easier, clearer, faster ccall out args

[vtjnash]

ccall supports the following coding pattern. I propose adding this code to base/c.jl:

type ByRef{T} # T enforced to be any BitsKind
   p::T
   ByRef{S}(p::S) = (@assert isa(S,BitsKind); new(p))
end
ByRef{T}(::Type{T}) = ByRef{T}(zero(T))
typealias _out_{T} Ptr{ByRef{T}}

x = ByRef(Int)
ccall(:fname, Void, ( _out_{T}, ), &x)

edit: updated the example to use ByRef rather than OutArg

[vtjnash]

@JeffBezanson thoughts?
note: OutArg could also be described as / named ByRef

[JeffBezanson]

I'm not sure heap-allocating an object to return an integer qualifies as fast! :)
It would be best to have some syntax for this that causes the space written to by the C function to be assigned back to x.

[vtjnash]

compared to the current strategy of heap-allocating an array, it seems much faster to me to ask the gc for two words.
anyways, yes, ideally ByRef() would be dropped and the definition changed compatibly as follows:

ByRef{T}(::Type{T}) = zero(T)
typealias _out_{T} Ptr{T}

x = ByRef(Int)
ccall(:fname, Void, ( _out_{T}, ), &x)

which could also be written:

x::Int
ccall(:fname, Void, ( Ptr{Int}, ), &x)

[stevengj]

+1 on having a syntax to cause the space written by the C function to be written back to x. Currently, this is probably the single biggest annoyance in ccall.

[vtjnash]

implemented by 3387e77

[stevengj]

Is it documented yet?

[vtjnash]

most of the documentation is in #7906