Custom Serialization

The API is simple enough, to enable custom serialization for your type A you define a new type e.g. ASerialization that contains the fields you want to store and define JLD2.writeas(::Type{A}) = ASerialization. Internally JLD2 will call Base.convert when writing and loading, so you need to make sure to extend that for your type.

struct A
    x::Int
end

struct ASerialization
    x::Vector{Int}
end

JLD2.writeas(::Type{A}) = ASerialization
Base.convert(::Type{ASerialization}, a::A) = ASerialization([a.x])
Base.convert(::Type{A}, a::ASerialization) = A(only(a.x))

Already defined custom serialization

Take care, some Julia internal types already use a CustomSerialization and JLD2.jl cannot nest them. In order to avoid unexpected behavior you should define a wrapper type, such as in the example above ASerialization even if you could use a simple Julia built in type (as in this case Vector{Int}). Target types to avoid are <: AbstractDict and <:Array.

If you do not want to overload Base.convert then you can also define

JLD2.wconvert(::Type{ASerialization}, a::A) = ASerialization([a.x])
JLD2.rconvert(::Type{A}, a::ASerialization) = A(only(a.x))

instead. This may be particularly relevant when types are involved that are not your own.

struct B
    x::Float64
end

JLD2.writeas(::Type{B}) = Float64
JLD2.wconvert(::Type{Float64}, b::B) = b.x
JLD2.rconvert(::Type{B}, x::Float64) = B(x)

arr = [B(rand()) for i=1:10]

@save "test.jld2" arr

In this example JLD2 converts the array of B structs to a plain Vector{Float64} prior to storing to disk.