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Fl_Slider

The Fl_Slider widget contains a sliding knob inside a box that allows users to select a value from a continuous range. It’s commonly used for volume controls, scrollbars, and value selection.

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Header File

#include <FL/Fl_Slider.H>

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Class Overview

Fl_Slider is a valuator widget that provides a graphical slider control. Features include:
  • Sliding knob for value selection
  • Horizontal or vertical orientation
  • Multiple visual styles (standard, fill, nice)
  • Linear or logarithmic scaling
  • Optional tick marks for nice sliders
  • Configurable range with minimum() and maximum()

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Inheritance

Fl_Widget
  └── Fl_Valuator
        └── Fl_Slider
Inherits from: Fl_Valuator (which inherits from Fl_Widget)

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Slider Types

Set the slider type using type(int):
  • FL_VERT_SLIDER (0) - Vertical slider (default)
  • FL_HOR_SLIDER (1) - Horizontal slider
  • FL_VERT_FILL_SLIDER (2) - Vertical filled slider (progress meter style)
  • FL_HOR_FILL_SLIDER (3) - Horizontal filled slider
  • FL_VERT_NICE_SLIDER (4) - Vertical slider with nice-looking knob
  • FL_HOR_NICE_SLIDER (5) - Horizontal slider with nice-looking knob

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Constructors

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Standard Constructor

Fl_Slider(int X, int Y, int W, int H, const char *L = 0)
Creates a new slider widget with default vertical orientation.
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X
int
The X coordinate of the slider relative to the enclosing window
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Y
int
The Y coordinate of the slider relative to the enclosing window
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W
int
Width of the slider in pixels
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H
int
Height of the slider in pixels
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L
const char*
default:"nullptr"
Label text for the slider

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Type Constructor

Fl_Slider(uchar t, int X, int Y, int W, int H, const char *L)
Creates a slider with a specific type.
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t
uchar
Slider type (e.g., FL_HOR_SLIDER, FL_VERT_NICE_SLIDER)
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X
int
The X coordinate of the slider
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Y
int
The Y coordinate of the slider
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W
int
Width in pixels
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H
int
Height in pixels
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L
const char*
Label text

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Public Methods

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slider_size()

float slider_size() const
void slider_size(double v)
Gets or sets the dimensions of the moving slider piece as a fraction of the entire widget.
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v
double
Fraction of widget size (0.0 to 1.0). Default is 0.08. Setting to 1.0 prevents movement.
Returns: For getter, current slider size as a float
For fill sliders, this controls the size of the area around the end that causes dragging rather than jumping to the mouse.

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slider()

Fl_Boxtype slider() const
void slider(Fl_Boxtype c)
Gets or sets the box type used for the slider knob.
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c
Fl_Boxtype
Box type for the slider knob (e.g., FL_UP_BOX, FL_ROUND_UP_BOX)
Returns: For getter, current slider box type

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scale()

Scale_Type scale() const
void scale(Scale_Type s)
Gets or sets the scale type (linear or logarithmic).
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s
Scale_Type
Either LINEAR_SCALE (0, default) or LOG_SCALE
Returns: For getter, current scale type

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ticks()

uchar ticks() const
void ticks(uchar t, uchar num_ticks = 11)
Gets or sets tick mark positions for nice sliders.
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t
uchar
Tick position bitset:
  • TICKS_NONE (0x00) - No ticks
  • TICKS_BELOW (0x01) - Below horizontal slider
  • TICKS_ABOVE (0x02) - Above horizontal slider
  • TICKS_LEFT (0x01) - Left of vertical slider
  • TICKS_RIGHT (0x02) - Right of vertical slider
Can combine with OR for both sides
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num_ticks
uchar
default:"11"
Number of tick marks to draw
Returns: For getter, current tick position bitset

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num_ticks()

uchar num_ticks() const
Gets the number of tick marks. Returns: uchar - Number of tick marks

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bounds()

void bounds(double a, double b)
Sets the minimum and maximum values for the slider.
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a
double
Minimum value
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b
double
Maximum value
The minimum may be greater than maximum to reverse the slider direction.

