C++ Variable Scope

Variable scope represents the scope of a variable, that is, where they can be used in a C++ program. Basically there can be two scope of a variable in a C++ program. The first one is local variables and the second one is global variables. Let's take a look.

C++ Local Variables

Local variables are those variables which are declared inside a function/block. Here is an example of local variables in C++, demonstrating the variable scope in C++:

/* C++ Variable Scope - Local Variable */

#include<iostream.h>
#include<conio.h>
void main()
{
   clrscr();
   int a = 100, b = 200, c;
   c = a + b;
   cout<<"a(100) + b(200) = c("<<c<<")";
   getch();
}

Here is the sample output of the above C++ program:

Let's take another program also demonstrating variable scope in C++:

/* C++ Variable Scope - Local Variable */

#include<iostream.h>
#include<conio.h>
void main()
{
   clrscr();
   int num1, num2;
   cout<<"Enter any two number: ";
   cin>>num1>>num2;
   cout<<"\nThe two numbers are:\n";
   cout<<num1<<"\t"<<num2;
   if(num1<100 || num2<100)
   {
      int localvar1, localvar2;   // local variables declared
      localvar1=num1;
      localvar2=num2;
      cout<<"\n\n"<<localvar1<<"\t"<<localvar2;
   }
   getch();
}

Here is the sample run of this C++ program:

If you try to use localvar1 and localvar2, out of the if block (where they are declared locally) like:

/* C++ Variable Scope - Wrong use of Local Variables */

#include<iostream.h>
#include<conio.h>
void main()
{
   clrscr();
   int num1, num2;
   cout<<"Enter any two number: ";
   cin>>num1>>num2;
   cout<<"\nThe two numbers are:\n";
   cout<<num1<<"\t"<<num2;
   if(num1<100 || num2<100)
   {
      int localvar1, localvar2;   // local variables declared
      localvar1=num1;
      localvar2=num2;
   }
   cout<<"\n\n"<<localvar1<<"\t"<<localvar2;   // error occurred..!!
   getch();
}

Then, four error message will be displayed as shown in this figure:

As you can see from the above C++ program, local variables are destroyed upon exit of the scope where they are declared. So they becomes unknown outside their block. Now let's discuss about global variables in C++

Global Variables

Global variables are those variables which are defined outside of all the functions/block. Here is an example using global variables, helps you in understanding variable scope in C++:

/* C++ Variable Scope - Global Variable */

#include<iostream.h>
#include<conio.h>

int gvar1, gvar2;

void main()
{
   clrscr();
   int lvar1, lvar2;

   gvar1 = 1;
   gvar2 = 2;
   lvar1 = 3;
   lvar2 = 4;

   cout<<"Global Variables:\n";
   cout<<"gvar1 = "<<gvar1<<"\n";
   cout<<"gvar2 = "<<gvar2<<"\n";
   cout<<"\nLocal Variables:\n";
   cout<<"lvar1 = "<<lvar1<<"\n";
   cout<<"lvar2 = "<<lvar2<<"\n";

   getch();
}

Here is the sample output of this C++ program:

Here is another C++ program, also uses local and global variables both:

/* C++ Variable Scope - Global Variable */

#include<iostream.h>
#include<conio.h>

int gvar1;

void main()
{
   clrscr();
   int lvar1, lvar2;
   lvar1 = 100;
   lvar2 = 200;
   gvar1 = lvar1 + lvar2;
   cout<<"lvar1(100) + lvar2(200) = gvar1("<<gvar1<<")";
   getch();
}

Here is the sample output of this C++ program:

You can also declare local and global variable with same name. But remember, the local variable will take higher preference. More local variable, more preference. Here is an example:

/* C++ Variable Scope - Global Variable */

#include<iostream.h>
#include<conio.h>

int var = 200;

void main()
{
   clrscr();
   
   /* local variable is going to declare
    * with same name as global variable
    * Local variables always having high
    * priority than global variables.
    * Therefore the value (100) initialized here
    * will be printed, not 200.
    */

    int var = 100;
    cout<<var;
    getch();
}

Below is the sample output of the above C++ program:

C++ Online Test

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