C++ introduces the reference operator, &, which works slightly different than it does in C.
double a = 1;
double b = &a; // b is now a pointer to a
b = 2;
// a now = 2
For those who come from a C background and understand pointers, treat it as though double &b = a goes to double *b = &a and all the subsequent usage of b is replaced with *b. Note that the double that b points to cannot be changed! This is unlike a normal pointer, double *b = &a, in where you could later write b = &c. This will not work with a reference.
What Are They Good For?
References can simplify the syntax of certain pointer manipulation. They also useful when creating operator overloads, (e.g. the ++ operator for an enumeration), so that the syntax remains consistent with the base types in C++.
Undefined behaviour occurs if you ever return a local variable by reference, even if it’s an rvalue reference.