Why not just profile it? Set up a loop of 100,000 or so with each method and
time it.
Ryan
> On 4 Nov 2017, at 6:42 am, Jens Alfke via swift-users <swift-users@swift.org>
> wrote:
>
> I’m working with a C API that represents strings as UTF-8 data tagged with a
> length but **without a trailing NUL byte**. In other words, its string type
> is basically a tuple {const char*, size_t}. I need to convert this
> representation to and from Swift 4 strings.
>
> This needs to be efficient, as these calls will occur in some areas of my
> project that are known to be performance-critical. (Equivalent conversions in
> my Obj-C code have already shown up as hot-spots and been carefully
> optimized.)
>
> For String-to-UTF-8, I’m using String.withCString():
> _ = str.withCString { bytes in c_function(bytes, strlen(bytes)) }
> An alternative is
> let bytes = [UInt8](str.utf8)
> c_function(&bytes, bytes.count)
> Any idea which of these is more optimal? The former has to call strlen, but I
> suspect the latter may incur more heap allocation.
>
> For UTF-8-to-String I use this, where `stringPointer` is an UnsafeRawPointer
> and stringLen is an Int:
> let data = Data(bytes: stringPointer, count: stringLen)
> return String(data: data, encoding: String.Encoding.utf8)
> I’m unhappy about this because it incurs both heap allocation and copying the
> string bytes. But Data doesn’t seem to have the “noCopy” options that NSData
> does. Any way to pass the bytes directly to String without an intermediate
> copy?
>
> —Jens
>
> PS: I’m aware this is an FAQ, but I’ve already put in time searching. Most of
> the hits are obsolete because the damn String API keeps changing, or else
> they assume NUL-terminated C strings; and the remainder don’t consider
> performance.
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