How to instantiate a struct in Julia where I have the struct name in a string variable? - julia

Question

The name of the struct to instantiate will be passed by the caller to my program. Then I would need to instantiate the corresponding struct for the same for further processing.
For example, if the struct is defined like this
struct A end
and I have a function defined as
function load(struct_name::AbstractString)
if struct_name == "A"
return A()
elseif struct_name == "B"
return B()
elseif ..... # and so on
end
end
it will work. But is there a more direct way like return struct_name() instead of having n number of if else statements? I see that Julia supports reflection. How can that be used to support the above use case?

Answer 1

I would recommend not doing it in production code, but you can do the following:
function load(struct_name::AbstractString)
invoke(eval(Symbol(struct_name)),Tuple{})
end
strut_name via eval will get resolved in the global scope of the module.
It is safer to use a dictionary as #EPo suggested.

Answer 2

An example of dictionary-based dispatch. Dict("a" => A, "b" => B)[tag] selects a constructor, and () calls it.
struct A end
struct B end
function dispatch(tag)
return Dict("a" => A, "b" => B)[tag]()
end
#assert dispatch("a") == A()
If you care about default values to handle unexpected parameter, for example dispatch('zzz'),
you can make recourse to get().
As a side note about risks of eval() there is a small collection of powerful warning references in a neighboring Python question. In short, eval() is a big security hole and a 'smell' (warning sign) for a questionable design of a program.

Answer 3

You could use a macro instead:
julia> module Load
export #load
macro load(struct_name::Symbol)
return :($(esc(struct_name))())
end
end
Main.Load
julia> using Main.Load: #load
julia> struct A end
julia> struct B end
julia> #load A
A()
julia> #macroexpand #load B
:(B())
julia> #load C
ERROR: UndefVarError: C not defined
Stacktrace:
[1] top-level scope at none:0