Improve docs and add two new Fl_Menu_Item methods (#875)

This addresses some issues pointed out by GitHub Issue #875.

Documentation lacked details about Fl_Multi_Label assignment and
correct memory handling.

The new methods
 - Fl_Menu_Item::image_label(const Fl_Image *) and
 - Fl_Menu_Item::multi_label(const Fl_Multi_Label *)
provide a cleaner interface to assign images and Fl_Multi_Label's
to menu items.

examples/howto-menu-with-images.cxx: carify some issues, fix leak,
  and use new Fl_Menu_Item::multi_label(const Fl_Multi_Label *).

4 files changed, 220 insertions, 65 deletions

diff --git a/FL/Fl_Menu_Item.H b/FL/Fl_Menu_Item.H
index 4bca0e1ce..94035d097 100644
--- a/FL/Fl_Menu_Item.H
+++ b/FL/Fl_Menu_Item.H
@@ -17,9 +17,10 @@
 #ifndef Fl_Menu_Item_H
 #define Fl_Menu_Item_H
 
-#  include "Fl_Widget.H"
-#  include "Fl_Image.H"
-#  include <FL/platform_types.h> // for FL_COMMAND and FL_CONTROL
+#include <FL/Fl_Widget.H>
+#include <FL/Fl_Image.H>
+#include <FL/Fl_Multi_Label.H>
+#include <FL/platform_types.h> // for FL_COMMAND and FL_CONTROL
 
 // doxygen needs the following line to enable e.g. ::FL_MENU_TOGGLE to link to the enums
 /// @file
@@ -141,19 +142,100 @@ struct FL_EXPORT Fl_Menu_Item {
 
   // methods on menu items:
   /**
-    Returns the title of the item.
+    Returns the title (label) of the menu item.
+
     A NULL here indicates the end of the menu (or of a submenu).
     A '&' in the item will print an underscore under the next letter,
-    and if the menu is popped up that letter will be a "shortcut" to pick
-    that item.  To get a real '&' put two in a row.
+    and if the menu is popped up that letter will be a "shortcut" to
+    pick that item. To get a real '&' put two in a row.
+
+    The returned value depends on the labeltype() that has been used
+    to store the label.
+
+    \returns  A pointer to the label cast to (const char *)
+
+    \retval (a)   A pointer to text (const char *)
+    \retval (b)   A pointer to an image (Fl_Image *)
+    \retval (c)   A pointer to a "multi label" (Fl_Multi_Label *)
+    \retval NULL  End of menu or submenu
+
+    \see Fl_Menu_Item::label(const char *)
+    \see Fl_Menu_Item::label(Fl_Labeltype, const char *)
+    \see Fl_Menu_Item::image_label(const Fl_Image *)
+    \see Fl_Menu_Item::multi_label(const Fl_Multi_Label *)
+    \see Fl_Multi_Label::label(Fl_Menu_Item *)
+  */
+  const char* label() const { return text; }
+
+  /**
+    Sets the title (label) of the menu item.
+
+    \note This does \b not change the labeltype() for backwards compatibility.
+      Take care to assign the correct labeltype() if you assign different
+      label types to the same menu item sequentially.
+
+    The default labeltype() is FL_NORMAL_LABEL.
+
+    \see label(Fl_Labeltype, const char*)
+    \see const char* Fl_Menu_Item::label() const
+  */
+  void label(const char* a) { text = a; }
+
+  /**
+    Sets the title (label) and the label type of the menu item.
+
+    The default Fl_Labeltype when using Fl_Menu_Item::label(const char* a)
+    is FL_NORMAL_LABEL but you can set any other label type, for instance
+    FL_EMBOSSED_LABEL.
+
+    Special labeltypes are:
+
+      - FL_ICON_LABEL:  draws the icon (Fl_Image) associated with the text
+      - FL_IMAGE_LABEL: draws the image (Fl_Image) associated with the text
+      - FL_MULTI_LABEL: draws multiple parts side by side, see Fl_Multi_Label
+
+    \see const char* Fl_Menu_Item::label() const
   */
-  const char* label() const {return text;}
+  void label(Fl_Labeltype a, const char* b) {
+    labeltype_ = a;
+    text = b;
+  }
+
+  /**
+    Sets the title (label()) and labeltype() to an Fl_Multi_Label.
+
+    This sets the labeltype() to \_FL_MULTI_LABEL (note the leading
+    underscore).
 
