fltk  1.3.8
About: FLTK (Fast Light Tool Kit) is a cross-platform C++ GUI toolkit for UNIX/Linux (X11), Microsoft Windows, and MacOS X.
  Fossies Dox: fltk-1.3.8-source.tar.bz2  ("unofficial" and yet experimental doxygen-generated source code documentation)  

Common Widgets and Attributes

This chapter describes many of the widgets that are provided with FLTK and covers how to query and set the standard attributes.


FLTK provides many types of buttons:

Figure 3-1: FLTK Button Widgets

All of these buttons just need the corresponding <FL/Fl_xyz_Button.H> header file. The constructor takes the bounding box of the button and optionally a label string:

Fl_Button *button = new Fl_Button(x, y, width, height, "label");
Fl_Light_Button *lbutton = new Fl_Light_Button(x, y, width, height);
Fl_Round_Button *rbutton = new Fl_Round_Button(x, y, width, height, "label");
Buttons generate callbacks when they are clicked by the user.
Definition: Fl_Button.H:76
This subclass displays the "on" state by turning on a light, rather than drawing pushed in.
Buttons generate callbacks when they are clicked by the user.
static Fl_Button * button[3]
Definition: fl_ask.cxx:44

Each button has an associated type() which allows it to behave as a push button, toggle button, or radio button:

value() toggles between 0 and 1 at every click of the button
Definition: Fl_Button.H:32
is set to 1 at button press, and all other buttons in the same group with type() == FL_RADIO_BUTTON a...
Definition: Fl_Button.H:33
value() will be set to 1 during the press of the button and reverts back to 0 when the button is rele...
Definition: Fl_Button.H:30
uchar type() const
Gets the widget type.
Definition: Fl_Widget.H:274

For toggle and radio buttons, the value() method returns the current button state (0 = off, 1 = on). The set() and clear() methods can be used on toggle buttons to turn a toggle button on or off, respectively. Radio buttons can be turned on with the setonly() method; this will also turn off other radio buttons in the same group.


FLTK provides several text widgets for displaying and receiving text:

  • Fl_Input - A one-line text input field.

The Fl_Output and Fl_Multiline_Output widgets allow the user to copy text from the output field but not change it.

The value() method is used to get or set the string that is displayed:

Fl_Input *input = new Fl_Input(x, y, width, height, "label");
input->value("Now is the time for all good men...");
int value(const char *)
Changes the widget text.
Definition: Fl_Input_.cxx:1284
This is the FLTK text input widget.
Definition: Fl_Input.H:222
static Fl_Input * input
Definition: fl_ask.cxx:45

The string is copied to the widget's own storage when you set the value() of the widget.

The Fl_Text_Display and Fl_Text_Editor widgets use an associated Fl_Text_Buffer class for the value, instead of a simple string.


Unlike text widgets, valuators keep track of numbers instead of strings. FLTK provides the following valuators:

  • Fl_Counter - A widget with arrow buttons that shows the current value.
Figure 3-2: FLTK valuator widgets

The value() method gets and sets the current value of the widget. The minimum() and maximum() methods set the range of values that are reported by the widget.


The Fl_Group widget class is used as a general purpose "container" widget. Besides grouping radio buttons, the groups are used to encapsulate windows, tabs, and scrolled windows. The following group classes are available with FLTK:

  • Fl_Group - The base container class; can be used to group any widgets together.
  • Fl_Pack - A collection of widgets that are packed into the group area.
  • Fl_Tabs - Displays child widgets as tabs.
  • Fl_Wizard - Displays one group of widgets at a time.

Setting the Size and Position of Widgets

The size and position of widgets is usually set when you create them. You can access them with the x(), y(), w(), and h() methods.

You can change the size and position by using the position(), resize(), and size() methods:

group->resize(x, y, width, height);
window->size(width, height);
static Fl_TooltipBox * window
Definition: Fl_Tooltip.cxx:75
void position(int X, int Y)
Repositions the window or widget.
Definition: Fl_Widget.H:332
void size(int W, int H)
Changes the size of the widget.
Definition: Fl_Widget.H:341

If you change a widget's size or position after it is displayed you will have to call redraw() on the widget's parent.


FLTK stores the colors of widgets as an 32-bit unsigned number that is either an index into a color palette of 256 colors or a 24-bit RGB color. The color palette is not the X or MS Windows colormap, but instead is an internal table with fixed contents.

See the Colors section of Drawing Things in FLTK for implementation details.

There are symbols for naming some of the more common colors:

  • FL_RED

Other symbols are used as the default colors for all FLTK widgets.


The full list of named color values can be found in FLTK Enumerations.

