I’m very much +1 on this idea.
> On Mar 17, 2016, at 6:59 PM, Jordan Rose via swift-evolution
> <[email protected]> wrote:
> I consider this a bit more difficult to understand than the original code, at
> least at a glance. We should therefore add new initializers of the following
> form:
>
> init?<OtherPointee>(_ otherPointer: UnsafePointer<OtherPointee>?) {
> guard let nonnullPointer = otherPointer else {
> return nil
> }
> self.init(nonnullPointer)
> }
> The body is for explanation purposes only; we'll make sure the actual
> implementation does not require an extra comparison.
>
> (This would need to be an overload rather than replacing the previous
> initializer because the "non-null-ness" should be preserved through the type
> conversion.)
>
> The alternative is to leave this initializer out, and require the nil case to
> be explicitly handled or mapped away.
>
>
This should definitely be included. Everyone is used to “casting” by calling
the appropriate initializers, and
UnsafePointer<SpecificType>(someUnsafeVoidPointer) maps directly to what a
C/Objective-C programmer would expect.
>
> <https://github.com/jrose-apple/swift-evolution/tree/optional-pointers#open-issue-unsafebufferpointer>Open
> Issue: UnsafeBufferPointer
>
> The type UnsafeBufferPointer represents a bounded typed memory region with no
> ownership or lifetime semantics; it is logically a bare typed pointer (its
> baseAddress) and a length (count). For a buffer with 0 elements, however,
> there's no need to provide the address of allocated memory, since it can't be
> read from. Previously this case would be represented as a nil base address
> and a count of 0.
>
> With optional pointers, this now imposes a cost on clients that want to
> access the base address: they need to consider the nil case explicitly, where
> previously they wouldn't have had to. There are several possibilities here,
> each with their own possible implementations:
>
> Like UnsafePointer, UnsafeBufferPointer should always have a valid base
> address, even when the count is 0. An UnsafeBufferPointer with a
> potentially-nil base address should be optional.
>
> UnsafeBufferPointer's initializer accepts an optional pointer and becomes
> failable, returning nil if the input pointer is nil.
>
> UnsafeBufferPointer's initializer accepts an optional pointer and synthesizes
> a non-null aligned pointer value if given nil as a base address.
>
> UnsafeBufferPointer's initializer only accepts non-optional pointers. Clients
> such as withUnsafeBufferPointermust synthesize a non-null aligned pointer
> value if they do not have a valid pointer to provide.
>
> UnsafeBufferPointer's initializer only accepts non-optional pointers. Clients
> using withUnsafeBufferPointermust handle a nil buffer.
>
> UnsafeBufferPointer should allow nil base addresses, i.e. the baseAddress
> property will be optional. Clients will need to handle this case explicitly.
>
> UnsafeBufferPointer's initializer accepts an optional pointer, but no other
> changes are made.
>
> UnsafeBufferPointer's initializer accepts an optional pointer. Additionally,
> any buffers initialized with a count of 0 will be canonicalized to having a
> base address of nil.
>
> I'm currently leaning towards option (2i). Clients that expect a pointer and
> length probably shouldn't require the pointer to be non-null, but if they do
> then perhaps there's a reason for it. It's also the least work.
>
> Chris (Lattner) is leaning towards option (1ii), which treats
> UnsafeBufferPointer similar to UnsafePointer while not penalizing the common
> case of withUnsafeBufferPointer.
What’s the use of an UnsafeBufferPointer with zero count? Semantically that is
making a claim that it can’t back up (“I have allocated memory at location X”
which isn’t compatible with the idea of “zero count/size").
Without knowing more context I’d strongly favor (1i). If an array is empty the
natural expectation for withUnsafeBufferPointer is you get
UnsafeBufferPointer<Element>? = nil, which follows the behavior of the rest of
the language and things like guard let make it trivial to handle properly. If
someone really has a problem with it they can add ifUnsafeBufferPointer() that
returns a non-optional pointer and skips executing the closure if the Array is
empty (which is the behavior of your standard for loop).
Russ
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