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scrollvalue()

int scrollvalue(int pos, int size, int first, int total)
Sets the slider position and size for use as a scrollbar.
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pos
int
Current scroll position
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size
int
Size of visible area
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first
int
First position in scrollable area
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total
int
Total size of scrollable area
Returns: int - 1 if slider changed, 0 otherwise

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handle()

int handle(int event) override
Handles events for the slider.
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event
int
The FLTK event type
Returns: int - 1 if handled, 0 otherwise

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Protected Methods

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draw()

void draw() override
void draw(int X, int Y, int W, int H)
Draws the slider. The parameterized version allows subclasses to draw in a smaller area.

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Inherited Methods from Fl_Valuator

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Value Management

  • value() / value(double) - Get or set current value
  • minimum() / minimum(double) - Get or set minimum value
  • maximum() / maximum(double) - Get or set maximum value
  • range(double min, double max) - Set min and max together
  • step(double) / step(double, int) - Set value stepping increment
  • precision(int) - Set number of digits for text display

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Callback Control

  • when() / when(uchar) - Control when callbacks occur
  • clamp(double) - Clamp value to min/max range

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Usage Example

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Slider.H>
#include <FL/Fl_Box.H>
#include <stdio.h>

Fl_Box *output;

void slider_cb(Fl_Widget *w, void *) {
  Fl_Slider *slider = (Fl_Slider*)w;
  char buffer[32];
  sprintf(buffer, "Value: %.2f", slider->value());
  output->label(buffer);
  output->redraw();
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(400, 300);
  
  // Horizontal slider
  Fl_Slider *h_slider = new Fl_Slider(FL_HOR_NICE_SLIDER, 
                                       50, 50, 300, 30, "Volume");
  h_slider->align(FL_ALIGN_TOP);
  h_slider->bounds(0.0, 100.0);
  h_slider->value(50.0);
  h_slider->callback(slider_cb);
  h_slider->ticks(TICKS_BELOW, 11);
  
  // Vertical slider
  Fl_Slider *v_slider = new Fl_Slider(FL_VERT_FILL_SLIDER,
                                       50, 120, 40, 150, "Level");
  v_slider->align(FL_ALIGN_LEFT);
  v_slider->bounds(0.0, 100.0);
  v_slider->value(75.0);
  
  // Output display
  output = new Fl_Box(150, 180, 200, 30, "Value: 50.00");
  output->box(FL_DOWN_BOX);
  
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

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Logarithmic Scale Example

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Slider.H>
#include <FL/Fl_Box.H>
#include <stdio.h>
#include <math.h>

Fl_Box *freq_display;

void freq_cb(Fl_Widget *w, void *) {
  Fl_Slider *slider = (Fl_Slider*)w;
  double freq = slider->value();
  char buffer[64];
  sprintf(buffer, "Frequency: %.0f Hz", freq);
  freq_display->copy_label(buffer);
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(450, 200);
  
  // Logarithmic frequency slider (20Hz to 20kHz)
  Fl_Slider *freq = new Fl_Slider(FL_HOR_NICE_SLIDER,
                                    50, 50, 350, 30, 
                                    "Frequency (Logarithmic)");
  freq->align(FL_ALIGN_TOP);
  freq->bounds(20.0, 20000.0);
  freq->value(1000.0);
  freq->scale(Fl_Slider::LOG_SCALE);
  freq->callback(freq_cb);
  freq->ticks(TICKS_BELOW | TICKS_ABOVE, 10);
  
  freq_display = new Fl_Box(100, 120, 250, 40, "Frequency: 1000 Hz");
  freq_display->box(FL_FLAT_BOX);
  freq_display->labelsize(16);
  
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

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Common Use Cases

  • Volume controls - Audio level adjustment
  • Brightness/contrast - Image adjustment controls
  • Progress indicators - Using fill sliders as progress meters
  • Scrollbars - Custom scrollbar implementation
  • Parameter adjustment - Fine-tuning numeric values
  • Color pickers - RGB/HSV component selection

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See Also