Overriding OM SET() - Revision history

Krashan: Formatting.

2011-01-26T07:39:16Z

Formatting.

Krashan: Minor style fixes, formatting.

2011-01-26T07:32:21Z

Minor style fixes, formatting.

Krashan: Formatting.

2011-01-26T07:23:28Z

Formatting.

Krashan: Link to Polish version.

2011-01-26T06:53:50Z

Link to Polish version.

AusPPC at 05:57, 7 January 2011

2011-01-07T05:57:01Z

Krashan: Code fixed (NextTagItem() call).

2011-01-05T21:15:17Z

Code fixed (NextTagItem() call).

Krashan: New article.

2011-01-03T13:37:09Z

New article.

New page

''Grzegorz Kraszewski''

The ''OM_SET()'' method receives an ''opSet'' structure as its message. The structure is defined in the ''<intuition/classusr.h>'' header.

struct opSet
{
ULONG MethodID; /* always OM_SET (0x103) */
struct TagItem *ops_AttrList;
struct GadgetInfo *ops_GInfo;
};

The most important field is ''ops_AttrList''. It is a pointer to a [[Taglists|taglist]] containing attributes and values to be set. The ''ops_GInfo'' field is an obsolete legacy thing and is not used by modern components like MUI or Reggae. The method implementation should iterate the taglist and set all attributes recognized. The operation of setting an attribute may be just setting some field in an object instance data, it may also trigger some actions (like for example object redrawing). It is recommended however that complex actions are implemented as methods rather than attribute changes. A reference implementation of ''OM_SET()'' may look like this:

IPTR MyClassSet(Class *cl, Object *obj, struct opSet *msg)
{
struct TagItem *tag, *tagptr;
IPTR tagcount = 0;

tagptr = msg->ops_AttrList;

while (tag = '''NextTagItem'''(&tagptr))
{
switch (tag->ti_Tag)
{
case SOME_TAG:
/* attribute setting actions for SOME_TAG */
tagcount++;
break;

/* more tags here */
}
}

tagcount += '''DoSuperMethodA'''(cl, obj, (Msg)msg);
return tagcount;
}

The taglist iteration is done with ''NextTagItem()'' function from utility.library. The function returns pointer to a next tag each time it is called and keeps the current position in ''tagptr''. The advantage of this function is automatic handling of special tag values (''TAG_MORE'', ''TAG_IGNORE'', ''TAG_SKIP''), they are not returned, but their actions are performed instead.

The ''OM_SET()'' function returns the total number of recognized tags. It is implemented with ''tagcounter''. It gets incremented on every tag recognized and finally number of tags recognized by superclass(es) is added.

Common bugs in ''OM_SET()'' implementation are: ignoring tag counting and calling super method in the ''default'' case of ''switch'' statement. The second bug causes the supermethod to be called multiple times, once for every tag not handled by the subclass.

----
<small>In some rare cases a subclass may want to override an attribute completely, so it is not passed to superclasses. It can be done by replacing the tag (not value!) by ''TAG_IGNORE''. There is one caveat however. It most cases in C and C++, the taglist is built dynamically on the stack from variable arguments of a function like ''SetAttrs()''. It is possible however, that a taglist is a static object (for example a global one, or created in an allocated chunk of free memory). In this case changing a tag is a '''permanent''' operation, which may have unexpected results. This remark also applies for changing a value of a tag before passing it to a superclass. A safe solution is to clone the taglist with ''CloneTagItems()'' function from ''utility.library''. Then changes are made in the copy and this copy is passed to superclass. The copy is then freed with ''FreeTagItems()''. The disadvantage of this solution is that cloning a taglist may fail due to lack of free memory and this possibility must be handled somehow.</small>

← Older revision		Revision as of 07:39, 26 January 2011
Line 49:		Line 49:

