C++11 makes it possible to overload member functions based on reference qualifiers:
class Foo {
public:
void f() &; // for when *this is an lvalue
void f() &&; // for when *this is an rvalue
};
Foo obj;
obj.f(); // calls lvalue overload
std::move(obj).f(); // calls rvalue overload
I understand how this works, but what is a use case for it?
I see that N2819 proposed limiting most assignment operators in the standard library to lvalue targets (i.e., adding "&
" reference qualifiers to assignment operators), but this was rejected. So that was a potential use case where the committee decided not to go with it. So, again, what is a reasonable use case?
In a class that provides reference-getters, ref-qualifier overloading can activate move semantics when extracting from an rvalue. E.g.:
class some_class {
huge_heavy_class hhc;
public:
huge_heavy_class& get() & {
return hhc;
}
huge_heavy_class const& get() const& {
return hhc;
}
huge_heavy_class&& get() && {
return std::move(hhc);
}
};
some_class factory();
auto hhc = factory().get();
This does seem like a lot of effort to invest only to have the shorter syntax
auto hhc = factory().get();
have the same effect as
auto hhc = std::move(factory().get());
The original proposal paper N1821 provides three motivating examples:
Constraining operator =
to lvalues (TemplateRex's answer)
Enabling move for members (this answer)
Constraining operator &
to lvalues. I suppose this is sensible to ensure that the "pointee" is more likely to be alive when the "pointer" is eventually dereferenced:
struct S {
T operator &() &;
};
int main() {
S foo;
auto p1 = &foo; // Ok
auto p2 = &S(); // Error
}
Can't say I've ever personally used an operator&
overload.