Re: [boost] Re: is_pointer Bug

From: John Maddock (John_Maddock_at_[hidden])
Date: 2000-10-22 05:51:15

>Right, there's one trivial error (is_POD needs a const int 
>specialization) and a more serious one where I try to get the value_type 
>using iterator_traits (because int* const is not a valid iterator type).

Again there are sublties here: is_POD<const int> is deliberately false,
otherwise you may find yourself memcpy'ing over a const array of int's.  In
this context we are saying that const int is not a POD because it is not
trivially assignable.

>Yes. :-) Why wouldn't it? You're explicitly telling the compiler to load 
>the pointer whenever it has to access it and save it whenever it is 
>modified. It seems to me that the compiler can do this easily enough with 
>the element by element version and with the memcpy version.

I realised afterwards that copy was a bad example, consider:

extern const volatile int vi;

int a[3];

std::fill(a, a+3, vi);

Would you expect a[0] to have the same value as a[2]?  If so you're likely
to be surprised (remember that volatile qualified types may be changed by
events outside your programs control).
Now imagine that fill was implemented using a block fill for integer types
(load value once and then "rep stosd [addr]"), in this case the effect will
be different from the "assign one at a time" version, and it's not clear
which if either of these is correct.

-John.