	----		----
−	<small>In some rare cases a subclass may want to override an attribute completely, so it is not passed to superclasses. This can be done by replacing the tag (not value!) by ''TAG_IGNORE''. There is one caveat however. In most cases in C and C++, the taglist is built dynamically on the stack from variable arguments of a function like ''SetAttrs()''. It is possible however, that a taglist is a static object (for example a global one, or created in an allocated chunk of free memory). In this case changing a tag is a '''permanent''' operation, which may have unexpected results. This remark also applies for changing a value of a tag before passing it to a superclass. A safe solution is to clone the taglist with the ''CloneTagItems()'' function from the ''utility.library''. Then changes are made in the copy and this copy is passed to the superclass. The copy is then freed with ''FreeTagItems()''. The disadvantage of this solution is that cloning a taglist may fail due to lack of free memory and this possibility must be handled somehow.</small>	+	<small>In some rare cases a subclass may want to override an attribute completely, so it is not passed to superclasses. This can be done by replacing the tag (not value!) by ''TAG_IGNORE''. There is one caveat however. In most cases in C and C++, the taglist is built dynamically on the stack from variable arguments of a function like ''SetAttrs()''. It is possible however, that a taglist is a static object (for example a global one, or created in an allocated chunk of free memory). In this case changing a tag is a '''permanent''' operation, which may have unexpected results. This remark also applies for changing a '''value''' of a tag before passing it to a superclass. A safe solution is to clone the taglist with the ''CloneTagItems()'' function from the ''utility.library''. Then changes are made in the copy and this copy is passed to the superclass. The copy is then freed with ''FreeTagItems()''. The disadvantage of this solution is that cloning a taglist may fail due to lack of free memory and this possibility must be handled somehow.</small>

@@ Line 43: / Line 43: @@
 The ''OM_SET()'' function returns the total number of recognized tags. It is implemented with ''tagcounter''. It gets incremented on every tag recognized and finally the number of tags recognized by superclass(es) is added.
-Common bugs in ''OM_SET()'' implementation are: ignoring tag counting and calling the super method in the ''default'' case of a ''switch'' statement. The second bug causes the supermethod to be called multiple times, once for every tag not handled by the subclass.
+Common bugs in ''OM_SET()'' implementation are:
 ----
 <small>In some rare cases a subclass may want to override an attribute completely, so it is not passed to superclasses. This can be done by replacing the tag (not value!) by ''TAG_IGNORE''. There is one caveat however. In most cases in C and C++, the taglist is built dynamically on the stack from variable arguments of a function like ''SetAttrs()''. It is possible however, that a taglist is a static object (for example a global one, or created in an allocated chunk of free memory). In this case changing a tag is a '''permanent''' operation, which may have unexpected results. This remark also applies for changing a value of a tag before passing it to a superclass. A safe solution is to clone the taglist with the ''CloneTagItems()'' function from the ''utility.library''. Then changes are made in the copy and this copy is passed to the superclass. The copy is then freed with ''FreeTagItems()''. The disadvantage of this solution is that cloning a taglist may fail due to lack of free memory and this possibility must be handled somehow.</small>

@@ Line 39: / Line 39: @@
+  }
-The taglist iteration is done with the ''NextTagItem()'' function from the utility.library. The function returns a pointer to the next tag each time it is called and keeps the current position in ''tagptr''. The advantage of this function is automatic handling of special tag values (''TAG_MORE'', ''TAG_IGNORE'', ''TAG_SKIP''), they are not returned, but their actions are performed instead.
+The taglist iteration is done with the ''NextTagItem()'' function from the ''utility.library''. The function returns a pointer to the next tag each time it is called and keeps the current position in ''tagptr''. The advantage of this function is automatic handling of special tag values (''TAG_MORE'', ''TAG_IGNORE'', ''TAG_SKIP''), they are not returned, but their actions are performed instead.
 The ''OM_SET()'' function returns the total number of recognized tags. It is implemented with ''tagcounter''. It gets incremented on every tag recognized and finally the number of tags recognized by superclass(es) is added.

