Programming Tutorials

Demonstrating global and local variables in C++

By: Priya in C++ Tutorials on 2007-09-04  

Variables defined outside of any function have global scope and thus are available from any function in the program, including main().

Local variables with the same name as global variables do not change the global variables. A local variable with the same name as a global variable hides the global variable, however. If a function has a variable with the same name as a global variable, the name refers to the local variable--not the global--when used within the function. Program below illustrates these points.

Demonstrating global and local variables.

1:   #include <iostream.h>
2:   void myFunction();           // prototype
3:
4:   int x = 5, y = 7;            // global variables
5:   int main()
6:   {
7:
8:        cout << "x from main: " << x << "\n";
9:        cout << "y from main: " << y << "\n\n";
10:       myFunction();
11:       cout << "Back from myFunction!\n\n";
12:       cout << "x from main: " << x << "\n";
13:       cout << "y from main: " << y << "\n";
14:       return 0;
15:  }
16:
17:  void myFunction()
18:  {
19:       int y = 10;
20:
21:       cout << "x from myFunction: " << x << "\n";
22:       cout << "y from myFunction: " << y << "\n\n";
23: }

Output: x from main: 5
y from main: 7

x from myFunction: 5
y from myFunction: 10

Back from myFunction!

x from main: 5
y from main: 7

Analysis: This simple program illustrates a few key, and potentially confusing, points about local and global variables. On line 1, two global variables, x and y, are declared. The global variable x is initialized with the value 5, and the global variable y is initialized with the value 7.
On lines 8 and 9 in the function main(), these values are printed to the screen. Note that the function main() defines neither variable; because they are global, they are already available to main().

When myFunction() is called on line 10, program execution passes to line 18, and a local variable, y, is defined and initialized with the value 10. On line 21, myFunction() prints the value of the variable x, and the global variable x is used, just as it was in main(). On line 22, however, when the variable name y is used, the local variable y is used, hiding the global variable with the same name.

The function call ends, and control returns to main(), which again prints the values in the global variables. Note that the global variable y was totally unaffected by the value assigned to myFunction()'s local y variable.