Fl_Flex

Overview

Fl_Flex is a container (layout) widget for one row or one column of widgets. It provides flexible positioning of its children either in one row (HORIZONTAL) or in one column (VERTICAL). Since: FLTK 1.4.0 Header: <FL/Fl_Flex.H> Inherits: Fl_Group

Key Features

Simple flexible row or column layout
Configurable margins on all four sides
Gap spacing between children
Fixed and flexible widget sizing
Drop-in replacement for Fl_Pack (recommended)
Predictable resizing behavior

Use Cases

Toolbar layouts with flexible spacing
Button rows with consistent sizing
Form layouts with aligned controls
Single-row or single-column arrangements
Nested layouts for complex UIs

Layout Algorithm

Fl_Flex positions widgets in a row or column:

Margins are applied around all children at the inner side of the box frame
Gaps are inserted between each child widget
Fixed-size widgets maintain their specified width (HORIZONTAL) or height (VERTICAL)
Flexible widgets share remaining space equally

For HORIZONTAL layout:

All widgets are resized to full height minus margins
Flexible widgets share available width equally
Use fixed() to set specific widget widths

For VERTICAL layout:

All widgets are resized to full width minus margins
Flexible widgets share available height equally
Use fixed() to set specific widget heights

Constructor

Standard Constructor

Fl_Flex(int X, int Y, int W, int H, const char *L = 0)

Creates a vertical Fl_Flex container with standard FLTK coordinates.

int

X position of the widget

int

Y position of the widget

int

Width of the widget

int

Height of the widget

const char*

Optional label string (default: NULL)

Simplified Constructors

Fl_Flex(int direction)
Fl_Flex(int w, int h, int direction)
Fl_Flex(int x, int y, int w, int h, int direction)

Alternate constructors that specify layout direction directly.

direction

int

Layout direction: Fl_Flex::HORIZONTAL, Fl_Flex::VERTICAL (or aliases ROW, COLUMN)

Layout Direction

Type Constants

enum {
  VERTICAL   = 0,  // One column
  HORIZONTAL = 1,  // One row
  COLUMN     = 0,  // Alias for VERTICAL
  ROW        = 1   // Alias for HORIZONTAL
}

horizontal()

int horizontal() const

Returns non-zero if layout is horizontal. Returns: 1 if type() == HORIZONTAL, 0 if VERTICAL

fixed()

void fixed(Fl_Widget *w, int size)
void fixed(Fl_Widget &w, int size)
int fixed(Fl_Widget *w) const

Sets or gets the fixed size of a child widget.

Fl_Widget*

Widget to affect

size

int

Fixed width (HORIZONTAL) or height (VERTICAL) in pixels

For HORIZONTAL: Sets fixed width; widget height adjusts to container
For VERTICAL: Sets fixed height; widget width adjusts to container Widgets without a fixed size share remaining space equally.

Margins

margin() - Get

int margin() const

Returns the left margin size (useful if all margins are equal).

margin() - Get All Four

int margin(int *left, int *top, int *right, int *bottom) const

Returns all four margin sizes.

left

int*

Pointer to receive left margin (can be NULL)

top

int*

Pointer to receive top margin (can be NULL)

right

int*

Pointer to receive right margin (can be NULL)

bottom

int*

Pointer to receive bottom margin (can be NULL)

Returns: 1 if all margins are equal, 0 otherwise

margin() - Set All Equal

void margin(int m, int g = -1)

Sets all four margins to the same size, optionally sets gap.

int

Margin size in pixels (must be >= 0)

int

Optional gap size (if >= 0); negative values leave gap unchanged

margin() - Set Individual

void margin(int left, int top, int right, int bottom)

Sets each margin individually.

left

int

Left margin in pixels (negative values set to 0)

top

int

Top margin in pixels (negative values set to 0)

right

int

Right margin in pixels (negative values set to 0)

bottom

int

Bottom margin in pixels (negative values set to 0)

Gap Spacing

gap()

int gap() const
void gap(int g)

Gets or sets the gap size between children.

int

Gap size in pixels (must be >= 0; negative values clamped to 0)

spacing()

int spacing() const
void spacing(int i)

Synonym for gap(). Provided for compatibility with Fl_Pack.

Layout Management

layout()

void layout()

Calculates the layout of children and redraws the widget.

need_layout()

void need_layout(int set)
bool need_layout() const

Sets or gets whether layout calculation is required.

set

int

1 to request layout recalculation, 0 to reset (rarely use 0)

Call with set=1 if you change child attributes or sizes to ensure proper layout calculation.

resize()

void resize(int x, int y, int w, int h) override

Resizes the Fl_Flex container and recalculates child layout.

