Re:Overload functions in Python
by stabbles on Monday February 17, 2020 @08:07AM
(#22347383)
C++ does not have multiple dispatch, though.
Historically in C++ it was considered to be more performant to dispatch function calls on the base type of its arguments, not the concrete types.
Consider for simplicity member functions A::f(x...) in C++ as free functions where the first argument is fixed to be of type A, as in f(A, x...). If you want to emulate multiple dispatch for just 2 arguments f(A, B), you already find yourself in visitor pattern-land.
And the other issue with C++ is that it does not allow open functions. In the visitor pattern you necessarily have to implement f(A, B) for _all_ subtypes of A and B.
In Julia open functions + multiple dispatch make life quite a bit easier than in C++:
julia> abstract type Animal end
julia> abstract type Color end
julia> struct Dog <: Animal end
julia> struct Cat <: Animal end
julia> struct Brown <: Color end
julia> struct White <: Color end
julia> f(::White, ::Cat) = println("Hello white cat")
julia> f(::Brown, ::Dog) = println("Hello brown dog")
julia> for (color, animal) in [(White(), Cat()), (Brown(), Dog())]
f(color, animal)
end
Hello white cat
Hello brown dog
Note that array type is here is Vector{Tuple{Color,Animal}}, which would correspond to type erasure in C++.
Lastly, the modern C++ version with std::variant + std::visitor is probably not really multiple dispatch, since it has a union storage type under the hood.
Re:Overload functions in Python
by jampekka on Monday February 17, 2020 @08:17AM
(#22347437)
I tend to think of C++ function templates to have (static) multiple dispatch. With runtime polymorphism you do end up with visitors or some other hackery.
Historically in C++ it was considered to be more performant to dispatch function calls on the base type of its arguments, not the concrete types.
Consider for simplicity member functions A::f(x...) in C++ as free functions where the first argument is fixed to be of type A, as in f(A, x...). If you want to emulate multiple dispatch for just 2 arguments f(A, B), you already find yourself in visitor pattern-land.
And the other issue with C++ is that it does not allow open functions. In the visitor pattern you necessarily have to implement f(A, B) for _all_ subtypes of A and B.
In Julia open functions + multiple dispatch make life quite a bit easier than in C++:
Note that array type is here is Vector{Tuple{Color,Animal}}, which would correspond to type erasure in C++.Lastly, the modern C++ version with std::variant + std::visitor is probably not really multiple dispatch, since it has a union storage type under the hood.