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design-patterns - What is delegation in julia?

问题描述

I see occational references to delegation, or the delegation design pattern in Julia.

What is this?

E.g. I see it mentioned in

标签： design-patternspolymorphismjuliacomposition

解决方案

This is a form of polymorphism via composition (rather than inheritance)

Say one has a wrapper type, wrapping some instance of a concrete subtype of AbstractT where the wrapper itself is intended to be a subtype of AbstractT (Not nesc always true, but in general).

To add all the methods one exacts such a subtype of AbstractT to have, we want to delegate some or all of those methods to the wrapped object. We do this via metaprogramming. There are a few varients on how to do this. But in general it is a hard pattern to abstract so people often write their own.

Say that that all AbstractT subtypes should implement 1arg length, size and mean

struct WrappedT{T<:AbstractT} <: AbstractT
    backing ::T
    ...
end

for fun in (:(Base.length), :(Base.size), :(Statistics.mean))
    @eval ($fun)(x::WrappedT, args...) = ($fun(x.backing, args...))
end

Generally you won't delegate all the methods, since some you will want to do differently, that is the point of creating the new type after all.

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