Why this two rvalue references examples have different behavior?

Question

First example

int a = 0;
auto && b = ++a;
++a;
cout << a << b << endl;

prints 22

Second example

int a = 0;
auto && b = a++;
++a;
cout << a << b << endl;

prints 20

Question: Why in first example ++a in 3rd line also increments b, and why there is no such behavior in second example?

Update: New question arised.

Answer 1

Because pre-increment (++a) first increments the value of a, stores the result, and then returns the reference to a. Now a and b effectively point to the same object.

Post-increment (a++), however, first stores the current value of a in a temporary, increments a, and returns this temporary - to which your rvalue ref points. a and b point to different objects, more specifically - b is a temporary holding the value of a prior to incrementing.

This is the reason why it's encouraged to use ++it over it++ for iterators and other complex objects that define increment / decrement: the latter creates a temporary copy and thus may be slower.