C string to D without memory allocation?

Jonathan M Davis via Digitalmars-d-learn digitalmars-d-learn at puremagic.com
Mon Dec 21 00:27:23 PST 2015

On Monday, December 21, 2015 05:43:59 Jakob Ovrum via Digitalmars-d-learn wrote:
> On Monday, 21 December 2015 at 05:41:31 UTC, Shriramana Sharma
> wrote:
> > Rikki Cattermole wrote:
> >
> >> string myCString = cast(string)ptr[0 .. strLen];
> >
> > Thanks but does this require that one doesn't attempt to append
> > to the returned string using ~= or such? In which case it is
> > not safe, right?
> Growing operations like ~= will copy the array to a GC-allocated,
> druntime-managed array if it isn't one already.

Exactly. As long as the GC has not been disabled, that there is sufficient
memory to allocate, and that appending elements does not result in an
exception being thrown (which it wouldn't with arrays of char) ~= should
always work. When ~= is used, the runtime looks at the capacity of the
dynamic array to see whether it has enough room to grow to fit the new
elements. If it does, then the array is grown into that space. If it
doesn't, then a block of GC memory is allocated, the elements are copied
into that memory, and the dynamic array is set to point to that block of
memory.  Whether the array is pointing to a GC-allocated block of memory or
not when ~= is called is irrelevant. If it isn't, all that means is that the
array's capacity will be 0, so it's going to have to reallocate, whereas if
it were GC-allocated, it might have enough capacity to not need to
reallocate. In either case, the operation will work.

- Jonathan M Davis

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