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