A color value can be created from its RGB components by using the fl_rgb_color() function, and decomposed again with Fl::get_color():

Fl_Color c = fl_rgb_color(85, 170, 255); // RGB to Fl_Color
Fl::get_color(c, r, g, b); // Fl_Color to RGB
Fl_Color fl_rgb_color(uchar r, uchar g, uchar b)
Returns the 24-bit color value closest to r, g, b.
Definition: Enumerations.H:995
unsigned int Fl_Color
An FLTK color value; see also Colors
Definition: Enumerations.H:932
static unsigned get_color(Fl_Color i)
Returns the RGB value(s) for the given FLTK color index.
Definition: fl_color.cxx:359
long b
Definition: jpegint.h:417

The widget color is set using the color() method:

button->color(FL_RED); // set color using named value
const Fl_Color FL_RED
Definition: Enumerations.H:955
Fl_Color color() const
Gets the background color of the widget.
Definition: Fl_Widget.H:378

Similarly, the label color is set using the labelcolor() method:

const Fl_Color FL_WHITE
Definition: Enumerations.H:969
Fl_Color labelcolor() const
Gets the label color.
Definition: Fl_Widget.H:472

The Fl_Color encoding maps to a 32-bit unsigned integer representing RGBI, so it is also possible to specify a color using a hex constant as a color map index:

button->color(0x000000ff); // colormap index #255 (FL_WHITE)

or specify a color using a hex constant for the RGB components:

button->color(0xff000000); // RGB: red
button->color(0x00ff0000); // RGB: green
button->color(0x0000ff00); // RGB: blue
button->color(0xffffff00); // RGB: white
If TrueColor is not available, any RGB colors will be set to the nearest entry in the colormap.

Box Types

The type Fl_Boxtype stored and returned in Fl_Widget::box() is an enumeration defined in Enumerations.H.

Figure 3-3 shows the standard box types included with FLTK.

Figure 3-3: FLTK box types

FL_NO_BOX means nothing is drawn at all, so whatever is already on the screen remains. The FL_..._FRAME types only draw their edges, leaving the interior unchanged. The blue color in Figure 3-3 is the area that is not drawn by the frame types.

Making Your Own Boxtypes

You can define your own boxtypes by making a small function that draws the box and adding it to the table of boxtypes.

The Drawing Function

The drawing function is passed the bounding box and background color for the widget:

void xyz_draw(int x, int y, int w, int h, Fl_Color c) {

A simple drawing function might fill a rectangle with the given color and then draw a black outline:

void xyz_draw(int x, int y, int w, int h, Fl_Color c) {
fl_rectf(x, y, w, h);
fl_rect(x, y, w, h);
const Fl_Color FL_BLACK
Definition: Enumerations.H:954
void fl_color(Fl_Color c)
Sets the color for all subsequent drawing operations.
Definition: fl_draw.H:52
void fl_rectf(int x, int y, int w, int h)
Colors with current color a rectangle that exactly fills the given bounding box.
Definition: fl_draw.H:206
void fl_rect(int x, int y, int w, int h)
Draws a 1-pixel border inside the given bounding box.
Definition: fl_draw.H:201

Fl_Boxtype fl_down(Fl_Boxtype b)

fl_down() returns the "pressed" or "down" version of a box. If no "down" version of a given box exists, the behavior of this function is undefined and some random box or frame is returned. See Drawing Functions for more details.

Fl_Boxtype fl_frame(Fl_Boxtype b)

fl_frame() returns the unfilled, frame-only version of a box. If no frame version of a given box exists, the behavior of this function is undefined and some random box or frame is returned. See Drawing Functions for more details.

Fl_Boxtype fl_box(Fl_Boxtype b)

fl_box() returns the filled version of a frame. If no filled version of a given frame exists, the behavior of this function is undefined and some random box or frame is returned. See Drawing Functions for more details.
Adding Your Box Type

The Fl::set_boxtype() method adds or replaces the specified box type:

Fl::set_boxtype(XYZ_BOX, xyz_draw, 1, 1, 2, 2);
static void set_boxtype(Fl_Boxtype, Fl_Box_Draw_F *, uchar, uchar, uchar, uchar)
Sets the function to call to draw a specific boxtype.
Definition: fl_boxtype.cxx:416

The last 4 arguments to Fl::set_boxtype() are the offsets for the x, y, width, and height values that should be subtracted when drawing the label inside the box.

A complete box design contains four box types in this order: a filled, neutral box (UP_BOX), a filled, depressed box (DOWN_BOX), and the same as outlines only (UP_FRAME and DOWN_FRAME). The function fl_down(Fl_Boxtype) expects the neutral design on a boxtype with a numerical value evenly dividable by two. fl_frame(Fl_Boxtype) expects the UP_BOX design at a value dividable by four.

