(Julia 1.x) Getting type of #undef variable

Question

I am looking to get the types of the fields within a struct in order to set the field values correspondingly. Some data types initialise values on instantiation (e.g. Int64, Float64), whereas other types initialise to #undef (e.g. String, Array). Whilst typeof(getfield()) works for the former types, it throws UndefRefError for the latter:

julia> mutable struct MyStruct
           a::Int64
           b::String
           MyStruct() = new()
       end

julia> foo = MyStruct()
MyStruct(0, #undef)

julia> typeof(getfield(foo, :a))
Int64

julia> typeof(getfield(foo, :b))
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] top-level scope at none:0

Is there is way to get the type of an uninitialised variable or does #undef indicate the distinct lack of a type? Alternatively, is it possible to initialise default values using the inner constructor? e.g.

julia> mutable struct MyStruct
           a::Int64
           b::String
           MyStruct() = new(b = "")
       end

Answer 1

You're looking for the fieldtype function:

julia> fieldtype(MyStruct, :a)
Int64                         

julia> fieldtype(MyStruct, :b)
String                        

To your other question, you surely can initialize fields.

mutable struct MyStruct
    a::Int64
    b::String
    MyStruct() = new(0,"") # will initialize a as 0 and b as ""
end

Answer 2

Just a follow-on, you can get a tuple of all field types with fieldtypes:

julia> fieldtypes(MyStruct)
(Int64, String)