Casting Pointers?

[Anon via Digitalmars-d]

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.