Input Field Examples

What This Example Demonstrates

This comprehensive example shows:

Different types of input fields (text, numeric, password, multiline)
Input validation (float, integer)
Callback timing options (when callbacks fire)
Color customization
Keyboard navigation options
Getting and setting input values

Source file: test/input.cxx (simplified for clarity)

Complete Source Code

#include <stdio.h>
#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Input.H>
#include <FL/Fl_Float_Input.H>
#include <FL/Fl_Int_Input.H>
#include <FL/Fl_Secret_Input.H>
#include <FL/Fl_Multiline_Input.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_Toggle_Button.H>

void cb(Fl_Widget *ob) {
  printf("Callback for %s '%s'\n", ob->label(), ((Fl_Input*)ob)->value());
}

int when = 0;
Fl_Input *input[5];

void toggle_cb(Fl_Widget *o, long v) {
  if (((Fl_Toggle_Button*)o)->value()) 
    when |= v; 
  else 
    when &= ~v;
  for (int i=0; i<5; i++) 
    input[i]->when(when);
}

int main(int argc, char **argv) {
  Fl::args(argc, argv);
  Fl::get_system_colors();
  
  Fl_Window *window = new Fl_Window(400, 350);

  int y = 10;
  input[0] = new Fl_Input(70, y, 300, 30, "Normal:"); y += 35;
  input[0]->tooltip("Normal input field");
  
  input[1] = new Fl_Float_Input(70, y, 300, 30, "Float:"); y += 35;
  input[1]->tooltip("Input field for floating-point number (F1)");
  input[1]->shortcut(FL_F+1);
  
  input[2] = new Fl_Int_Input(70, y, 300, 30, "Int:"); y += 35;
  input[2]->tooltip("Input field for integer number (F2)");
  input[2]->shortcut(FL_F+2);
  
  input[3] = new Fl_Secret_Input(70, y, 300, 30, "&Secret:"); y += 35;
  input[3]->tooltip("Input field for password (Alt-S)");
  
  input[4] = new Fl_Multiline_Input(70, y, 300, 100, "&Multiline:"); y += 105;
  input[4]->tooltip("Input field for short text with newlines (Alt-M)");
  input[4]->wrap(1);

  for (int i = 0; i < 5; i++) {
    input[i]->when(0); 
    input[i]->callback(cb);
  }

  Fl_Button *b;
  b = new Fl_Toggle_Button(10, y, 200, 25, "FL_WHEN_CHANGED");
  b->callback(toggle_cb, FL_WHEN_CHANGED); y += 25;
  b->tooltip("Do callback each time the text changes");
  
  b = new Fl_Toggle_Button(10, y, 200, 25, "FL_WHEN_RELEASE");
  b->callback(toggle_cb, FL_WHEN_RELEASE); y += 25;
  b->tooltip("Do callback when widget loses focus");
  
  b = new Fl_Toggle_Button(10, y, 200, 25, "FL_WHEN_ENTER_KEY");
  b->callback(toggle_cb, FL_WHEN_ENTER_KEY); y += 25;
  b->tooltip("Do callback when user hits Enter key");

  window->end();
  window->resizable(window);
  window->show(argc, argv);
  return Fl::run();
}

Compilation Command

fltk-config --compile input.cxx

Expected Behavior

The program displays:

Normal - Accepts any text
Float - Only accepts floating-point numbers (e.g., 3.14, -2.5)
Int - Only accepts integers (e.g., 42, -17)
Secret - Password field (shows dots instead of characters)
Multiline - Multi-line text input with word wrapping
Toggle buttons to control when callbacks fire

Key Concepts

Input Field Types

Class	Purpose	Example
`Fl_Input`	General text	”Hello World”
`Fl_Float_Input`	Floating-point	3.14159
`Fl_Int_Input`	Integers only	42
`Fl_Secret_Input`	Password	•••••••••
`Fl_Multiline_Input`	Multiple lines	Text with\nnewlines

Getting and Setting Values

// Set value
input->value("Initial text");

// Get value
const char *text = input->value();
printf("Current value: %s\n", text);

// Check if value changed
if (input->changed()) {
  printf("Value was modified\n");
  input->clear_changed();  // Reset flag
}

Callback Timing (when)

Control when callbacks are triggered:

// Callback on every keystroke
input->when(FL_WHEN_CHANGED);

// Callback when Enter is pressed
input->when(FL_WHEN_ENTER_KEY);

// Callback when widget loses focus
input->when(FL_WHEN_RELEASE);

// Callback even if not changed
input->when(FL_WHEN_NOT_CHANGED);

// Combine multiple conditions
input->when(FL_WHEN_CHANGED | FL_WHEN_ENTER_KEY);

Keyboard Shortcuts

input->shortcut(FL_F+1);        // F1 key
input->shortcut(FL_CTRL+'s');   // Ctrl+S
input->shortcut(FL_ALT+'a');    // Alt+A

Or use & in the label:

new Fl_Input(10, 10, 100, 30, "&Name:");  // Alt+N focuses this field

Variations and Extensions

Input Validation

// Limit input length
input->maximum_size(20);

