I wonder what the best way would be to call a specialized function dependent on
the size of `Int`. Let's say that I have two implementations
func foo_impl(value: Int32) { /* ... */ }
func foo_impl(value: Int64) { /* ... */ }
and I want
func foo(value: Int)
to call the "right one" of them, according to the architecture (32-bit or
64-bit).
func foo(value: Int) { foo_impl(value: value) }
does not compile. (I assume that is because `Int` is not a type alias to
`Int32` or `Int64` but an independent type.)
This works:
func foo1(value: Int) {
if MemoryLayout<Int>.size == 4 {
foo_impl(value: Int32(value))
} else {
foo_impl(value: Int64(value))
}
}
or
func foo2(value: Int) {
switch MemoryLayout<Int>.size {
case 4: foo_impl(value: Int32(value))
case 8: foo_impl(value: Int64(value))
default:
abort()
}
}
But a typo in the constants would go unnoticed and just call the wrong
function, or cause a runtime error instead of a compile-time error. And perhaps
`Int` can be an 128-bit integer in the future? The compiler would not warn that
the code needs to be updated.
This seems to be more promising:
func foo3(value: Int) {
switch (__WORDSIZE) {
case 32: foo_impl(value: Int32(value)) // Warning: Will never be
executed
case 64: foo_impl(value: Int64(value))
}
}
Apparently the compiler "knows" which case will be executed, `foo3` does not
compile if there is no case matching actual integer size. But there is always
an annoying warning for the unused case. And I am not sure if it is guaranteed
that __WORDSIZE is the number of bits in an `Int`.
So my question is: What would be the best way to dispatch dependent on the size
of `Int`, such that
- The compiler checks the correctness.
- The compiler optimizes the code so that no runtime check is done.
- No warnings are produced.
If `Int` had a `native` property like `CGFloat` then I could simply call
func foo(value: Int) { foo_impl(value: value.native) }
but I could not find such a property. (Would that be a reasonable thing to ask
on swift-evolution?)
Background: I am asking this just out of curiosity, but the question came up
when looking at the `hash_combine` function in the Boost library:
http://www.boost.org/doc/libs/1_62_0/boost/functional/hash/hash.hpp
with quite different implementations
inline void hash_combine_impl(boost::uint32_t& h1, boost::uint32_t k1)
inline void hash_combine_impl(boost::uint64_t& h, boost::uint64_t k)
and I wondered how this would be done in Swift.
Regards,
Martin
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