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;

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.

Example
#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

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;