TITLE: casting between structs and arrays

(Source: comp.lang.c++.moderated, 8 Oct 2000)

WILLIAMS: Jim Williams <williajp@my-deja.com>

>If I'm not mistaken, the language guarantees the following strangeness
>is safe because the two structs begin with the same fields
>(reinterpret_cast is the appropriate choice here, right?).
>
>   struct Foo {
>      int a,b;
>   };
>
>   struct Bar {
>      int a,b,c;
>   };
>
>   Bar b;
>   Foo *f=reinterpret_cast<Foo *>(&b);
>   f->a=10; //safe
>   f->b=20; //safe

CLAMAGE: Stephen Clamage <stephen.clamage@sun.com>

I don't find such a guarantee in the standard, but the code will
probably work.

What is guaranteed is in section 9.2p16:
"If a POD-union contains two or more POD-structs that share a common
initial sequence, and if the POD-union object currently contains one
of these POD-structs, it is permitted to inspect the common initial
part of any of them. Two POD-structs share a common initial sequence
if corresponding members have layout-compatible types (and, for
bit-fields, the same widths) for a sequence of one or more initial
members."

If Foo and Bar are members of a union, your code is safe.

WILLIAMS:

>I'm wondering if the following is also safe.  Since all of the fields
>are the same type, there should be no need for padding within the
>structs.
>
>   int *p=reinterpret_cast<int *>(&b);
>   p[0]=10; //safe, modifies b.a
>   p[1]=20; //safe, modifies b.b
>   p[2]=30; //safe, modifies b.c

CLAMAGE:

You cannot assume that no padding is needed. For example, a system
with 64-bit words might make ints 32 bits and align them on 8-byte
boundaries. Since Bar does not contain an array of ints, the effect of
your code is undefined.

WILLIAMS:

>If this is not safe, would the following alternate definition of Bar
>make it so, with obvious adjustments to the preceding code?
>
>   struct Bar {
>      int a[3];
>   };

CLAMAGE:

I don't find any guarantee that 3 int struct members will have the
same offsets as elements of an int array. For example, I don't find a
requirement that the alignment of array elements be the same as the
alignment of struct members. This example does not even pass the
"common initial sequence" test.



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