dart:ffi Void size one byte or unknown

[dcharkes]

Should Void have a size of 1 byte, or an unknown size.

Defining a size would allow the following

Pointer<Void> p = allocate(count: 20);
Pointer<Void> p2 = p.elementAt(10);
sizeOf<Void>(); // returns 1

Keeping Void opaque, not knowing its size would disallow allocate and sizeOf:

Pointer<Void> p = allocate<Int8>(count: 20).cast();
Pointer<Void> p2 = p.offsetBy(10);
sizeOf<Void>(); // throws exception

cc @sjindel-google

[sjindel-google]

We can allow pointer arithmetic on a Pointer<Void> without necessarily allowing allocate and sizeOf.

[dcharkes]

We already allow offsetBy on Pointer<Void>.

[sjindel-google]

Right, I'm just saying that we could allow elementAt as well, for consistency with C.

[dcharkes]

It is not C or C++ semantics, it's a non standard extension in GNU C.

6.24 Arithmetic on void- and Function-Pointers

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.

A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.

The option -Wpointer-arith requests a warning if these extensions are used.

https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html

I prefer keeping things consistent with regard to being sized or not.

Sized:

• Int8, ..., Double, Float
• Pointer
• subtypes of Struct

Not sized:

• NativeType
• NativeInt
• NativeDouble
• NativeFunction
• Void
• Struct

Everything that is sized can be used in allocate, sizeOf, elementAt, load, and store.

Note that in our implementation before #37254 we require load and store to be statically typed as a sized thing, while we fail sizeOf, elementAt, and allocate dynamically. We do the latter so that you can write generic code. After landing #37254 all of these will be dynamic.

If we use extension methods we could even try using NativeSized in the hierarchy, and only define these 5 operations on those. This would almost work: you could still try to call them on Struct and NativeSized even though these are not sized themselves.

[sjindel-google]

👍