Text Display and Editor

FLTK provides powerful multi-line text widgets for displaying and editing text documents, with support for syntax highlighting, line numbers, word wrap, and more.

Fl_Text_Display

Header: FL/Fl_Text_Display.H Displays multi-line text with optional syntax highlighting, line numbers, and scrolling. This is a display-only widget - use Fl_Text_Editor for editing.

Constructor

Fl_Text_Display(int x, int y, int w, int h, const char *label = 0);

Key Methods

buffer() - Set Text Buffer

Associate a text buffer containing the text to display.

void buffer(Fl_Text_Buffer *buf);
Fl_Text_Buffer* buffer() const;

Fl_Text_Buffer *textbuf = new Fl_Text_Buffer();
Fl_Text_Display *display = new Fl_Text_Display(10, 10, 640, 480);
display->buffer(textbuf);

textbuf->text("Line one\nLine two\nLine three");

Important: The buffer is not owned by the display widget. You must manage its lifetime.

insert_position() - Cursor Position

Get or set the text insertion cursor position.

void insert_position(int pos);
int insert_position() const;

display->insert_position(0);  // Move to start
display->show_insert_position();  // Scroll to cursor

textfont/textsize/textcolor() - Text Style

Control the appearance of displayed text.

void textfont(Fl_Font font);
Fl_Font textfont() const;

void textsize(Fl_Fontsize size);
Fl_Fontsize textsize() const;

void textcolor(Fl_Color color);
Fl_Color textcolor() const;

display->textfont(FL_COURIER);
display->textsize(14);
display->textcolor(FL_BLACK);

wrap_mode() - Word Wrapping

Control how long lines are wrapped.

void wrap_mode(int wrap, int wrap_margin);

display->wrap_mode(Fl_Text_Display::WRAP_AT_BOUNDS, 0);  // Wrap at widget edge
display->wrap_mode(Fl_Text_Display::WRAP_AT_COLUMN, 80); // Wrap at column 80
display->wrap_mode(Fl_Text_Display::WRAP_NONE, 0);       // No wrapping

Wrap modes:

WRAP_NONE - No wrapping, horizontal scrollbar appears
WRAP_AT_COLUMN - Wrap at specified column
WRAP_AT_PIXEL - Wrap at pixel position
WRAP_AT_BOUNDS - Wrap to fit widget width

linenumber_width() - Show Line Numbers

Display line numbers in left margin.

void linenumber_width(int width);  // Width in pixels (0 = hide)
int linenumber_width() const;

void linenumber_font(Fl_Font font);
void linenumber_size(Fl_Fontsize size);
void linenumber_fgcolor(Fl_Color color);
void linenumber_bgcolor(Fl_Color color);

display->linenumber_width(50);
display->linenumber_bgcolor(FL_BACKGROUND_COLOR);
display->linenumber_fgcolor(FL_DARK3);

cursor_style() - Cursor Appearance

Set cursor style (for editor use).

void cursor_style(int style);

// Cursor styles
NORMAL_CURSOR   // I-beam
CARET_CURSOR    // Caret under text
DIM_CURSOR      // Dim I-beam
BLOCK_CURSOR    // Block under character
HEAVY_CURSOR    // Thick I-beam
SIMPLE_CURSOR   // Like Fl_Input cursor

highlight_data() - Syntax Highlighting

Add syntax highlighting using a style buffer.

void highlight_data(Fl_Text_Buffer *styleBuffer,
                   const Style_Table_Entry *styleTable,
                   int nStyles, char unfinishedStyle,
                   Unfinished_Style_Cb unfinishedHighlightCB,
                   void *cbArg);

// Define styles
Fl_Text_Display::Style_Table_Entry styletable[] = {
  { FL_BLACK,      FL_COURIER, 14 },  // A - Plain
  { FL_DARK_GREEN, FL_COURIER, 14 },  // B - Comments
  { FL_BLUE,       FL_COURIER, 14 },  // C - Keywords
  { FL_RED,        FL_COURIER, 14 },  // D - Strings
};

