if a class uses virtual functions then ...

...of course resource consumption will increase a lot. ;-)  But then,
if someone is using "true OO programming", he must be prepared to pay
for it.  Unlike languages like Java, C++ leaves it to the programmer
whether the true OO model (late binding) is used or whether everything
is bound at compile-time.

>   - if templates are used there maybe functions generated for every
> combination of argument type - ...

Sure.  But if you use C where you don't have templates, you just have
to write it multiple times should you need the functionality.

> And implementing C++ means all or nothing, not only a subset. 

But the generated code only contains the subset required, even though
the implementation is complete and were able to produce the full
complexity.  That's why i wrote that one needs to be overly cautious
to avoid waste of memory and code.  (It's usually a good idea to
verify the compiled assembler code.)

> One could argue that a smart compiler could resolve the overhead for
> simple classes, but regarding the origin of gcc I doubt ...

Why do you doubt instead of looking at the generated code?

-- 
J"org Wunsch                                           Unix support engineer
address@hidden        http://www.interface-systems.de/~j/
avr-gcc-list at http://avr1.org