On 8/26/15 8:14 AM, John Burton wrote:
This would be undefined behavior in c++ due to aliasing rules on
pointers. It appears to "work" reliably in D when I try it, but that's
obviously no guarantee that it's correct or will continue to do so.
Is this correct code in D? And if not, what should I do instead to
cleanly and efficiently extract structured data from a sequence of bytes?
import std.stdio;
struct data
{
int a;
int b;
}
void main()
{
byte[] x = [1, 2, 3, 4, 5, 6, 7, 8];
data* ptr = cast(data*)(x);
printf("%x %x\n", ptr.a, ptr.b);
}
This should be defined behavior (but there is an issue that will
probably eventually be fixed -- you shouldn't cast an array to a
pointer, use x.ptr instead). I think strict aliasing only comes into
play when something is *changed*. For example:
byte[] x = [1, 2, 3, 4, 5, 6, 7, 8];
data* ptr = cast(data *)(x.ptr);
ptr.a = 4;
writefln("%x %x\n", x[0], x[1]);
However, if we consider the strict aliasing rule from C++ allows casting
to char * and unsigned char *, D's byte[] and ubyte[] should be
equivalent to those. So if anything, that should be defined.
-Steve