Fl_Text_Buffer *stylebuf = new Fl_Text_Buffer();
display->highlight_data(stylebuf, styletable, 4, 'A', 0, 0);

Example

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

int main() {
  Fl_Window *win = new Fl_Window(640, 480, "Text Display");
  
  // Create text buffer
  Fl_Text_Buffer *buff = new Fl_Text_Buffer();
  
  // Create display widget
  Fl_Text_Display *disp = new Fl_Text_Display(10, 10, 620, 460);
  disp->buffer(buff);
  disp->textfont(FL_COURIER);
  disp->textsize(14);
  
  // Load some text
  buff->text("This is a multi-line\n"
             "text display widget.\n"
             "It supports:\n"
             "  - Word wrapping\n"
             "  - Syntax highlighting\n"
             "  - Line numbers\n"
             "  - Scrolling\n");
  
  // Enable line numbers
  disp->linenumber_width(50);
  disp->linenumber_bgcolor(FL_BACKGROUND_COLOR);
  
  win->end();
  win->show();
  return Fl::run();
}

Visual Description

Fl_Text_Display appears as a rectangular text area with scrollbars (if needed). Text is displayed in monospace or proportional font. Optional line numbers appear in a shaded left margin. Selected text is highlighted.

Fl_Text_Editor

Header: FL/Fl_Text_Editor.H Extends Fl_Text_Display with editing capabilities - keyboard input, mouse selection, undo/redo.

Constructor

Fl_Text_Editor(int x, int y, int w, int h, const char *label = 0);

Additional Features

All Fl_Text_Display features plus:

Keyboard editing - Type, delete, insert text
Mouse selection - Click and drag to select
Undo/redo - Ctrl+Z / Ctrl+Shift+Z
Cut/copy/paste - Standard clipboard operations
Find/replace - Text search functionality

Example

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

Fl_Text_Buffer *textbuf = 0;

void save_cb(Fl_Widget *, void *) {
  const char *text = textbuf->text();
  // Save text to file...
  printf("Saving: %s\n", text);
  free((void*)text);
}

int main() {
  Fl_Window *win = new Fl_Window(640, 500, "Text Editor");
  
  textbuf = new Fl_Text_Buffer();
  
  Fl_Text_Editor *editor = new Fl_Text_Editor(10, 10, 620, 440);
  editor->buffer(textbuf);
  editor->textfont(FL_COURIER);
  editor->textsize(14);
  
  Fl_Button *save = new Fl_Button(270, 460, 100, 30, "Save");
  save->callback(save_cb);
  
  win->end();
  win->resizable(editor);
  win->show();
  return Fl::run();
}

Fl_Text_Buffer

Header: FL/Fl_Text_Buffer.H Manages the actual text data for text widgets. Provides text manipulation methods.

Key Methods

// Get/set entire text
const char* text() const;
void text(const char *txt);

// Length
int length() const;

// Insert/remove
void insert(int pos, const char *text);
void remove(int start, int end);
void replace(int start, int end, const char *text);

// Search
int search_forward(int startPos, const char *searchString, int *foundPos);
int search_backward(int startPos, const char *searchString, int *foundPos);

// Selection
void select(int start, int end);
void unselect();
int selection_position(int *start, int *end);
char* selection_text();

// Undo
void canundo(char flag);
int undo(int *cursorPos);
int redo(int *cursorPos);

// Load/save
int loadfile(const char *file);
int savefile(const char *file);

Text Display

Fl_Text_Display - Read-only textMulti-line text viewer with scrolling and highlighting.Use for: Log viewers, read-only documents, formatted output

Text Editor

Fl_Text_Editor - Editable textFull-featured text editor with undo, selection, clipboard.Use for: Code editors, note taking, text documents

Syntax Highlighting

Style Buffers - Colored textStyle buffer parallels text buffer to assign colors/fonts.Use for: Source code, markup languages, logs