#include <FL/Fl.H>
#include <FL/Fl_Double_Window.H>
#include <FL/Fl_Flex.H>
#include <FL/Fl_Box.H>
#include <FL/Fl_Button.H>

int main(int argc, char **argv) {
  Fl_Double_Window window(410, 40, "Simple Fl_Flex Demo");
  
  // Create horizontal flex container
  Fl_Flex flex(5, 5, 400, 30, Fl_Flex::HORIZONTAL);
  
  // Add buttons - position/size will be managed by Fl_Flex
  Fl_Button b1(0, 0, 0, 0, "File");
  Fl_Button b2(0, 0, 0, 0, "Save");
  Fl_Box    bx(0, 0, 0, 0);  // Invisible spacer
  Fl_Button b3(0, 0, 0, 0, "Exit");
  
  // Configure layout
  flex.fixed(bx, 60);  // Fixed width spacer
  flex.gap(10);        // 10 pixel gap between widgets
  flex.end();
  
  window.resizable(flex);
  window.end();
  window.size_range(300, 30);  // Prevent too small
  window.show(argc, argv);
  return Fl::run();
}

Example: Vertical Form Layout

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Flex.H>
#include <FL/Fl_Input.H>
#include <FL/Fl_Button.H>

int main() {
  Fl_Window win(300, 250, "Form Example");
  
  // Vertical flex container with margins
  Fl_Flex *vbox = new Fl_Flex(0, 0, 300, 250, Fl_Flex::VERTICAL);
  vbox->margin(10);
  vbox->gap(5);
  
  Fl_Input *name = new Fl_Input(0, 0, 0, 30, "Name:");
  Fl_Input *email = new Fl_Input(0, 0, 0, 30, "Email:");
  
  // Flexible spacer
  Fl_Box *spacer = new Fl_Box(0, 0, 0, 0);
  
  // Fixed height button
  Fl_Button *submit = new Fl_Button(0, 0, 0, 30, "Submit");
  vbox->fixed(submit, 30);
  
  vbox->end();
  win.resizable(vbox);
  win.show();
  return Fl::run();
}

Best Practices

Size Constraints

Always set a minimum window size to prevent resizing below designed dimensions:

window.size_range(min_width, min_height);

Nested Layouts

Nest Fl_Flex containers for complex multi-row/column layouts:

// Outer vertical container
Fl_Flex *vbox = new Fl_Flex(0, 0, 300, 200, Fl_Flex::VERTICAL);

// Top horizontal row
Fl_Flex *hbox1 = new Fl_Flex(0, 0, 0, 0, Fl_Flex::HORIZONTAL);
hbox1->gap(5);
// Add buttons to hbox1...
vbox->fixed(hbox1, 40);  // Fixed height row
hbox1->end();

// Middle flexible area
Fl_Box *content = new Fl_Box(0, 0, 0, 0);

// Bottom horizontal row
Fl_Flex *hbox2 = new Fl_Flex(0, 0, 0, 0, Fl_Flex::HORIZONTAL);
// Add buttons to hbox2...
vbox->fixed(hbox2, 40);
hbox2->end();

vbox->end();

Invisible Spacers

Use Fl_Box widgets for flexible or fixed spacing:

// Flexible spacer (takes remaining space)
Fl_Box *spacer1 = new Fl_Box(0, 0, 0, 0);

// Fixed spacer
Fl_Box *spacer2 = new Fl_Box(0, 0, 0, 0);
flex.fixed(spacer2, 50);

Replacement for Fl_Pack

Fl_Flex is the recommended replacement for Fl_Pack since FLTK 1.4.0:

// Old Fl_Pack code:
Fl_Pack *pack = new Fl_Pack(x, y, w, h);
pack->type(Fl_Pack::HORIZONTAL);
pack->spacing(10);

// New Fl_Flex code:
Fl_Flex *flex = new Fl_Flex(x, y, w, h, Fl_Flex::HORIZONTAL);
flex->gap(10);

Notes

Default box type is FL_NO_BOX (inherited from Fl_Group)
Default margin is 0 on all sides
Default gap is 0
Default type is VERTICAL
Undefined behavior if resized below minimum child sizes
For complex grids, consider using Fl_Grid instead

Core Classes

Basic Widgets

Menu Widgets

Container Widgets

Complex Widgets

Drawing & Graphics

Overview

Key Features

Use Cases

Layout Algorithm

Constructor

Standard Constructor

Simplified Constructors

Layout Direction

Type Constants

horizontal()

Widget Sizing

fixed()

Margins

margin() - Get

margin() - Get All Four

margin() - Set All Equal

margin() - Set Individual

Gap Spacing

gap()

spacing()

Layout Management

layout()

need_layout()

resize()

Example: Vertical Form Layout

Best Practices

Size Constraints

Nested Layouts

Invisible Spacers

Replacement for Fl_Pack

Notes

See Also

Core Classes

Basic Widgets

Menu Widgets

Container Widgets

Complex Widgets

Drawing & Graphics

Documentation Index

​Overview

​Key Features

​Use Cases

​Layout Algorithm

​Constructor

​Standard Constructor

​Simplified Constructors

​Layout Direction

​Type Constants

​horizontal()

​Widget Sizing

​fixed()

​Margins

​margin() - Get

​margin() - Get All Four

​margin() - Set All Equal

​margin() - Set Individual

​Gap Spacing

​gap()

​spacing()

​Layout Management

​layout()

​need_layout()

​resize()

​Example: Simple Toolbar

​Example: Vertical Form Layout

​Best Practices

​Size Constraints

​Nested Layouts

​Invisible Spacers

​Replacement for Fl_Pack

​Notes

​See Also

Overview

Key Features

Use Cases

Layout Algorithm

Constructor

Standard Constructor

Simplified Constructors

Layout Direction

Type Constants

horizontal()

Widget Sizing

fixed()

Margins

margin() - Get

margin() - Get All Four

margin() - Set All Equal

margin() - Set Individual

Gap Spacing

gap()

spacing()

Layout Management

layout()

need_layout()

resize()

Example: Simple Toolbar

Example: Vertical Form Layout

Best Practices

Size Constraints

Nested Layouts

Invisible Spacers

Replacement for Fl_Pack

Notes

See Also