2 - FLTK Basics

This chapter will teach you the basics of compiling programs that use FLTK.


All public symbols in FLTK start with the characters 'F' and 'L':

Header Files

The proper way to include FLTK header files is: Microsoft Windows developers please note: case *is* significant under other operating systems, and the C standard uses the forward slash (/) to separate directories. Do not do any of the following:

Compiling Programs with Standard Compilers

Under UNIX (and under Microsoft Windows when using the GNU development tools) you will probably need to tell the compiler where to find the header files. This is usually done using the -I option: Similarly, when linking your application you will need to tell the compiler to use the FLTK library:

Compiling Programs with Microsoft Visual C++

In Visual C++ you will need to tell the compiler where to find the FLTK header files. This can be done by selecting "Settings" from the "Project" menu and then changing the "Preprocessor" settings under the "C/C++" tab. You will also need to add the FLTK and WinSock (WSOCK32.LIB) libraries to the "Link" settings.

You can build your Microsoft Windows applications as Console or WIN32 applications. If you want to use the standard C main() function as the entry point, FLTK includes a WinMain() function that will call your main() function for you.

Note: The Visual C++ 5.0 optimizer is known to cause problems with many programs. We only recommend using the "Favor Small Code" optimization setting. The Visual C++ 6.0 optimizer seems to be much better and can be used with the "optimized for speed" setting.

Writing Your First FLTK Program

All programs must include the file <FL/Fl.H>. In addition the program must include a header file for each FLTK class it uses. Listing 1 shows a simple "Hello, World!" program that uses FLTK to display the window. After including the required header files, the program then creates a window: and a box with the "Hello, World!" string in it: Next, we set the type of box and the size, font, and style of the label: Finally, we show the window and enter the FLTK event loop: The resulting program will display the window below. You can quit the program by closing the window or pressing the ESCape key.

Hello, World! Window

Creating the Widgets

The widgets are created using the C++ new operator. For most widgets the arguments to the constructor are:

The x and y parameters determine where the widget or window is placed on the screen. In FLTK the top left corner of the window or screen is the origin (i.e. x = 0, y = 0) and the units are in pixels.

The width and height parameters determine the size of the widget or window in pixels. The maximum widget size is typically governed by the underlying window system or hardware.

label is a pointer to a character string to label the widget with or NULL. If not specified the label defaults to NULL. The label string must be in static storage such as a string constant because FLTK does not make a copy of it - it just uses the pointer.

Get/Set Methods

box->box(FL_UP_BOX) sets the type of box the Fl_Box draws, changing it from the default of FL_NO_BOX, which means that no box is drawn. In our "Hello, World!" example we use FL_UP_BOX, which means that a raised button border will be drawn around the widget. You can learn more about boxtypes in Chapter 3.

You could examine the boxtype in by doing box->box(). Fltk uses method name overloading to make short names for get/set methods. A "set" method is always of the form "void name(type)", and a "get" method is always of the form "type name() const".

Redrawing After Changing Attributes

Almost all of the set/get pairs are very fast, short inline functions and thus very efficient. However, the "set" methods do not call redraw() - you have to call it yourself. This greatly reduces code size and execution time. The only common exception is value() which calls redraw() if necessary.


All widgets support labels. In the case of window widgets, the label is used for the label in the title bar. Our example program calls the labelfont, labelsize, and labeltype methods.

The labelfont method sets the typeface and style that is used for the label, which for this example we are using FL_BOLD and FL_ITALIC. You can also specify typefaces directly.

The labelsize method sets the height of the font in pixels.

The labeltype method sets the type of label. FLTK supports normal, embossed, shadowed, symbol, and image labels internally, and more types can be added as desired.

A complete list of all label options can be found in Chapter 3.

Showing the Window

The show() method shows the widget or window. For windows you can also provide the command-line arguments to allow users to customize the appearance, size, and position of your windows.

The Main Event Loop

FLTK provides the Fl:run() method to enter a standard event processing loop. This is equivalent to the following code: Fl::run() does not return until all of the windows under FLTK control are closed by the user or your program.