-  /**    See const char* Fl_Menu_Item::label() const   */
-  void label(const char* a) {text=a;}
+    \see const char* Fl_Menu_Item::label() const
 
-  /**    See const char* Fl_Menu_Item::label() const   */
-  void label(Fl_Labeltype a,const char* b) {labeltype_ = a; text = b;}
+    \since 1.4.0
+
+    \internal Overloading Fl_Menu_Item::image() would lead to ambiguities
+      when used like `item->label(0)`, hence we need our own method name.
+      Otherwise existing programs might not compile w/o error (e.g. fluid).
+  */
+  void multi_label(const Fl_Multi_Label *ml) {
+    label(FL_MULTI_LABEL, (const char *)ml);
+  }
+
+  /**
+    Sets the title (label()) to an icon or image.
+
+    This sets the labeltype() to \_FL_IMAGE_LABEL (note the leading
+    underscore).
+
+    \see const char* Fl_Menu_Item::label() const
+
+    \since 1.4.0
+
+    \internal Overloading Fl_Menu_Item::image() would lead to ambiguities
+      when used like `item->label(0)`, hence we need our own method name.
+      Otherwise existing programs might not compile w/o error (e.g. fluid).
+  */
+  void image_label(const Fl_Image *image) {
+    label(FL_IMAGE_LABEL, (const char *)image);
+  }
 
   /**
     Returns the menu item's labeltype.
diff --git a/FL/Fl_Multi_Label.H b/FL/Fl_Multi_Label.H
index ff46e32df..7c7f60a5a 100644
--- a/FL/Fl_Multi_Label.H
+++ b/FL/Fl_Multi_Label.H
@@ -1,7 +1,7 @@
 //
 // Multi-label header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2015 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -22,51 +22,57 @@ struct Fl_Menu_Item;
 
 /** Allows a mixed text and/or graphics label to be applied to an Fl_Menu_Item or Fl_Widget.
 
-    Most regular FLTK widgets now support the ability to associate both images and text
-    with a label but some special cases, notably the non-widget Fl_Menu_Item objects, do not.
-    Fl_Multi_Label may be used to create menu items that have an icon and text, which would
-    not normally be possible for an Fl_Menu_Item.
-    For example, Fl_Multi_Label is used in the New->Code submenu in fluid, and others.
+  Most regular FLTK widgets now support the ability to associate both images and text
+  with a label but some special cases, notably the non-widget Fl_Menu_Item objects, do not.
+  Fl_Multi_Label may be used to create menu items that have an icon and text, which would
+  not normally be possible for an Fl_Menu_Item.
+  For example, Fl_Multi_Label is used in the New->Code submenu in fluid, and others.
 
-    \image html Fl_Multi_Label-menu-item.png "Menu items with icons using Fl_Multi_Label"
-    \image latex Fl_Multi_Label-menu-item.png "Menu items with icons using Fl_Multi_Label" width=4cm
+  \image html Fl_Multi_Label-menu-item.png "Menu items with icons using Fl_Multi_Label"
+  \image latex Fl_Multi_Label-menu-item.png "Menu items with icons using Fl_Multi_Label" width=4cm
 
-    Each Fl_Multi_Label holds two elements, labela and labelb; each may hold either a
-    text label (const char*) or an image (Fl_Image*). When displayed, labela is drawn first
-    and labelb is drawn immediately to its right.
+  Each Fl_Multi_Label holds two elements, \p labela and \p labelb; each may hold either a
+  text label (const char*) or an image (Fl_Image*). When displayed, \p labela is drawn first
+  and \p labelb is drawn immediately to its right.
 
