On 23 May 2014 13:01, Chris Hall wrote:
> Where I have something along the lines of:
>
> typedef struct foo* foo
(I really hope you don't use the same name for a type and a pointer
to that type in real code!)
> void
> create(void** p_n, size_t s)
> {
> *p_n = malloc(s) ;
> } ;
>
> .....
> {
> foo bar ;
> .....
> create(&bar, 86) ;
> .....
> } ;
>
> I get the helpful warning "passing argument 1 of 'create' from incompatible pointer type". (This is, obviously, an artificial example. The problem arises with any function which takes a void** argument.)
>
> Don't get me wrong, I'm a big fan of this warning, in general. I suspect the reason is something deep in C... but it doesn't make full sense to me.
You can do it via a series of valid (and in this case, safe) conversions:
foo bar ;
void* vbar = bar;
create(&vbar, 86) ;
bar = vbar;
> And (bizarrely, IMHO):
>
> create(&((void*)bar), 86) ;
>
> is an error: "lvalue required as unary '&' operand" ?!?
The cast creates a temporary object of type void*, the error is
because you cannot take the address of a temporary.
> Pointers to pointers and, worse yet, chains of pointers to pointers require care (and my head hurts). So I appreciate all the help the compiler can give me to spot problems early. Having to cast to (void**) feels like taking the seat belt off and pressing the pedal to the metal :-(
I seriously suggest using C++ instead, the compiler gives you more
help with type safety, and provides features to avoid many common
mistakes while fiddling with pointers:
template<typename T>
void create(T** p)
{
*p = (T*)malloc(sizeof(T));
}
foo bar;
create(&bar);
That gets the size and the type correct automatically (assuming the
magic number 86 in your code was sizeof(struct foo)). But in C++
there's no need to write that code anyway:
struct foo { };
foo* bar = new foo;
(People will tell you that C++ is over-complicated and gives you more
than enough rope to hang yourself with ... fine, don't use all those
features. It's still a better C even if you only use a fraction of the
language.)
