Declaration of variables

In C++ (as in many other programming languages) all the variables that a program is going to use must be declared prior to use. Declaration of a variable serves two purposes:

• It associates a type and an identifier (or name) with the variable. The type allows the compiler to interpret statements correctly. For example in the CPU the instruction to add two integer values together is different from the instruction to add two floating-point values together. Hence the compiler must know the type of the variables so it can generate the correct add instruction.
• It allows the compiler to decide how much storage space to allocate for storage of the value associated with the identifier and to assign an address for each variable which can be used in code generation.

For the moment only four variable types are considered, namely, int, float, bool and char. These types hold values as follows:

int

variables can represent negative and positive integer values (whole numbers). There is a limit on the size of value that can be represented, which depends on the number of bytes of storage allocated to an int variable by the computer system and compiler being used. On a PC most compilers allocate two bytes for each int which gives a range of -32768 to +32767. On workstations, four bytes are usually allocated, giving a range of -2147483648 to 2147483647. It is important to note that integers are represented exactly in computer memory.

float

variables can represent any real numeric value, that is both whole numbers and numbers that require digits after the decimal point. The accuracy and the range of numbers represented is dependent on the computer system. Usually four bytes are allocated for float variables, this gives an accuracy of about six significant figures and a range of about $-10 ^ {38}$ to $+10 ^ {38}$. It is important to note that float values are only represented approximately.

bool

variables can only hold the values true or false. These variables are known as boolean variables in honour of George Boole, an Irish mathematician who invented boolean algebra.

char

variables represent a single character -- a letter, a digit or a punctuation character. They usually occupy one byte, giving 256 different possible characters. The bit patterns for characters usually conform to the American Standard Code for Information Interchange (ASCII).

Examples of values for such variables are:

int    123      -56    0      5645

float  16.315  -0.67   31.567

char   '+'     'A'     'a'    '*'    '7'

A typical set of variable declarations that might appear at the beginning of a program could be as follows:

int i, j, count;
float sum, product;
char ch;
bool passed_exam;

which declares integer variables i, j and count, real variables sum and product, a character variable ch, and a boolean variable pass_exam.
A variable declaration has the form:

type identifier-list;

type specifies the type of the variables being declared. The identifier-list is a list of the identifiers of the variables being declared, separated by commas.

Variables may be initialised at the time of declaration by assigning a value to them as in the following example:

int i, j, count = 0;
float sum = 0.0, product;
char ch = '7';
bool passed_exam = false;

which assigns the value 0 to the integer variable count and the value 0.0 to the real variable sum. The character variable ch is initialised with the character 7. i, j, and product have no initial value specified, so the program should make no assumption about their contents.