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Fl_Pack automatically arranges child widgets horizontally or vertically and resizes itself to fit them. It’s a legacy widget retained for compatibility.
For new code, use Fl_Flex instead. Fl_Flex provides more predictable resize behavior and is the recommended single-row/column container since FLTK 1.4.0.

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Key Features

  • Automatic horizontal or vertical packing
  • Self-resizing to wrap children
  • Configurable spacing between children
  • Simple API for basic layouts

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Creating Fl_Pack

Fl_Pack(int x, int y, int w, int h, const char *label = 0)
Default type: Fl_Pack::VERTICAL (stacks children vertically) 
Fl_Pack *pack = new Fl_Pack(10, 10, 200, 0);  // Height auto-calculated
pack->type(Fl_Pack::VERTICAL);
pack->spacing(5);

pack->begin();
  new Fl_Button(0, 0, 200, 30, "Button 1");
  new Fl_Button(0, 0, 200, 30, "Button 2");
  new Fl_Button(0, 0, 200, 30, "Button 3");
pack->end();

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Pack Types

enum {
  VERTICAL = 0,    // Stack vertically (default)
  HORIZONTAL = 1   // Arrange horizontally
};

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Vertical Packing

pack->type(Fl_Pack::VERTICAL);
Behavior:
  • Widgets stacked top to bottom
  • Each widget keeps its height
  • All widgets resized to pack’s width
  • Pack height = sum of child heights + spacing

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Horizontal Packing

pack->type(Fl_Pack::HORIZONTAL);
Behavior:
  • Widgets arranged left to right
  • Each widget keeps its width
  • All widgets resized to pack’s height
  • Pack width = sum of child widths + spacing

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Check Direction

uchar horizontal() const

if (pack->horizontal()) {
  // Horizontal layout
} else {
  // Vertical layout
}

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Spacing

Set space between adjacent children: 
void spacing(int s)
int spacing() const

pack->spacing(10);  // 10 pixels between children

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Resize Behavior

Fl_Pack has unique resize behavior:
  1. Self-resizing: Pack resizes itself to “shrink-wrap” around children
  2. Child sizing:
    • Vertical: Children get pack’s width
    • Horizontal: Children get pack’s height
  3. resizable(): Default is NULL (fixed positioning)
  4. Special case: If resizable is the last child, it extends to fill available space
pack->resizable(NULL);  // Default: fixed children

// OR make last child fill remaining space
Fl_Box *filler = new Fl_Box(0, 0, 0, 0);
pack->resizable(filler);  // filler extends to fill

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Complete Example

Vertical button list: 
#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Pack.H>
#include <FL/Fl_Button.H>

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(220, 300, "Fl_Pack Demo");
  
  // Vertical pack
  Fl_Pack *pack = new Fl_Pack(10, 10, 200, 0);
  pack->type(Fl_Pack::VERTICAL);
  pack->spacing(5);
  pack->box(FL_DOWN_FRAME);
  
  pack->begin();
  for (int i = 1; i <= 8; i++) {
    char label[20];
    snprintf(label, sizeof(label), "Button %d", i);
    Fl_Button *btn = new Fl_Button(0, 0, 200, 30, label);
  }
  pack->end();
  
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

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Common Patterns

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Scrollable List

Combine with Fl_Scroll for long lists: 
Fl_Scroll *scroll = new Fl_Scroll(10, 10, 200, 280);
scroll->box(FL_DOWN_BOX);

Fl_Pack *pack = new Fl_Pack(10, 10, 180, 0);
pack->type(Fl_Pack::VERTICAL);
pack->spacing(2);

pack->begin();
for (int i = 0; i < 50; i++) {
  char label[32];
  snprintf(label, sizeof(label), "Item %d", i + 1);
  new Fl_Button(0, 0, 180, 25, label);
}
pack->end();

scroll->end();

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Horizontal Button Bar

Fl_Pack *buttons = new Fl_Pack(0, 0, 0, 35);
buttons->type(Fl_Pack::HORIZONTAL);
buttons->spacing(5);

buttons->begin();
  new Fl_Button(0, 0, 80, 30, "OK");
  new Fl_Button(0, 0, 80, 30, "Cancel");
  new Fl_Button(0, 0, 80, 30, "Apply");
buttons->end();

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Mixed with Fl_Group

Fl_Group *container = new Fl_Group(0, 0, 400, 300);

// Fixed header
Fl_Box *header = new Fl_Box(0, 0, 400, 40, "Header");
header->box(FL_UP_BOX);

// Packed content
Fl_Pack *content = new Fl_Pack(0, 40, 400, 260);
content->type(Fl_Pack::VERTICAL);
content->spacing(10);
content->begin();
  // ... add widgets ...
content->end();

container->resizable(content);
container->end();

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Clearing Contents

void clear()
Removes all children and sets resizable to NULL: 
pack->clear();  // Remove all children

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Limitations

Fl_Pack resizes itself during draw(), which can cause:
  • Unexpected size changes
  • Complex interactions with parent containers
  • Difficulty achieving specific layouts
Solution: Use Fl_Flex for predictable fixed-size containers
Changing type() after the widget is displayed requires manual resize:
for (int i = 0; i < pack->children(); i++) {
  pack->child(i)->resize(0, 0, 25, 25);
}
pack->type(Fl_Pack::HORIZONTAL);
pack->parent()->redraw();
Nesting Fl_Pack widgets often leads to surprising behavior due to self-resizing.Solution: Use Fl_Flex or Fl_Grid for nested layouts
Works best with frame-only boxes (FL_*_FRAME). Background boxes (FL_*_BOX) can be slower with many children.Avoid: Irregular boxes like FL_DIAMOND_BOX

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Fl_Flex vs Fl_Pack

FeatureFl_PackFl_Flex
SizeSelf-resizingFixed size
MarginsNoYes
Fixed widgetsNoYes
PredictableNoYes
RecommendedLegacy codeNew code
SinceFLTK 1.0FLTK 1.4.0
Migration example: 
// OLD: Fl_Pack
Fl_Pack *pack = new Fl_Pack(10, 10, 200, 0);
pack->type(Fl_Pack::VERTICAL);
pack->spacing(5);

// NEW: Fl_Flex (drop-in replacement)
Fl_Flex *flex = new Fl_Flex(10, 10, 200, 300, Fl_Flex::VERTICAL);
flex->gap(5);

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Best Practices

Use Inside Fl_Scroll

Fl_Pack works well for scrollable content:
Fl_Scroll *scroll = new Fl_Scroll(...);
Fl_Pack *pack = new Fl_Pack(...);

Set resizable(NULL)

Keep children fixed:
pack->resizable(NULL);

Prefer Fl_Flex

Use Fl_Flex for new projects:
Fl_Flex *flex = new Fl_Flex(...);

Frame Boxes

Use frame-style boxes:
pack->box(FL_DOWN_FRAME);

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Reference

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Header File

#include <FL/Fl_Pack.H>

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Key Methods

MethodDescription
type(int)Set VERTICAL or HORIZONTAL
spacing(int)Set space between children
horizontal()Check if horizontal layout
clear()Remove all children
resize(x,y,w,h)Resize pack (triggers reflow)

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Enumerations

enum {
  VERTICAL = 0,    // Stack vertically (default)
  HORIZONTAL = 1   // Arrange horizontally
};

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See Also

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