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A C++ identifier is a name consisting of letters and
digits, the first of which must be a letter, with underscore,
_, counting as a letter.
A variable is an identifier that serves as the name of a chunk of computer memory. Thus, each variable refers to a chunk of memory.
The variable's data type determines the size of the chunk and the
way the bits in the chunk are interpreted. If the variable belongs to the
int data type, a kind of integer, the chunk of memory involved
is likely to contain 32 bits, one of which determines the integer's sign
and 31 of which determine the integer's absolute value.
The chunk of memory named by a variable is said to hold that
variable's
value. As a program runs, a variable's value may change,
but a variable's data type never changes. Thus, the value of an
integer variable named length could be the integer
40 at one time and the integer 41 at another, but
length's value could never be a floating-point number,
such as 40.5.