CODEWITHSUNDEEP
julia
Damian Pasierowski
Damian Pasierowski

Reputation: 113

How to create constructor with Null argumnet in Julia

I want to create type Family with argument type Family but i want to create possibility with Null argument

type Family
    name:: AbstracDtring
    people:: Int
    dad:: Family
    mom:: Family

Family(name:: AbstractString, people::Int ) = new (name, people , NULL, NULL)


end

Can i do somthing with this i want create "object" with references to another object or without references

Upvotes: 3

Views: 894

Answers (2)

StefanKarpinski
StefanKarpinski

Reputation: 33249

You can call new with fewer arguments:

type Family
    name::AbstractString
    people::Int
    dad::Family
    mom::Family

    Family(name::AbstractString, people::Int) = new(name, people)
end

You can construct instances but until you've assigned the .dad and .mom fields, accessing them will cause an error:

julia> fam = Family("Jones", 3)
Family("Jones",3,#undef,#undef)

julia> fam.dad
ERROR: UndefRefError: access to undefined reference
 in eval(::Module, ::Any) at ./boot.jl:225
 in macro expansion at ./REPL.jl:92 [inlined]
 in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46

julia> fam.mom
ERROR: UndefRefError: access to undefined reference
 in eval(::Module, ::Any) at ./boot.jl:225
 in macro expansion at ./REPL.jl:92 [inlined]
 in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46

julia> fam.dad = fam
Family("Jones",3,Family(#= circular reference @-1 =#),#undef)

julia> fam.mom
ERROR: UndefRefError: access to undefined reference
 in eval(::Module, ::Any) at ./boot.jl:225
 in macro expansion at ./REPL.jl:92 [inlined]
 in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46

You can check if a field is defined or not using the isdefined function:

julia> isdefined(fam, :dad)
true

julia> isdefined(fam, :mom)
false

The Nullable approach works too but this is somewhat lighter weight.

Upvotes: 5

colinfang
colinfang

Reputation: 21717

Use Nullable (http://docs.julialang.org/en/release-0.4/manual/types/#nullable-types-representing-missing-values)

type Family
    name:: AbstractString
    people:: Int
    dad:: Nullable{Family}
    mom:: Nullable{Family}

Family(name:: AbstractString, people::Int ) = new(name, people, Nullable{Family}(), Nullable{Family}())

end

Upvotes: 4

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