C++ Reference Declarations

C++ allows reference declarations^[1] so you can create aliases for variables. These declarations are typically of the form:

type& identifier = variable

int  n;
int&  nn = n;

double  a[10];
double& last = a[9];

The name nn is an alias for n. They refer to the same place in memory, and modifying the value of nn is equivalent to modifying n and vice versa.
The name last is an alternative way of referring to the single array element a[9].
An alias, once declared, cannot be changed. In other words, if you declared last as an alias for the array element a[9], you could not also declare last as an alias for the array element a[1].
Reference declarations that are definitions must be initialized, usually as simple variables. The initializer is the lvalue expression, which gives the variable's location in memory.
When a variable i is declared, it has an address and memory location associated with it. When a reference variable r is declared and initialized to i, it is identical to i. It does not have a separate identity from the i.

[1]: Reference declarations declare the identifier to be an alternative name or alias for a variable specified in an initialization of the reference.