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...)
This is a Bad Idea in C because types in C aren't actually
well-defined. `long` is required to be at least 32 bits. `double`
is required to be a floating point type. They will often not
match up in sizes the way you might think. The spec doesn't even
require `double` to be IEEE (that's an optional annex).
Clang on Linux x86_64 has both `long` and `double` as 64-bit
types.
Clang on Linux x86 has `long` as 32-bit, `double` as 64-bit.
Is this legal / valid in D and if not what is the appropriate
way to efficiently access data like this?
In D, both `long` and `double` are defined in the spec to be
64-bits, regardless of compiler/os/arch. It isn't "safe" (because
casting pointers is never safe), but it should behave predictably.
Note that D does have its own poorly-defined types, such as
`real`, that you will need to be careful with.