Labels and Label Types

The label(), align(), labelfont(), labelsize(), labeltype(), image(), and deimage() methods control the labeling of widgets.


The label() method sets the string that is displayed for the label. Symbols can be included with the label string by escaping them using the "@" symbol - "@@" displays a single at sign. Figure 3-4 shows the available symbols.

Figure 3-4: FLTK label symbols

The @ sign may also be followed by the following optional "formatting" characters, in this order:

  • '#' forces square scaling, rather than distortion to the widget's shape.
  • +[1-9] or -[1-9] tweaks the scaling a little bigger or smaller.
  • '$' flips the symbol horizontally, '%' flips it vertically.
  • [0-9] - rotates by a multiple of 45 degrees. '5' and '6' do no rotation while the others point in the direction of that key on a numeric keypad. '0', followed by four more digits rotates the symbol by that amount in degrees.

Thus, to show a very large arrow pointing downward you would use the label string "@+92->".


The align() method positions the label. The following constants are defined and may be OR'd together as needed:

  • FL_ALIGN_CENTER - center the label in the widget.
  • FL_ALIGN_TOP - align the label at the top of the widget.
  • FL_ALIGN_BOTTOM - align the label at the bottom of the widget.
  • FL_ALIGN_LEFT - align the label to the left of the widget.
  • FL_ALIGN_RIGHT - align the label to the right of the widget.
  • FL_ALIGN_LEFT_TOP - The label appears to the left of the widget, aligned at the top. Outside labels only.
  • FL_ALIGN_RIGHT_TOP - The label appears to the right of the widget, aligned at the top. Outside labels only.
  • FL_ALIGN_LEFT_BOTTOM - The label appears to the left of the widget, aligned at the bottom. Outside labels only.
  • FL_ALIGN_RIGHT_BOTTOM - The label appears to the right of the widget, aligned at the bottom. Outside labels only.
  • FL_ALIGN_INSIDE - align the label inside the widget.
  • FL_ALIGN_CLIP - clip the label to the widget's bounding box.
  • FL_ALIGN_WRAP - wrap the label text as needed.
  • FL_ALIGN_TEXT_OVER_IMAGE - show the label text over the image.
  • FL_ALIGN_IMAGE_OVER_TEXT - show the label image over the text (default).
  • FL_ALIGN_IMAGE_NEXT_TO_TEXT - The image will appear to the left of the text.
  • FL_ALIGN_TEXT_NEXT_TO_IMAGE - The image will appear to the right of the text.
  • FL_ALIGN_IMAGE_BACKDROP - The image will be used as a background for the widget.


The labeltype() method sets the type of the label. The following standard label types are included:

  • FL_NORMAL_LABEL - draws the text.
  • FL_NO_LABEL - does nothing.
  • FL_SHADOW_LABEL - draws a drop shadow under the text.
  • FL_ENGRAVED_LABEL - draws edges as though the text is engraved.
  • FL_EMBOSSED_LABEL - draws edges as thought the text is raised.
  • FL_ICON_LABEL - draws the icon associated with the text.
image() and deimage()

The image() and deimage() methods set an image that will be displayed with the widget. The deimage() method sets the image that is shown when the widget is inactive, while the image() method sets the image that is shown when the widget is active.

To make an image you use a subclass of Fl_Image.

Making Your Own Label Types

Label types are actually indexes into a table of functions that draw them. The primary purpose of this is to use this to draw the labels in ways inaccessible through the fl_font() mechanism (e.g. FL_ENGRAVED_LABEL) or with program-generated letters or symbology.

Label Type Functions

To setup your own label type you will need to write two functions: one to draw and one to measure the label. The draw function is called with a pointer to a Fl_Label structure containing the label information, the bounding box for the label, and the label alignment:

void xyz_draw(const Fl_Label *label, int x, int y, int w, int h, Fl_Align align) {
unsigned Fl_Align
FLTK type for alignment control.
Definition: Enumerations.H:826
This struct stores all information for a text or mixed graphics label.
Definition: Fl_Widget.H:65

The label should be drawn inside this bounding box, even if FL_ALIGN_INSIDE is not enabled. The function is not called if the label value is NULL.

The measure function is called with a pointer to a Fl_Label structure and references to the width and height:

void xyz_measure(const Fl_Label *label, int &w, int &h) {

The function should measure the size of the label and set w and h to the size it will occupy.

