variable in C++

In C++, all the variables must be declared before use.
A variable name can consist of alphabets (both upper and lower case), numbers and the underscore ‘_’ character. However, the name must not start with a number.
The variable declaration refers to the part where a variable is first declared or introduced before its first use. A variable definition is a part where the variable is assigned a memory location and a value. Most of the time, variable declaration and definition are done together.
```
#include <iostream>
using namespace std;
 
int main()
{
    // this is declaration of variable a
    int a;
    // this is initialisation of a
    a = 10;
    // this is definition = declaration + initialisation
    int b = 20;
   
    return 0;
}
```
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