← Older revision		Revision as of 06:53, 26 January 2011
Line 1:		Line 1:
	''Grzegorz Kraszewski''		''Grzegorz Kraszewski''
		+	----
		+	<small>This article in other languages: [[Przeciążanie metody OM_SET()\|Polish]]</small>

@@ Line 37: / Line 37: @@
+  }
-The taglist iteration is done with ''NextTagItem()'' function from utility.library. The function returns pointer to a next tag each time it is called and keeps the current position in ''tagptr''. The advantage of this function is automatic handling of special tag values (''TAG_MORE'', ''TAG_IGNORE'', ''TAG_SKIP''), they are not returned, but their actions are performed instead.
+The taglist iteration is done with the ''NextTagItem()'' function from the utility.library. The function returns a pointer to the next tag each time it is called and keeps the current position in ''tagptr''. The advantage of this function is automatic handling of special tag values (''TAG_MORE'', ''TAG_IGNORE'', ''TAG_SKIP''), they are not returned, but their actions are performed instead.
-The ''OM_SET()'' function returns the total number of recognized tags. It is implemented with ''tagcounter''. It gets incremented on every tag recognized and finally number of tags recognized by superclass(es) is added.
+The ''OM_SET()'' function returns the total number of recognized tags. It is implemented with ''tagcounter''. It gets incremented on every tag recognized and finally the number of tags recognized by superclass(es) is added.
-Common bugs in ''OM_SET()'' implementation are: ignoring tag counting and calling super method in the ''default'' case of ''switch'' statement. The second bug causes the supermethod to be called multiple times, once for every tag not handled by the subclass.
+Common bugs in ''OM_SET()'' implementation are: ignoring tag counting and calling the super method in the ''default'' case of a ''switch'' statement. The second bug causes the supermethod to be called multiple times, once for every tag not handled by the subclass.
 ----
-<small>In some rare cases a subclass may want to override an attribute completely, so it is not passed to superclasses. It can be done by replacing the tag (not value!) by ''TAG_IGNORE''. There is one caveat however. It most cases in C and C++, the taglist is built dynamically on the stack from variable arguments of a function like ''SetAttrs()''. It is possible however, that a taglist is a static object (for example a global one, or created in an allocated chunk of free memory). In this case changing a tag is a '''permanent''' operation, which may have unexpected results. This remark also applies for changing a value of a tag before passing it to a superclass. A safe solution is to clone the taglist with ''CloneTagItems()'' function from ''utility.library''. Then changes are made in the copy and this copy is passed to superclass. The copy is then freed with ''FreeTagItems()''. The disadvantage of this solution is that cloning a taglist may fail due to lack of free memory and this possibility must be handled somehow.</small>
+<small>In some rare cases a subclass may want to override an attribute completely, so it is not passed to superclasses. This can be done by replacing the tag (not value!) by ''TAG_IGNORE''. There is one caveat however. In most cases in C and C++, the taglist is built dynamically on the stack from variable arguments of a function like ''SetAttrs()''. It is possible however, that a taglist is a static object (for example a global one, or created in an allocated chunk of free memory). In this case changing a tag is a '''permanent''' operation, which may have unexpected results. This remark also applies for changing a value of a tag before passing it to a superclass. A safe solution is to clone the taglist with the ''CloneTagItems()'' function from the ''utility.library''. Then changes are made in the copy and this copy is passed to the superclass. The copy is then freed with ''FreeTagItems()''. The disadvantage of this solution is that cloning a taglist may fail due to lack of free memory and this possibility must be handled somehow.</small>

@@ Line 20: / Line 20: @@
     tagptr = msg->ops_AttrList;
-    while (tag = '''NextTagItem'''(&tagptr))
+    while ((tag = '''NextTagItem'''(&tagptr)) != NULL)
+    {
       switch (tag->ti_Tag)