-    More complex labels might be constructed by setting labelb as another Fl_Multi_Label and
-    thus chaining up a series of label elements.
+  More complex labels can be constructed by setting labelb as another Fl_Multi_Label and
+  thus chaining up a series of label elements.
 
-    When assigning a label element to one of labela or labelb, they should be explicitly cast
-    to (const char*) if they are not of that type already.
+  When assigning a label element to one of \p labela or \p labelb, they should be
+  explicitly cast to (const char*) if they are not of that type already.
 
-    Example Use: Fl_Menu_Bar
-    \code
-      Fl_Pixmap *image = new Fl_Pixmap(..);     // image for menu item; any Fl_Image based widget
-      Fl_Menu_Bar *menu = new Fl_Menu_Bar(..);  // can be any Fl_Menu_ oriented widget (Fl_Choice, Fl_Menu_Button..)
+  \note An Fl_Multi_Label object and all its components (label text, images, chained
+    Fl_Multi_Label's and their linked objects) must exist during the lifetime of the
+    widget or menu item they are assigned to. It is the responsibility of the user's
+    code to release these linked objects if necessary after the widget or menu is deleted.
 
-      // Create a menu item
-      int i = menu->add("File/New", ..);
-      Fl_Menu_Item *item = (Fl_Menu_Item*)&(menu->menu()[i]);
+  Example Use: Fl_Menu_Bar
+  \code
+    Fl_Pixmap *image = new Fl_Pixmap(..);     // image for menu item; any Fl_Image based widget
+    Fl_Menu_Bar *menu = new Fl_Menu_Bar(..);  // can be any Fl_Menu_ oriented widget (Fl_Choice, Fl_Menu_Button..)
 
-      // Create a multi label, assign it an image + text
-      Fl_Multi_Label *ml = new Fl_Multi_Label;
+    // Create a menu item
+    int i = menu->add("File/New", ..);
+    Fl_Menu_Item *item = (Fl_Menu_Item*)&(menu->menu()[i]);
 
-      // Left side of label is an image
-      ml->typea  = FL_IMAGE_LABEL;
-      ml->labela = (const char*)image;           // any Fl_Image widget: Fl_Pixmap, Fl_PNG_Image, etc..
+    // Create a multi label, assign it an image + text
+    Fl_Multi_Label *ml = new Fl_Multi_Label;
 
-      // Right side of label is label text
-      ml->typeb  = FL_NORMAL_LABEL;
-      ml->labelb = item->label();
+    // Left side of label is an image
+    ml->typea  = FL_IMAGE_LABEL;
+    ml->labela = (const char*)image;           // any Fl_Image widget: Fl_Pixmap, Fl_PNG_Image, etc..
 
-      // Assign the multilabel to the menu item
-      ml->label(item);
-    \endcode
+    // Right side of label is label text
+    ml->typeb  = FL_NORMAL_LABEL;
+    ml->labelb = item->label();
 
-    \see Fl_Label and Fl_Labeltype and examples/howto-menu-with-images.cxx
- */
+    // Assign the multilabel to the menu item
+    // ml->label(item); // deprecated since 1.4.0; backwards compatible with 1.3.x
+    item->label(ml);    // new method since 1.4.0
+  \endcode
+
+  \see Fl_Label and Fl_Labeltype and examples/howto-menu-with-images.cxx
+*/
 struct FL_EXPORT Fl_Multi_Label {
   /** Holds the "leftmost" of the two elements in the composite label.
       Typically this would be assigned either a text string (const char*),
@@ -87,9 +93,10 @@ struct FL_EXPORT Fl_Multi_Label {
     if "chaining" multiple Fl_Multi_Label elements together. */
   uchar typeb;
 
