const&&

[waxen parcel]

is a const&& method overload a code smell? ie is there a way of calling such an overload from somewhere that is itself not a code smell? eg this overload set should be sufficient for anything irl:

struct S {
  auto foo() &;
  auto foo() const&;
  auto foo() &&;
  // auto foo() const&&; // is there any use for this?
};

are there legitimate ways to call foo() const&& except from, eg, withing an expression in which a function returns const S, which is already smelly?

const S bar(); // this is already smelly
int main() {
  bar().foo(); // will call const&& if it exists, otherwise const&.
}

[south mesa]

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[copper carbon]

you can try to add reference and const to member functions, but that can still be easily broken by doing a use after free, so none of these are perfect

[waxen parcel]

UAF is already UB, so why do I care which will be chosen?

[copper carbon]

whose code is this and why did they write so many member function overloads?

how much do the different overloads differ?

[waxen parcel]

there are so many overloads because they are necessary to optimize for different cases. ie if foo is called on a temporary, then i want the && overload to be called.

tbh all of this is going to be kinda moot eventually anyway, because of deducing this. but for pre-C++23 code, it has its relevance

[copper carbon]

if someone does

int main()
{
  S().foo();
}

is this one going to use const&& overload?

or the && overload?

[waxen parcel]

that would use the normal && overload

[copper carbon]

well then maybe you should get rid of the const&&

since, you can't move things away from a const anyways

[ripe kindle]

not sure I'd consider it a code smell. it's just not very useful 🤷‍♂️

like, what would you do with such a thing?

[waxen parcel]

well isn't that the point? either it's unused, or is indicative of code elsewhere that is probably (though not necessarily) wrong

[rigid ivy]

const&& makes basically no sense, you only use it to explicitly disallow something

for example, std::ref and std::cref have deleted const&& overloads

without it, it would simply call the const& overload which would silently work and may be a mistake

[waxen parcel]

ok, yea that makes sense. it's like calling out a code smell explicitly

if you somehow got a const&&, that's wrong and should be called out

[rigid ivy]

kinda depends, it may be convenient to std::move a T which can potentially be const in some places

which would produce a const&& but that's not necessarily a mistake, not if your container supports const template arguments

[waxen parcel]

ok, thanks. that clears things up a bit

!solved