[swift-evolution] Proposal: One-past-end array pointers and convertibility of nil for interaction with C APIs

Árpád Goretity arpad.goretity at gmail.com
Wed Dec 23 19:35:09 CST 2015

Hi everyone,

I was recently trying to use a C API (LLVM for the record) that required
passing an array to a function in the form of a pointer and a size. I
couldn't find a straightforward way to pass a null pointer to the function
in question conditionally (when the array is empty), since the following –
simplified – code doesn't currently typecheck:

    // C function with signature: void foo(T *ptr, unsigned size)
    // imported into Swift as: (UnsafeMutablePointer<T>, UInt32) -> ()
    var arr: [T] = []
    foo(arr.count > 0 ? &arr[0] : nil, UInt32(arr.count))

The error is: result values in '? :' expression have mismatching types
'inout T' and '_'

This does not make sense since although `nil` is typeless by itself, its
concrete type should still be able to be inferred from the context (just
like it is inferred correctly if one writes

    condition ? 1 as Optional<Int> : nil

which is an analogous scenario.)

Since the inout operator (&) can only be used in function call arguments
(so it's not exactly C's address-of), I believe that there's no easy way of
elegantly passing a null pointer when the array is empty. (Yes, I could
write two almost-identical calls, but meh…) And even if there is one (and
I'm just missing it), the fact that the above code does not work seems
inconsistent to me.

I also realized that this specific issue generalizes to the (in)ability of
passing one-past-end pointers – which would be equally valid and even more
convenient in the above case, as the callee does not dereference the passed
pointer when the count is 0, but in general, it can be applied to functions
accepting [begin, end + 1) ranges.

The problem here is that a one-past-end pointer does not reside at a valid
index (pretty much by definition), so bounds checking kicks in and kills
the program.

My proposed solutions:

 – Extend type inference for unsafe pointers and nil, so that when a value
is passed by address to a function, it's not only the result of an
&-expression that has its type inferred to be (or implicitly converted to)
Unsafe[Mutable]Pointer, but if there's a nil somewhere around, such as the
one in the example above, it gets promoted to that type too, just like NULL
in C or nullptr in C++.

 – Stop overloading the inout '&' operator and using it for C-style
address-of operations. I could imagine a similar, but distinct operator or
even a library function (something along the lines of unsafeAddressOf) that
specifically yields the physical address of its operand as an unsafe C
pointer, and which is thus first-class in the sense that it may be used
anywhere other expressions may be, not just as immediate call arguments.

 – Make array bounds checking more lenient when passing pointers to array
elements into C functions. Bounds checking should, in these cases, allow
indexing the one-past-end element of an array if (and only if) it is the
argument of the address-of operator.

Comments and questions are welcome (you might need clarification, as it's
2:35 AM here when I'm writing this…)


Author of the Sparkling language
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