Casting Pointers?

I've been unable to find a clear definitive answer to this so please point me to one if it already exists in the manual or the forums. Is it safe to cast pointer types? double data; long p = *cast(long*)&data; (Excuse any silly syntax errors as I'm doing this on my phone). This used to be possible in C until people recently decided it not only was wrong, it had always been wrong :P (Ok I'm not entirely serious there but that's what this issue feels like...) Is this legal / valid in D and if not what is the appropriate way to efficiently access data like this?

On Thursday, 12 May 2016 at 08:41:25 UTC, John Burton wrote: > I've been unable to find a clear definitive answer to this so please point me to one if it already exists in the manual or the forums. > > Is it safe to cast pointer types? > > double data; > long p = *cast(long*)&data; > > (Excuse any silly syntax errors as I'm doing this on my phone). > > This used to be possible in C until people recently decided it not only was wrong, it had always been wrong :P (Ok I'm not entirely serious there but that's what this issue feels like...) > > Is this legal / valid in D and if not what is the appropriate way to efficiently access data like this? Yes, it's perfectly legal in @system code, but only if you know what you're doing ;) Take a look at std.math for example: https://github.com/dlang/phobos/blob/v2.071.0/std/math.d#L4839

On Thursday, 12 May 2016 at 08:41:25 UTC, John Burton wrote: > Is this legal / valid in D and if not what is the appropriate way to efficiently access data like this? Phobos / druntime use casts like this as well, but there has been some discussion as to whether it should be guaranteed to be correct. There are good reasons the C folks disallow it (well, performance of course, what else? ^^), and the reasoning applies to D as well. In this case, you can use a union: union DL { double d; long l; } auto dl = DL(1.0); writeln(dl.l);

On Thursday, 12 May 2016 at 08:41:25 UTC, John Burton wrote: > Is this legal / valid in D and if not what is the appropriate way to efficiently access data like this? We rely on this cast to be legal and well-defined in a lot of Sociomantic code, banning would be a real disaster :X

On Thursday, 12 May 2016 at 09:33:54 UTC, Dicebot wrote: > On Thursday, 12 May 2016 at 08:41:25 UTC, John Burton wrote: >> Is this legal / valid in D and if not what is the appropriate way to efficiently access data like this? > > We rely on this cast to be legal and well-defined in a lot of Sociomantic code, banning would be a real disaster :X (most common case is casting between `char[]` and `struct S { char[]; }`)

On Thursday, 12 May 2016 at 09:25:49 UTC, Stefan Koch wrote: > On Thursday, 12 May 2016 at 08:41:25 UTC, John Burton wrote: >> I've been unable to find a clear definitive answer to this so please point me to one if it already exists in the manual or the forums. >> >> Is it safe to cast pointer types? >> >> double data; >> long p = *cast(long*)&data; >> >> (Excuse any silly syntax errors as I'm doing this on my phone). >> >> This used to be possible in C until people recently decided it not only was wrong, it had always been wrong :P (Ok I'm not entirely serious there but that's what this issue feels like...) >> >> Is this legal / valid in D and if not what is the appropriate way to efficiently access data like this? > > How would such a cast be safe? "Safe" as in it produces a valid long value by interpreting the bytes of the stored double. Obviously if it works it's very system dependent. But in C it's simply undefined behavior to read a value stored using one pointer type using a pointer to a different type. (With some additional rules about char in a few cases). The whole strict aliasing businesss