How to declare functions with example

Functions either return a value or they return void, meaning they return nothing. A function that adds two integers might return the sum, and thus would be defined to return an integer value. A function that just prints a message has nothing to return and would be declared to return void.

Functions consist of a header and a body. The header consists, in turn, of the return type, the function name, and the parameters to that function. The parameters to a function allow values to be passed into the function. Thus, if the function were to add two numbers, the numbers would be the parameters to the function. Here's a typical function header:
int Sum(int a, int b)
A parameter is a declaration of what type of value will be passed in; the actual value passed in by the calling function is called the argument. Many programmers use these two terms, parameters and arguments, as synonyms. Others are careful about the technical distinction. This book will use the terms interchangeably.

The body of a function consists of an opening brace, zero or more statements, and a closing brace. The statements constitute the work of the function. A function may return a value, using a return statement. This statement will also cause the function to exit. If you don't put a return statement into your function, it will automatically return void at the end of the function. The value returned must be of the type declared in the function header.

NOTE: Functions are covered in more detail on Day 5, "Functions." The types that can be returned from a function are covered in more det+[radical][Delta][infinity]on Day 3. The information provided today is to present you with an overview, because functions will be used in almost all of your C++ programs.

Listing 2.5 demonstrates a function that takes two integer parameters and returns an integer value. Don't worry about the syntax or the specifics of how to work with integer values (for example, int x) for now; that is covered in detail on Day 3.

Listing 2.5. FUNC.CPP demonstrates a simple function.
1:    #include <iostream.h>
2:    int Add (int x, int y)
3:    {
5:      cout << "In Add(), received " << x << " and " 
        << y << "\n";
6:      return (x+y);
7:    }
9:    int main()
10:    {
11:         cout << "I'm in main()!\n";
12:         int a, b, c;
13:         cout << "Enter two numbers: ";
14:         cin >> a;
15:         cin >> b;
16:         cout << "\nCalling Add()\n";
17:         c=Add(a,b);
18:         cout << "\nBack in main().\n";
19:         cout << "c was set to " << c;
20:         cout << "\nExiting...\n\n";
21:         return 0;
22: }

I'm in main()!
Enter two numbers: 3 5

Calling Add()
In Add(), received 3 and 5

Back in main().
c was set to 8

The function Add() is defined on line 2. It takes two integer parameters and returns an integer value. The program itself begins on line 9 and on line 11, where it prints a message. The program prompts the user for two numbers (lines 13 to 15). The user types each number, separated by a space, and then presses the Enter key. main() passes the two numbers typed in by the user as arguments to the Add() function on line 17.
Processing branches to the Add() function, which starts on line 2. The parameters a and b are printed and then added together. The result is returned on line 6, and the function returns.
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