Allocations for abstract field of struct

General Usage
6

Usually structs with many fields don’t have such many types, that is, many fields share the same type, such that something like struct A{T1,T2,T3} ... end tends to suffice.

Now, if you really want all possible flexibility without annotating anything, but still keeping fields concrete, you man just want to define a named tuple:

julia> test1(x::String) = (; :x => _TestStringReference(x))
test1 (generic function with 1 method)

julia> test1("foobar")
(x = _TestStringReference("foobar"),)

julia> @btime test1("foobar")
  2.186 ns (0 allocations: 0 bytes)
(x = _TestStringReference("foobar"),)

and, eventually wrap the named tuple in a struct for dispatch, such as

julia> struct _MyTest1{T<:NamedTuple}
           x::T
       end

and define getproperty operations for the _MyTest1 type.

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