Re: typedefs and structs

["J.A. Magallon" <jamagallon@able.es>]

On Wed, 9 Nov 2005 14:12:38 -0800 (PST), Vadim Lobanov <vlobanov@speakeasy.net> wrote:

> On Wed, 9 Nov 2005, linux-os \(Dick Johnson\) wrote:
> 
> >
> > On Wed, 9 Nov 2005, linas wrote:
> >
> > > On Wed, Nov 09, 2005 at 08:22:15AM -0800, Vadim Lobanov was heard to remark:
> > >> On Wed, 9 Nov 2005, J.A. Magallon wrote:
> > >>
> > >>> void do_some_stuff(T& arg1,T&  arg2)
> > >>
> > >> A diligent C programmer would write this as follows:
> > >> 	void do_some_stuff (struct T * a, struct T * b);
> > >> So I don't see C++ winning at all here.
> > >
> > > I guess the real point that I'd wanted to make, and seems
> > > to have gotten lost, was that by avoiding using pointers,
> > > you end up designing code in a very different way, and you
> > > can find out that often/usually, you don't need structs
> > > filled with a zoo of pointers.
> > >
> >
> > But you can't avoid pointers unless you make your entire
> > program have global scope. That may be great for performance,
> > but a killer if for have any bugs.
> 
> Just to extract some useful technical knowledge from the current ongoing
> "flamewar"...
> I'm not entirely sure if the above statement regarding performance is
> correct. Some enlightenment would be appreciated.
> 
> Suppose you have the following code:
> 	int myvar;
> 	void foo (void) {
> 		printf("%d\n", myvar);
> 		bar();
> 		printf("%d\n", myvar);
> 	}
> If bar is declared in _another_ file as
> 	void bar (void);
> then I believe the compiler has to reread the global 'myvar' from memory
> for the second printf().
> 
> However, if the code is as follows:
> 	void foo (void) {
> 		int myvar = 0;
> 		printf("%d\n", myvar);
> 		bar(&myvar);
> 		printf("%d\n", myvar);
> 	}
> If bar is declared in _another_ file as
> 	void bar (const int * var);
> then I think the compiler can validly cache the value of 'myvar' for the
> second printf without re-reading it. Correct/incorrect?
> 

Nope. You can't trust bar() not doing something like

bar(const int* local_var)
{
   ... use local_var as ro...
   extern int myvar;
   myvar = 7;
}

For the compiler to do that, you must tag bar() with attribute(pure).

--
J.A. Magallon <jamagallon()able!es>     \               Software is like sex:
werewolf!able!es                         \         It's better when it's free
Mandriva Linux release 2006.1 (Cooker) for i586
Linux 2.6.14-jam1 (gcc 4.0.2 (4.0.2-1mdk for Mandriva Linux release 2006.1))

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