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Subject: Re: [std-proposals] operator ==/!= for empty struct
From: Jake Arkinstall (jake.arkinstall_at_[hidden])
Date: 2019-11-03 08:01:58


You can create this overload yourself. Make sure you arent just using
holds_alternative though, because equality implies that not only are the
objects the same (or both representable as the same common thing), but that
the value of that common representation is equal too (your idea needs to be
generalisable to non-empty structs). This is why holds_alternative is a
clearer syntax for this use case.

Off topic but relevant to your specific example: a variant is a fantastic
tool for storing a state that you're going to access at a later point, such
as a connection state, but it's a pretty bad tool if you only care about
its value within one iteration of a loop. Update() *knows* the type of what
it's giving Foo, but is having to wrap it into a more generic type and Foo
needs to then figure that type out again. For this pattern a function
object which accepts all of the possible output types is typically more
effective. Very much like running a std::visit on a variant, except you
pass the callable straight to the source, which already knows the type, and
the compiler can optimise it more effectively. Try it out on godbolt and
you'll see what I mean.

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