Line Numbers

Line Number Margin - Show line countOptional gutter showing line numbers.Use for: Code editors, technical documents

Common Patterns

Simple Text Editor

class TextEditor {
  Fl_Window *window;
  Fl_Text_Editor *editor;
  Fl_Text_Buffer *textbuf;
  
public:
  TextEditor() {
    window = new Fl_Window(640, 480, "Editor");
    
    textbuf = new Fl_Text_Buffer();
    textbuf->canundo(1);  // Enable undo
    
    editor = new Fl_Text_Editor(0, 30, 640, 450);
    editor->buffer(textbuf);
    editor->textfont(FL_COURIER);
    editor->textsize(14);
    
    create_menubar();
    
    window->end();
    window->resizable(editor);
  }
  
  void show() { window->show(); }
};

Syntax Highlighting for C++

// Define color styles
Fl_Text_Display::Style_Table_Entry styletable[] = {
  { FL_BLACK,      FL_COURIER, 14, 0, 0 }, // A - Plain
  { FL_DARK_GREEN, FL_COURIER, 14, 0, 0 }, // B - Line comments  
  { FL_BLUE,       FL_COURIER, 14, 0, 0 }, // C - Keywords
  { FL_RED,        FL_COURIER, 14, 0, 0 }, // D - Strings
  { FL_DARK_CYAN,  FL_COURIER, 14, 0, 0 }, // E - Directives
};

const char *keywords[] = {
  "if", "else", "while", "for", "return", "int", "void", 
  "char", "class", "public", "private", "namespace", NULL
};

void style_update(int pos, int nInserted, int nDeleted,
                 int nRestyled, const char *deletedText, void *cbArg) {
  Fl_Text_Buffer *textbuf = ((Fl_Text_Editor*)cbArg)->buffer();
  Fl_Text_Buffer *stylebuf = ((Fl_Text_Editor*)cbArg)->style_buffer();
  
  // Re-parse text and update styles
  // This is simplified - real implementation is more complex
  char *text = textbuf->text();
  char *style = new char[strlen(text) + 1];
  
  memset(style, 'A', strlen(text));  // Default to plain
  
  // Mark keywords
  for (int i = 0; keywords[i]; i++) {
    char *p = text;
    while ((p = strstr(p, keywords[i])) != NULL) {
      int offset = p - text;
      memset(style + offset, 'C', strlen(keywords[i]));
      p += strlen(keywords[i]);
    }
  }
  
  style[strlen(text)] = '\0';
  stylebuf->text(style);
  
  delete[] style;
  free(text);
}

// Apply to editor
Fl_Text_Buffer *stylebuf = new Fl_Text_Buffer();
editor->highlight_data(stylebuf, styletable, 
                      sizeof(styletable)/sizeof(styletable[0]),
                      'A', style_update, editor);
textbuf->add_modify_callback(style_update, editor);

Log Viewer with Auto-Scroll

void append_log(Fl_Text_Display *display, const char *msg) {
  Fl_Text_Buffer *buf = display->buffer();
  
  buf->append(msg);
  buf->append("\n");
  
  // Scroll to bottom
  display->insert_position(buf->length());
  display->show_insert_position();
  
  // Limit buffer size (keep last 1000 lines)
  if (buf->count_lines(0, buf->length()) > 1000) {
    int pos = buf->skip_lines(0, 100);  // Skip first 100 lines
    buf->remove(0, pos);
  }
}

Read-Only Display with Selection

Fl_Text_Display *display = new Fl_Text_Display(10, 10, 600, 400);
Fl_Text_Buffer *textbuf = new Fl_Text_Buffer();

display->buffer(textbuf);
textbuf->loadfile("document.txt");

// Allow selection and copying, but not editing
// (Fl_Text_Display is already read-only)
display->cursor_style(Fl_Text_Display::NORMAL_CURSOR);

