C++ – Understanding Rvalue References for *this

c++c++11rvalue-reference

What are the most typical use cases of "rvalue references for *this" which the standard also calls reference qualifiers for member functions?

By the way, there is a really good explanation about this language feature here.

Best Answer

When called, each member function has an implicit object parameter that *this references.

So (a) these normal function overloads:

void f(const T&);
void f(T&&);

when called like f(x); and (b) these member function overloads:

struct C
{
    void f() const &;
    void f() &&;
};

when called like x.f() - both (a) and (b) dispatch with similar viability and ranking.

So the use cases are essentially the same. They are to support move semantic optimization. In the rvalue member function you can essentially pillage the objects resources because you know that it is an expiring object (is about to be deleted):

int main()
{
    C c;
    c.f(); // lvalue, so calls lvalue-reference member f
    C().f(); // temporary is prvalue, so called rvalue-reference member f
    move(c).f(); // move changes c to xvalue, so again calls rvalue-reference member f
}

So for example:

struct C
{
    C operator+(const C& that) const &
    {
        C c(*this); // take a copy of this
        c += that;
        return c;
    }

    C operator+(const C& that) &&
    {
        (*this) += that;
        return move(*this); // moving this is ok here
    }
}
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