Why is this code compiling? I thought that rvalues returned by the constructor are not located in memory and therefore can't be used as lvalues.
class Y {
public :
explicit Y(size_t num = 0) {}
};
int main() {
Y(1) = Y(0); // WHAT?!?
return 0;
}
The synthesized assignment operator is declared as one of these (if it can be synthesized and isn't declared as deleted) according to see 12.8 [class.copy] paragraph 18:
Y& Y::operator=(Y const&)
Y& Y::operator=(Y&)
()That is, like for any other member function which isn't specifically declared with ref-qualifiers it is applicable to rvalues.
If you want to prevent a temporary object on the left hand side of the assignment you'd need to declare it correspondingly:
class Y {
public :
explicit Y(std::size_t num = 0);
Y& operator= (Y const&) & = default;
};
The standard uses the name ref-qualifier for the &
before the = default
. The relevant proposal is N2439. I don't know where there is a good description of ref-qualifiers. There is some information at this question.