Find and Replace

void find_next(Fl_Text_Editor *editor, const char *searchtext) {
  Fl_Text_Buffer *buf = editor->buffer();
  int pos = editor->insert_position();
  int found = 0;
  
  if (buf->search_forward(pos, searchtext, &found)) {
    buf->select(found, found + strlen(searchtext));
    editor->insert_position(found + strlen(searchtext));
    editor->show_insert_position();
  } else {
    fl_alert("Not found: %s", searchtext);
  }
}

void replace_all(Fl_Text_Editor *editor, 
                const char *find, const char *replace) {
  Fl_Text_Buffer *buf = editor->buffer();
  int pos = 0;
  int count = 0;
  
  while (buf->search_forward(pos, find, &pos)) {
    buf->select(pos, pos + strlen(find));
    buf->remove_selection();
    buf->insert(pos, replace);
    pos += strlen(replace);
    count++;
  }
  
  fl_message("Replaced %d occurrences", count);
}

Keyboard Shortcuts (Editor)

Fl_Text_Editor supports these default key bindings:

Action	Shortcut
Undo	Ctrl+Z
Redo	Ctrl+Shift+Z
Cut	Ctrl+X
Copy	Ctrl+C
Paste	Ctrl+V
Select All	Ctrl+A
Move to line start	Home
Move to line end	End
Move to doc start	Ctrl+Home
Move to doc end	Ctrl+End
Delete line	Ctrl+K

Important Notes

The text() method of Fl_Text_Buffer returns a malloc’d string that you must free() when done. Forgetting this causes memory leaks.

For syntax highlighting, the style buffer must be the same length as the text buffer, with one style character per text character.

Use wrap_mode(WRAP_AT_BOUNDS, 0) for a better user experience in editors - text wraps to window width automatically.

Performance Tips

For large files (>100K lines), consider lazy loading or pagination
Use canundo(0) if you don’t need undo to save memory
Disable syntax highlighting for very large files
Use suspend_undo() / resume_undo() during bulk updates

Reference

Source: FL/Fl_Text_Display.H (lines 32-674)
Source: FL/Fl_Text_Editor.H
Source: FL/Fl_Text_Buffer.H
Example: test/editor.cxx

Getting Started

Core Concepts

Widgets

Layout Management

Advanced Topics

FLUID Designer

Platform Guides

Text Display and Editor

Fl_Text_Display

Constructor

Key Methods

Example

Visual Description

Fl_Text_Editor

Constructor

Additional Features

Example

Fl_Text_Buffer

Key Methods

Text Widget Gallery

Text Display

Text Editor

Syntax Highlighting

Line Numbers

Common Patterns

Simple Text Editor

Syntax Highlighting for C++

Log Viewer with Auto-Scroll

Read-Only Display with Selection

Find and Replace

Keyboard Shortcuts (Editor)

Important Notes

Performance Tips

Reference

Getting Started

Core Concepts

Widgets

Layout Management

Advanced Topics

FLUID Designer

Platform Guides

Documentation Index

​Fl_Text_Display

​Constructor

​Key Methods

​Example

​Visual Description

​Fl_Text_Editor

​Constructor

​Additional Features

​Example

​Fl_Text_Buffer

​Key Methods

​Text Widget Gallery

Text Display

Text Editor

Syntax Highlighting

Line Numbers

​Common Patterns

​Simple Text Editor

​Syntax Highlighting for C++

​Log Viewer with Auto-Scroll

​Read-Only Display with Selection

​Find and Replace

​Keyboard Shortcuts (Editor)

​Important Notes

​Performance Tips

​Reference

Fl_Text_Display

Constructor

Key Methods

Example

Visual Description

Fl_Text_Editor

Constructor

Additional Features

Example

Fl_Text_Buffer

Key Methods

Text Widget Gallery

Common Patterns

Simple Text Editor

Syntax Highlighting for C++

Log Viewer with Auto-Scroll

Read-Only Display with Selection

Find and Replace

Keyboard Shortcuts (Editor)

Important Notes

Performance Tips

Reference