Adding Your Label Type

The Fl::set_labeltype() method creates a label type using your draw and measure functions:

Fl::set_labeltype(XYZ_LABEL, xyz_draw, xyz_measure);
static void set_labeltype(Fl_Labeltype, Fl_Label_Draw_F *, Fl_Label_Measure_F *)
Sets the functions to call to draw and measure a specific labeltype.

The label type number n can be any integer value starting at the constant FL_FREE_LABELTYPE. Once you have added the label type you can use the labeltype() method to select your label type.

The Fl::set_labeltype() method can also be used to overload an existing label type such as FL_NORMAL_LABEL.

Making your own symbols

It is also possible to define your own drawings and add them to the symbol list, so they can be rendered as part of any label.

To create a new symbol, you implement a drawing function void drawit(Fl_Color c) which typically uses the functions described in Drawing Complex Shapes to generate a vector shape inside a two-by-two units sized box around the origin. This function is then linked into the symbols table using fl_add_symbol():

int fl_add_symbol(const char *name, void (*drawit)(Fl_Color), int scalable)
static const char * name
Definition: Fl_arg.cxx:53
int fl_add_symbol(const char *name, void(*drawit)(Fl_Color), int scalable)
Adds a symbol to the system.
Definition: fl_symbols.cxx:78

name is the name of the symbol without the "@"; scalable must be set to 1 if the symbol is generated using scalable vector drawing functions.

int fl_draw_symbol(const char *name,int x,int y,int w,int h,Fl_Color col)
int fl_draw_symbol(const char *label, int x, int y, int w, int h, Fl_Color)
Draw the named symbol in the given rectangle using the given color.
Definition: fl_symbols.cxx:103

This function draws a named symbol fitting the given rectangle.


Callbacks are functions that are called when the value of a widget changes. A callback function is sent a Fl_Widget pointer of the widget that changed and a pointer to data that you provide:

void xyz_callback(Fl_Widget *w, void *data) {
Fl_Widget is the base class for all widgets in FLTK.
Definition: Fl_Widget.H:101

The callback() method sets the callback function for a widget. You can optionally pass a pointer to some data needed for the callback:

int xyz_data;
button->callback(xyz_callback, &xyz_data);
Fl_Callback_p callback() const
Gets the current callback function for the widget.
Definition: Fl_Widget.H:561

Normally callbacks are performed only when the value of the widget changes. You can change this using the Fl_Widget::when() method:

Do the callback when the user presses the ENTER key and the value changes.
Definition: Enumerations.H:443
Do the callback when the button or key is released, even if the value doesn't change.
Definition: Enumerations.H:442
Do the callback when the user presses the ENTER key, even if the value doesn't change.
Definition: Enumerations.H:444
Do the callback whenever the user interacts with the widget.
Definition: Enumerations.H:440
Never call the callback.
Definition: Enumerations.H:438
Do the callback only when the widget value changes.
Definition: Enumerations.H:439
Do the callback when the button or key is released and the value changes.
Definition: Enumerations.H:441
Fl_When when() const
Returns the conditions under which the callback is called.
Definition: Fl_Widget.H:621


You cannot delete a widget inside a callback, as the widget may still be accessed by FLTK after your callback is completed. Instead, use the Fl::delete_widget() method to mark your widget for deletion when it is safe to do so.


Many programmers new to FLTK or C++ try to use a non-static class method instead of a static class method or function for their callback. Since callbacks are done outside a C++ class, the this pointer is not initialized for class methods.

To work around this problem, define a static method in your class that accepts a pointer to the class, and then have the static method call the class method(s) as needed. The data pointer you provide to the callback() method of the widget can be a pointer to the instance of your class.

class Foo {
void my_callback(Fl_Widget *w);
static void my_static_callback(Fl_Widget *w, void *f) { ((Foo *)f)->my_callback(w); }
w->callback(my_static_callback, (void *)this);
Fl_Box_Draw_F * f
Definition: fl_boxtype.cxx:285


Shortcuts are key sequences that activate widgets such as buttons or menu items. The shortcut() method sets the shortcut for a widget:

button->shortcut(0); // no shortcut
#define FL_CTRL
One of the ctrl keys is down.
Definition: Enumerations.H:557
#define FL_ALT
One of the alt keys is down.
Definition: Enumerations.H:558
#define FL_SHIFT
One of the shift keys is down.
Definition: Enumerations.H:555
#define FL_Enter
The enter key.
Definition: Enumerations.H:469
int shortcut() const
Returns the current shortcut key for the button.
Definition: Fl_Button.H:122

The shortcut value is the key event value - the ASCII value or one of the special keys described in Fl::event_key() Values combined with any modifiers like Shift , Alt , and Control.

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