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While
main() is a function, it is an unusual one. Typical
functions are called, or invoked, during the course
of your program. A program is executed line by line
in the order it appears in your source code, until
a function is reached. Then the program branches
off to execute the function. When the function finishes,
it returns control to the line of code immediately
following the call to the function.
A
good analogy for this is sharpening your pencil.
If you are drawing a picture, and your pencil breaks,
you might stop drawing, go sharpen the pencil, and
then return to what you were doing. When a program
needs a service performed, it can call a function
to perform the service and then pick up where it
left off when the function is finished running.
Listing 2.4 demonstrates this idea.
Listing
2.4. Demonstrating a call to a function.
1:
#include <iostream.h>
2:
3:
// function Demonstration Function
4:
// prints out a useful message
5:
void DemonstrationFunction()
6:
{
7:
cout << "In Demonstration Function\n";
8:
}
9:
10:
// function main - prints out a message, then
11:
// calls DemonstrationFunction, then prints out
12:
// a second message.
13:
int main()
14:
{
15:
cout << "In main\n" ;
16:
DemonstrationFunction();
17:
cout << "Back in main\n";
18:
return 0;
19:
}
OUTPUT:
In
main
In
Demonstration Function
Back
in main
The
function DemonstrationFunction() is defined on lines
5-7. When it is called, it prints a message to the
screen and then returns.
Line
13 is the beginning of the actual program. On line
15, main() prints out a message saying it is in
main(). After printing the message, line 16 calls
DemonstrationFunction(). This call causes the commands
in DemonstrationFunction() to execute. In this case,
the entire function consists of the code on line
7, which prints another message. When DemonstrationFunction()
completes (line 8), it returns back to where it
was called from. In this case the program returns
to line 17, where main() prints its final line.
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