Using external files for your functions
Create the project Create the function header file Create the functions source file Create the main source code file Add the code for main.cpp Add the code for functions.cpp Add the function header to functions.h Build and test the program Going further (adding more functions)
This tutorial will show you how to create a project that uses an external file to hold commonly used functions. This allows you to keep your core source code file (the one that contains the main function) as minimal as possible and to sort functions that do similar things into their own files.
1 : Create the project
Create a new empty project in the development environment of your choice. For the examples shown here I will be using Visual Studio.
2 : Create the function header file
First, you need to create the external functions' header file. This will be the file that the function headers will be placed in. This is the file that will be included in your main source code file.
Right-click on the Header files section of the Solution Explorer and select 
then 
.
The Add New Item window will open and display all of the possible resources that can be added. Make sure that Code is selected in the Categories list on the left and then Header File (.h) in the panel on the right.
Call the file functions.h and click on the Add button.
3 : Create the functions source file
You now need to create the source code file for the functions. This will be the file that the source code for your function bodies will be written in. Its name must match that of the header file so that when the header file is included in a souce code file, the function body code can be found in this file.
Right-click on 
in the Solution Explorer and select then to display the Add New Item window.
Again, make sure that Code is selected in the list on the left.
In the panel on the right, select C++ File (.cpp).
For the name you must use the same name as the header file but with a ".cpp" extension instead of ".h".
I called the header file, "functions.h" so I must call the source file, "functions.cpp".
Click the Add button to add the new source code file to the project.
4 : Create the main source code file
Follow the steps for step 3 above except this time call the file main.cpp.
Main.cpp should now be open for editing in the code editor. You can confirm this by looking at the tabs along the top of the editor. The source code file in bold is the one that you are currently editing.
If main.cpp is not currently shown in bold on the tabs, click on the main.cpp tab. You're going to need to put code in the main source code file in the next step.
5 : Add the code for main.cpp.
In the code editor, enter the following code for main.cpp. The comments in the code explain what each line of code does.
main.cpp
// Include the input/output stream library
#include <iostream>
// Include my own functions library
#include "functions.h"
// Use the standard namespace
using namespace std;
// Start the main function
int main(void){
// Using the power function from my functions library,
// display 2 to the power of 8 (which should be 256)
cout << "2 to the power of 8 is " << power(2,8) << endl;
// Wait for the user to press a key before ending
system("PAUSE");
// Return a success value from the main function
return EXIT_SUCCESS;
}
6 : Add the code for functions.cpp
On the tabs at the top of the code editor, select functions.cpp.
In the code editor, enter the following code:
functions.cpp
// Function to calculate an exponential number
// E.g. 2 to the power of 6 = 2x2x2x2x2x2 = 64
int power(int base, int power){
// Create an integer variable called total, initialised to 1
int total = 1;
// Use a for loop to loop through the powers,
// multiplying the running total by the base with each iteration
for (int loop = 1; loop <= power; loop++){
total = total * base;
}
// Return the contents of total
return total;
}
7 : Add the function header to functions.h
Right now, main.cpp has no way of knowing that the power function even exists in functions.cpp. To link functions.cpp to main.cpp you must add the function header for power to functions.h.
On the tabs at the top of the code editor, select functions.h.
In the code editor enter the following code:
functions.h
// Header for the power function
int power(int base, int power);
8 : Build and test the program
That's all the code done. All you need to do now is compile and run the program.
If you're using Visual Studio, press F5 to compile and run your program.
If you're using Dev-C++ press F9 to compile and run your program.
9 : Going further (using more functions)
It's very doubtful that your future programs will only ever use one function but expanding beyond a single function is easy - just remember to put your other function bodies in functions.cpp and the function header for each function in functions.h.
The function header is basically the first line of the function but instead of an opening curly brace, it ends in a semi-colon.
For example:
| First line of my function: | int power(int base, int power){ |
| Function header: | int power(int base, int power); |