-2- Unless described below, all operations have the same requirements and semantics as the
primary vector template, except that operations dealing with the bool value type
map to bit values in the container's storage.
-3- There is no requirement that the data is stored as a contiguous allocation of
bool values. A space-optimized representation of bits is recommended instead.
-4- reference is a class that simulates the behavior of references of a single
bit in vector<bool>. The conversion operator returns true
when the bit is set, and false otherwise. The assignment operator sets the bit
when the argument is (convertible to) true and clears it otherwise.
flip reverses the state of the bit.
void flip()
-5- Replaces each element in the container with its complement. It is unspecified if
it has any effect on allocated but unused bits.