C++ Variable Types

A variable provides us with named storage that our programs can manipulate. Each variable in C++ has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C++ is case-sensitive:

There are following basic types of variable in C++ as explained in last chapter:

Type	Description
bool	Stores either value true or false.
char	Typically a single octet(one byte). This is an integer type.
int	The most natural size of integer for the machine.
float	A single-precision floating point value.
double	A double-precision floating point value.
void	Represents the absence of type.
wchar_t	A wide character type.

C++ also allows to define various other type of variables which we will cover in subsequent chapters like Enumeration, Pointer, Array, Reference, Data structures, and Classes.

Following section will cover how to define, declare and use various type of variables.

Variable Declaration in C++:

All variables must be declared before use, although certain declarations can be made implicitly by content. A declaration specifies a type, and contains a list of one or more variables of that type as follows:

type variable_list;

Here, type must be a valid C++ data type including char, w_char, int, float, double, bool or any user defined object etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here:

int    i, j, k;
char   c, ch;
float  f, salary;
double d;

A variable declaration with an initializer is always a definition. This means that storage is allocated for the variable and could be declared as follows:

int    i = 100;

An extern declaration is not a definition and does not allocate storage. In effect, it claims that a definition of the variable exists elsewhere in the program. A variable can be declared multiple times in a program, but it must be defined only once. Following is the declaration of a variable with extern keyword:

extern int    i;

Though you can declare a variable multiple times in your C++ program but it can be decalred only once in a file, a function or a block of code.

Variable Initialization in C++:

Variables are initialized (assigned an value) with an equal sign followed by a constant expression. The general form of initialization is:

variable_name = value;

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

type variable_name = value;

Some examples are:

int d = 3, f = 5;    // initializing d and f.
byte z = 22;         // initializes z.
double pi = 3.14159; // declares an approximation of pi.
char x = 'x';        // the variable x has the value 'x'.

For declarations without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.

It is a good programming practice to initialize variables properly otherwise, sometime program would produce unexpected result. Try following example which makes use of various types of variables:

#include <iostream>
using namespace std;
 int main ()
{
  // Variable declaration:
  int a, b;
  int c;
  float f;
 
  // actual initialization
  a = 10;
  b = 20;
  c = a + b;
 
  cout << c << endl ;

  f = 70.0/3.0;
  cout << f << endl ;
 
  return 0;
}

When the above code is compiled and executed, it produces following result:

30
23.3333

Lvalues and Rvalues:

There are two kinds of expressions in C++:

• lvalue : An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.
• rvalue : An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement:

int g = 20;

But following is not a valid statement and would generate compile-time error:

10 = 20;