// Custom validation in callback
void validate_email(Fl_Widget *w, void*) {
  Fl_Input *inp = (Fl_Input*)w;
  const char *text = inp->value();
  if (strchr(text, '@') == NULL) {
    inp->color(FL_RED);  // Invalid: no @ symbol
  } else {
    inp->color(FL_WHITE);  // Valid
  }
  inp->redraw();
}

Read-Only Input

Fl_Input *readonly = new Fl_Input(10, 10, 200, 30);
readonly->value("Cannot edit this");
readonly->readonly(1);  // Make read-only

#include <FL/Fl_Output.H>

Fl_Output *output = new Fl_Output(10, 10, 200, 30, "Result:");
output->value("This is read-only output");
// User cannot edit, but can select and copy text

Multiline Features

Fl_Multiline_Input *multi = new Fl_Multiline_Input(10, 10, 300, 200);

// Enable word wrapping
multi->wrap(1);

// Allow Tab key to insert tabs instead of navigating
multi->tab_nav(0);

// Get cursor position
int pos = multi->position();
int line = multi->count_lines(0, pos, 1);
printf("Cursor at line %d\n", line);

// Insert text at cursor
multi->insert("inserted text");

Text Selection

// Select all text
input->position(input->size());  // End
input->mark(0);                  // Start of selection

// Get selected text
const char *selected = input->selection_text();

// Clear selection
input->position(input->position());

Colors and Styling

input->textcolor(FL_BLACK);           // Text color
input->color(FL_WHITE);               // Background
input->selection_color(FL_BLUE);      // Selection highlight
input->textfont(FL_COURIER);          // Use monospace font
input->textsize(14);                  // Font size
input->cursor_color(FL_RED);          // Cursor color

Input Masks and Filters

For phone numbers, dates, etc., create a custom input class:

class PhoneInput : public Fl_Input {
public:
  PhoneInput(int x, int y, int w, int h, const char *l=0) 
    : Fl_Input(x,y,w,h,l) {}
  
  int handle(int e) {
    if (e == FL_KEYBOARD) {
      char c = Fl::event_text()[0];
      // Only allow digits and dashes
      if (!isdigit(c) && c != '-' && c != '(' && c != ')' && c != ' ') {
        return 1;  // Consume event, don't insert
      }
    }
    return Fl_Input::handle(e);
  }
};

Complete Example: Form Validation

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Input.H>
#include <FL/Fl_Int_Input.H>
#include <FL/Fl_Button.H>
#include <FL/fl_ask.H>
#include <string.h>

Fl_Input *name_input;
Fl_Input *email_input;
Fl_Int_Input *age_input;

void submit_cb(Fl_Widget*, void*) {
  const char *name = name_input->value();
  const char *email = email_input->value();
  const char *age = age_input->value();
  
  if (strlen(name) == 0) {
    fl_alert("Please enter your name");
    return;
  }
  
  if (!strchr(email, '@')) {
    fl_alert("Please enter a valid email");
    return;
  }
  
  if (strlen(age) == 0 || atoi(age) < 1) {
    fl_alert("Please enter a valid age");
    return;
  }
  
  fl_message("Form submitted!\nName: %s\nEmail: %s\nAge: %s", 
             name, email, age);
}

int main() {
  Fl_Window *win = new Fl_Window(400, 200, "Form Example");
  
  name_input = new Fl_Input(100, 20, 280, 30, "Name:");
  email_input = new Fl_Input(100, 60, 280, 30, "Email:");
  age_input = new Fl_Int_Input(100, 100, 280, 30, "Age:");
  
  Fl_Button *submit = new Fl_Button(150, 150, 100, 30, "Submit");
  submit->callback(submit_cb);
  
  win->end();
  win->show();
  return Fl::run();
}

Next Steps

Learn about callbacks
Explore flex layouts
See custom widgets

Basic Examples

Layout Examples

Advanced Examples

Input Field Examples

What This Example Demonstrates

Complete Source Code

Compilation Command

Expected Behavior

Key Concepts

Input Field Types

Getting and Setting Values

Callback Timing (when)

Keyboard Shortcuts

Variations and Extensions

Input Validation

Read-Only Input

Output Widget (Non-Editable Display)

Multiline Features

Text Selection

Colors and Styling

Input Masks and Filters

Complete Example: Form Validation

Next Steps

Basic Examples

Layout Examples

Advanced Examples

Documentation Index

​What This Example Demonstrates

​Complete Source Code

​Compilation Command

​Expected Behavior

​Key Concepts

​Input Field Types

​Getting and Setting Values

​Callback Timing (when)

​Keyboard Shortcuts

​Variations and Extensions

​Input Validation

​Read-Only Input

​Output Widget (Non-Editable Display)

​Multiline Features

​Text Selection

​Colors and Styling

​Input Masks and Filters

​Complete Example: Form Validation

​Next Steps

What This Example Demonstrates

Complete Source Code

Compilation Command

Expected Behavior

Key Concepts

Input Field Types

Getting and Setting Values

Callback Timing (when)

Keyboard Shortcuts

Variations and Extensions

Input Validation

Read-Only Input

Output Widget (Non-Editable Display)

Multiline Features

Text Selection

Colors and Styling

Input Masks and Filters

Complete Example: Form Validation

Next Steps