Re: class layout and dummy template parameter
Mycroft Holmes wrote:
"Tom Widmer [VC++ MVP]" <tom_usenet@hotmail.com> wrote in message
news:u7RsBM7EHHA.3616@TK2MSFTNGP06.phx.gbl...
The two types are indeed layout compatible. However, that doesn't allow
you to reinterpret_cast between them, but only allows you to put them
together in a union and access the members through either struct.
union U
{
POD<1> p1;
POD<2> p2;
};
assume p1.A is the first member.
then if I interpret correctly, by 9.2/17 and what you mentioned, &p1 ==
&p1.A == &p2.A == &p2, so a reinterpret_cast should work (i.e. return the
right pointer -- aliasing problems not included :)
I've just done a bit more research. The C99 standard has this example:
[quote]
The following is not a valid fragment (because the union type is not
visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f(struct t1 *p1, struct t2 *p2)
{
if (p1->m < 0)
p2->m = -p2->m;
return p1->m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
} u;
/* ... */
return f(&u.s1, &u.s2);
}
[/quote]
I would suggest that the intent in C++ is the same, and in order to
avoid aliasing problems you have to access through the union directly
(or at least in the same function).
Tom
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