-  /** This method is used to associate a Fl_Multi_Label with a Fl_Widget. */
+  // This method is used to associate a Fl_Multi_Label with a Fl_Widget.
   void label(Fl_Widget*);
-  /** This method is used to associate a Fl_Multi_Label with a Fl_Menu_Item. */
+
+  // This method is used to associate a Fl_Multi_Label with a Fl_Menu_Item.
   void label(Fl_Menu_Item*);
 };
 
diff --git a/examples/howto-menu-with-images.cxx b/examples/howto-menu-with-images.cxx
index e0be5cad4..a5f7e70c0 100644
--- a/examples/howto-menu-with-images.cxx
+++ b/examples/howto-menu-with-images.cxx
@@ -1,10 +1,8 @@
 //
-// vim: autoindent tabstop=2 shiftwidth=2 expandtab softtabstop=2 filetype=cpp
-//
-//     How to use Fl_Multi_Label to make menu items with images and labels.
+// How to use Fl_Multi_Label to make menu items with images and labels.
 //
 // Copyright 2017 Greg Ercolano.
-// Copyright 1998-2021 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -28,6 +26,8 @@
 #include <FL/Fl_Pixmap.H>
 #include <FL/fl_message.H>
 
+#include <stdlib.h>   // free()
+
 // Document icon
 static const char *L_document_xpm[] = {
   "13 11 3 1",
@@ -120,14 +120,20 @@ int AddItemToMenu(Fl_Menu_   *menu,                   // menu to add item to
 
   // Right side of label is text
   ml->typeb  = FL_NORMAL_LABEL;
-  ml->labelb = item->label();
+  ml->labelb = item->label();     // transfers "ownership" to ml
 
   // Assign multilabel to item.
-  // Note: There are two documented ways to achieve this. Both of them
-  // are supported, but one of them is commented out intentionally.
-
-  // item->label(FL_MULTI_LABEL, (const char *)ml);
-  ml->label(item);
+  // There are three documented ways to achieve this. All of them are still
+  // supported but two of them are deprecated (see docs and comments below).
+  // The recommended way is to use Fl_Menu_Item::multi_label(Fl_Multi_Label *)
+  // which was added in FLTK 1.4.0 and sets the correct labeltype().
+  // All of these statements overwrite the old label (pointer) whose
+  // ownership has been "transferred" to the Fl_Multi_Label (correct).
+  // The old label is not released (as documented).
+
+  // ml->label(item);                                 // deprecated (1.3.x)
+  // item->label(FL_MULTI_LABEL, (const char *)ml);   // deprecated (1.3.x)
+  item->multi_label(ml);                              // new since 1.4.0
 
   return i;
 }
@@ -151,29 +157,37 @@ void CreateMenuItems(Fl_Menu_* menu) {
   //    This shows why you need Fl_Multi_Label; the item->label()
   //    gets clobbered by the item->image() setting.
   //
+  // In the following lines 'menu->add(...)' assigns a duplicated label
+  // string which must be released prior to assigning an image to the label
+  // with 'item->image()'.
+  // This is "not nice" but it prevents memory leaks (see GitHub Issue #875).
+
   int i;
   Fl_Menu_Item *item;
 
   i = menu->add("Images/One", 0, Menu_CB, (void*)"One");
   item = (Fl_Menu_Item*)&(menu->menu()[i]);
-  item->image(L_document_pixmap);   // note: this clobbers the item's label()
+  free((void*)item->label());
+  item->image(L_document_pixmap);
 
   i = menu->add("Images/Two", 0, Menu_CB, (void*)"Two");
   item = (Fl_Menu_Item*)&(menu->menu()[i]);
+  free((void*)item->label());
   item->image(L_folder_pixmap);
 
   i = menu->add("Images/Three", 0, Menu_CB, (void*)"Three");
   item = (Fl_Menu_Item*)&(menu->menu()[i]);
+  free((void*)item->label());
   item->image(L_redx_pixmap);
 }
 
-int main() {
+int main(int argc, char **argv) {
   Fl_Double_Window *win = new Fl_Double_Window(400, 400, "Menu items with images");
   win->tooltip("Right click on window background\nfor popup menu");
 
   // Help message
   Fl_Box *box = new Fl_Box(100,100,200,200);
-  box->copy_label(win->tooltip());
+  box->label(win->tooltip()); // no need to copy_label() because it's static
   box->align(FL_ALIGN_CENTER|FL_ALIGN_INSIDE);
 
