Re: Cannot understand the following codes

Ben C wrote:
On 2006-03-28, Alf P. Steinbach <alfps@xxxxxxxx> wrote:
* Ben C:
On 2006-03-28, Alf P. Steinbach <alfps@xxxxxxxx> wrote:
It's probably a void(*)(). But not matter what type it has, the cast is undefined behavior in C++, and I believe also in C.
Why's it undefined behaviour? I can see that it invites undefined
behaviour, but not that it entails it-- if the value of
stGOFuncs.p_go_Measurements is a function of the correct type won't it
be OK?
It will be OK, /if/ the type is some pointer to function (but not if the type is simply void*, in which case it's UB); I was too hasty there.

You're right, void * and function pointers are not compatible. I tried
this:

#include <stdio.h>

int f(int x)
{
return x * 2;
}

int main(void)
{
int (*q)(int) = f;
void *p = q; /* WARNING */
int (*r)(int) = p; /* WARNING */

int x = (*r)(3);

printf("Answer is %d\n", x);

return 0;
}

I had assumed void * was good for anything. But it's only good for data,
not functions.

I suppose on some machines functions might have a completely different
address space, be a completely different size etc., and the standard is
allowing for that...

Sorry about that... ;-)

Not at all, thank you, I was missing something.

Pedantically void pointers and function pointers are not compatible. But is a less pedantic world it might work.. if you change your line

int x = (*r)(3);

to

int x = (r)(3); or even

int x = r(3);

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---
.

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