🚀 go-pugleaf

In article <1992Nov1.201547.4661@cs.brown.edu> sdm@cs.brown.edu (Scott Meyers) writes:
>However, you don't need an explicit cast to go from a T* to a void*.
>Hence, given that this is legal,
>
>    T *p;
>    void *q = p;
>
>is there a good reason why this isn't?
>
>    void f(T*);
>    void (*pf)(void*) = f;

Certainly.  Consider the line that would be legal next:

T2* p;	// class T2 has no relation to class T
pf(p);

Now we have passed a pointer to T2 to a function that thinks it is getting
a pointer to T, and we have broken the type system.

It turns out that you've gotten things exactly backwards.  It would
be safer (although there is still a problem) to have

	void f(void*);
	void (*pf)(T*) = f;

Why is this better?  Because we can only pass T* pointers as arguments
to pf, and these are guaranteed to be legal to cast to void*.  Why is
it still a problem?  Because void* and T* may have different
representations.





--
Joe Buck	jbuck@ohm.berkeley.edu