C++ Programming Tutorial for Beginners: Learn in 2 Hours

A scope is a span of a program where a variable has a meaning. Mostly the same name can be used to refer to different entities within different scopes. Variables are visible from the point where they are declared until the end of the scope in which their declaration appears.

#include <iostream>	
int main()	
{	
    int sum = 0;	
    // sum values from 1 through 10 inclusive	
    for (int val = 1; val <= 10; ++val)	
        sum += val;  // equivalent to sum = sum + val	
    cout << "Sum of 1 to 10 inclusive is "<< sum <<endl;	
    return 0;	
}	 

This program defines 3 names, viz, main, sum, and val. It uses the namespace name std, along with two other names from that namespace—cout and endl.

• The function name “main” is defined outside the curly braces. The function name main—like most other names defined outside a function—has a global scope. Which means that once declared, names which are at the global scope are accessible throughout the program.
• The variable sum is defined within the scope of the block which is the body of the main function. It can be accessed from its point of declaration and throughout the rest of the main function’s body. However,not outside of it. This means that variable sum has block scope.
• The variable val is defined in the scope of “for statement”. It can easily be used in that statement but not elsewhere in the main function. It has local scope.

Nested Scope

The scope can contain other scopes. The contained (or nested) scope is referred to as an inner scope. The containing scope is the outer scope.

#include <iostream>	
using namespace std;	
// Program for illustration purposes only: It is bad style for a function	
// to use a global variable and also define a local variable with the same name	
int reused = 42;  // reused has global scope	
int main()	
{	
    int unique = 0; // unique has block scope	
    // output #1: uses global reused; prints 42 0	
    cout << reused << " " << unique << endl;	
    int reused = 0; // new, local object named reused hides global reused	
    // output #2: uses local reused; prints 0 0	
    cout << reused << " " << unique << endl;	
    // output #3: explicitly requests the global reused; prints 42 0	
    cout << ::reused << " " << unique << endl;	
    return 0;	
}	 

Output #1 appears before the local definition of reused. Thus, this output

statement is the one which uses the name reused that is defined in global scope. This statement outputs

42 0

Output #2 occurs after the local definition of reused. It is now in scope. Therefore, this second output statement simply uses the local object named reused rather than global one and outputs

0 0

Output #3 overrides the default scoping rules using the scope operator. The global scope has no name. Thus, when the scope operator(::) has an empty left-hand side. It interprets it as a request to fetch the name on the right-hand side of the global scope. Thus, the expression uses the global reused and outputs

42 0