   // Menu bar
@@ -192,12 +206,11 @@ int main() {
 
   // TODO: Show complex labels with Fl_Multi_Label. From docs:
   //
-  //     "More complex labels might be constructed by setting labelb as another
+  //     "More complex labels can be constructed by setting labelb as another
   //     Fl_Multi_Label and thus chaining up a series of label elements."
-  //
 
   win->end();
   win->resizable(win);
-  win->show();
+  win->show(argc, argv);
   return Fl::run();
 }
diff --git a/src/Fl_Multi_Label.cxx b/src/Fl_Multi_Label.cxx
index 72074bc12..29bbcfe61 100644
--- a/src/Fl_Multi_Label.cxx
+++ b/src/Fl_Multi_Label.cxx
@@ -1,7 +1,7 @@
 //
 // Multi-label widget for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2017 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -60,10 +60,63 @@ Fl_Labeltype fl_define_FL_MULTI_LABEL() {
   return _FL_MULTI_LABEL;
 }
 
+/**
+  Associate an Fl_Multi_Label with an Fl_Widget.
+
+  This method uses Fl_Widget::label(Fl_Labeltype, const char *) internally
+  to set the \e label of the widget, i.e. it stores a \e pointer to the
+  Fl_Multi_Label object (\e this).
+  An existing label that has been set using Fl_Widget::copy_label() will be
+  released prior to the assignment of the new label.
+
+  This sets the type of the widget's label to \_FL_MULTI_LABEL - note the
+  leading underscore ('_').
+
+  There is no way to use a method like Fl_Widget::copy_label() that transfers
+  ownership of the Fl_Multi_Label and its linked objects (images, text, and
+  chained Fl_Multi_Label's) to the widget.
+
+  The Fl_Multi_Label and all linked images, text labels, or chained Fl_Multi_Label
+  objects must exist during the lifetime of the widget and will not be released
+  when the widget is destroyed.
+
+  \note The user's code is responsible for releasing the Fl_Multi_Label and
+    all linked objects (images, text, chained Fl_Multi_Label's) after the
+    widget has been deleted. This may cause memory leaks if Fl_Multi_Label
+    is used and reassigned w/o releasing the objects assigned to it.
+*/
 void Fl_Multi_Label::label(Fl_Widget* o) {
   o->label(FL_MULTI_LABEL, (const char*)this); // calls fl_define_FL_MULTI_LABEL()
 }
 
+/**
+  Associate an Fl_Multi_Label with an Fl_Menu_Item.
+
+  This uses Fl_Menu_Item::label(Fl_Labeltype a, const char *b) internally to
+  set the \e label and the label type of the menu item, i.e. it stores a
+  \e pointer to the Fl_Multi_Label object (\e this).
+  An existing label (pointer) will be overwritten.
+
+  This sets the type of the menu item's label to \_FL_MULTI_LABEL - note the
+  leading underscore ('_').
+
+  There is no way to use a method like Fl_Widget::copy_label() that transfers
+  ownership of the Fl_Multi_Label and its linked objects (images, text, and
+  chained Fl_Multi_Label's) to the menu item.
+
+  The Fl_Multi_Label and all linked images, text labels, or chained
+  Fl_Multi_Label objects must exist during the lifetime of the menu and
+  will not be released when the menu item is destroyed.
+
+  \note The user's code is responsible for releasing the Fl_Multi_Label and
+    all linked objects (images, text, chained Fl_Multi_Label's) after the
+    menu has been deleted. This may cause memory leaks if Fl_Multi_Label
+    is used and reassigned w/o releasing the objects assigned to it.
+
+  \deprecated since 1.4.0: please use Fl_Menu_Item::label(Fl_Multi_Label *)
+
+  \see Fl_Menu_Item::label(Fl_Multi_Label *)
+*/
 void Fl_Multi_Label::label(Fl_Menu_Item* o) {
   o->label(FL_MULTI_LABEL, (const char*)this); // calls fl_define_FL_MULTI_LABEL()
 }