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 types 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 types of variables.
Variable Definition in C++:
A variable definition means to tell the compiler where and how much to create the storage for the variable. A variable definition specifies a data 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;
The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int.
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:
extern int d = 3, f = 5; // declaration of d and f. int d = 3, f = 5; // definition and initializing d and f. byte z = 22; // definition and initializes z. char x = 'x'; // the variable x has the value 'x'.
For definition 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.
Variable Declaration in C++:
A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable declaration at the time of linking of the program.
A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your C++ program, but it can be defined only once in a file, a function or a block of code.
Example
Try the following example where a variable has been declared at the top, but it has been defined inside the main function:
#include <iostream> using namespace std; // Variable declaration: extern int a, b; extern int c; extern float f; int main () { // Variable definition: 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 the following result:
30 23.3333
Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. For example:
// function declaration int func(); int main() { // function call int i = func(); } // function definition int func() { return 0; }
Lvalues and Rvalues:
There are two kinds of expressions in C++:
- lvalue : Expressions that refer to a memory location is called "lvalue" expression. An lvalue may appear as either the left-hand or right-hand side of an assignment.
- rvalue : The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means 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;
A scope is a region of the program and broadly speaking there are three places, where variables can be declared:
-
Inside a function or a block which is called local variables,
-
In the definition of function parameters which is called formal parameters.
-
Outside of all functions which is called global variables.
We will learn what is a function and it's parameter in subsequent chapters. Here let us explain what are local and global variables.
Local Variables:
Variables that are declared inside a function or block are local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. Following is the example using local variables:
#include <iostream>
using namespace std;
int main ()
{
// Local variable declaration:
int a, b;
int c;
// actual initialization
a = 10;
b = 20;
c = a + b;
cout << c;
return 0;
}
Global Variables:
Global variables are defined outside of all the functions, usually on top of the program. The global variables will hold their value throughout the life-time of your program.
A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration. Following is the example using global and local variables:
#include <iostream>
using namespace std;
// Global variable declaration:
int g;
int main ()
{
// Local variable declaration:
int a, b;
// actual initialization
a = 10;
b = 20;
g = a + b;
cout << g;
return 0;
}
A program can have same name for local and global variables but value of local variable inside a function will take preference. For example:
#include <iostream>
using namespace std;
// Global variable declaration:
int g = 20;
int main ()
{
// Local variable declaration:
int g = 10;
cout << g;
return 0;
}
When the above code is compiled and executed, it produces the following result:
10
Initializing Local and Global Variables:
When a local variable is defined, it is not initialized by the system, you must initialize it yourself. Global variables are initialized automatically by the system when you define them as follows:
Data Type Initializer int 0 char '\0' float 0 double 0 pointer NULL
It is a good programming practice to initialize variables properly, otherwise sometimes program would produce unexpected result.
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