Julia parametric constructor and incomplete initialization

Question

I'm new to programming in Julia (I'm coming from Java) and some Julia concepts are still difficult to understand to me. What I intend is to replicate this Java code:

public class Archive<T extends Solution> {
  public List<T> solutions;

  public Archive() {
    solutions = new ArrayList<>() ;
  }
}

Then, I have defined the following Julia struct:

mutable struct Archive{T <: Solution}
  solutions::Vector{T}
end

and I would like to define a constructor to inialize empty archives. In the case of an external constructor, I think that the code would look like this:

function Archive()
  return Archive([])
end

Would it be possible to modify the constructor somehow to include the parametric type T?.

Answer 1

This is most likely the constructor you want:

Archive(T::Type{<:Solution}=Solution) =Archive(T[])

Then you can write Archive() or Archive(SomeType) if SomeType is a subtype of Solution.

Note that your definition:

function Archive()
  return Archive([])
end

Is incorrect unless Solution is Any (and most likely it is not). You will get an error as [] is Vector{Any} and you require Archive to store subtypes of Vector{<:Solution}.