Fl_Output

API reference for the Fl_Output class in FLTK

Fl_Output

The Fl_Output widget displays a piece of text in a read-only field. Users can select and copy the text but cannot edit it.

Header File

#include <FL/Fl_Output.H>

Class Overview

Fl_Output is a read-only text display widget that:

• Displays static or program-generated text
• Allows users to select text with the mouse
• Supports copying selected text to clipboard
• Uses strcpy() to maintain its own copy of the value
• Handles UTF-8 and displays control characters using ^X notation

For multiline output, use Fl_Multiline_Output. For more advanced text display with scrolling, use Fl_Text_Display.

Inheritance

Fl_Widget
  └── Fl_Input_
        └── Fl_Input
              └── Fl_Output

Inherits from: Fl_Input (which inherits from Fl_Input_ and Fl_Widget)

Constructor

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

Creates a new Fl_Output widget with the given position, size, and label.

X · int

The X coordinate of the widget relative to the enclosing window

Y · int

The Y coordinate of the widget relative to the enclosing window

W · int

Width of the widget in pixels

H · int

Height of the widget in pixels

L · const char* · default: "nullptr"

Label text displayed next to the output field

The default box type is FL_DOWN_BOX, giving it a sunken appearance.

Inherited Methods

Since Fl_Output inherits from Fl_Input, it has access to all input methods, but editing operations are disabled. The most commonly used methods include:

Text Management

• value() / value(const char*) - Get or set the displayed text
• size() - Get the length of text in bytes

Selection

• position() / position(int) - Get or set cursor/selection position
• mark() / mark(int) - Get or set selection mark
• position(int, int) - Set selection range

Appearance

• textcolor() / textcolor(Fl_Color) - Get or set text color
• textfont() / textfont(Fl_Font) - Get or set text font
• textsize() / textsize(Fl_Fontsize) - Get or set text size
• color() / color(Fl_Color) - Get or set background color

Display Behavior

The text displayed in Fl_Output:

• Can contain any characters except \0
• Displays unprintable control characters using ^X notation (e.g., ^M for carriage return)
• Displays unprintable characters with the high bit set using \nnn notation
• Assumes the font can draw any characters in the ISO-Latin1 character set
• Supports full UTF-8 text rendering

Usage Example

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Output.H>
#include <FL/Fl_Button.H>
#include <stdio.h>
#include <time.h>

void update_time(void *data) {
  Fl_Output *output = (Fl_Output*)data;
  
  time_t now = time(0);
  char *timestr = ctime(&now);
  timestr[24] = '\0';  // Remove newline
  
  output->value(timestr);
  
  // Schedule next update in 1 second
  Fl::repeat_timeout(1.0, update_time, data);
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(400, 100);
  
  // Display current time (read-only)
  Fl_Output *time_display = new Fl_Output(100, 30, 280, 30, "Current Time:");
  time_display->textfont(FL_COURIER);
  time_display->textsize(14);
  
  window->end();
  window->show(argc, argv);
  
  // Start the timer
  Fl::add_timeout(0.0, update_time, time_display);
  
  return Fl::run();
}

Status Display Example

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Output.H>
#include <FL/Fl_Button.H>

Fl_Output *status_output;
int counter = 0;

void button_cb(Fl_Widget *, void *) {
  counter++;
  char buffer[100];
  sprintf(buffer, "Button clicked %d times", counter);
  status_output->value(buffer);
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(350, 120);
  
  Fl_Button *btn = new Fl_Button(100, 20, 150, 30, "Click Me");
  btn->callback(button_cb);
  
  status_output = new Fl_Output(50, 70, 250, 30, "Status:");
  status_output->value("Ready");
  status_output->color(FL_BACKGROUND2_COLOR);
  
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

Multiline Output Example

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Multiline_Output.H>

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(400, 300);
  
  Fl_Multiline_Output *info = new Fl_Multiline_Output(10, 10, 380, 280, 0);
  info->value(
    "System Information\n"
    "=================\n\n"
    "Application: FLTK Demo\n"
    "Version: 1.4.0\n"
    "Platform: Cross-platform\n\n"
    "This is a read-only multiline\n"
    "output field. You can select\n"
    "and copy this text, but you\n"
    "cannot edit it."
  );
  info->textfont(FL_COURIER);
  
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

Differences from Fl_Input

While Fl_Output inherits from Fl_Input, the key differences are:

Feature	Fl_Input	Fl_Output
Editing	Allowed	Not allowed
Text Selection	Yes	Yes
Copy to Clipboard	Yes	Yes
Paste from Clipboard	Yes	No
Keyboard Input	Accepted	Ignored (except copy commands)
Default Box	FL_DOWN_BOX	FL_DOWN_BOX
Primary Use	User input	Display program output

Common Use Cases

• Status displays - Show application status or messages
• Calculated results - Display computation results
• Read-only information - Show configuration or system info
• Log viewers - Display log messages (single line)
• Data presentation - Show formatted data to users

See Also

• Fl_Input - Editable text input widget
• Fl_Text_Display - Multi-line text display with scrolling
• Fl_Multiline_Output - Multi-line read-only output
• Fl_Box - Simple label display