mud/lib/doc/manual/chapter14

chapter 14 "Intermediate Inheritance"
Intermediate LPC
Descartes of Borg
November 1993

                       Chapter 6: Intermediate Inheritance

6.1 Basics of Inheritance
In the textbook LPC Basics, you learned how it is the mudlib maintains
consistency amoung mud objects through inheritance.  Inheritance
allows the mud administrators to code the basic functions and such that
all mudlib objects, or all mudlib objects of a certain type must have so
that you can concentrate on creating the functions which make these
objects different.  When you build a room, or a weapon, or a monster,
you are taking a set of functions already written for you and inheriting
them into your object.  In this way, all objects on the mud can count on
other objects to behave in a certain manner.  For instance, player objects
can rely on the fact that all room objects will have a function in them
called GetLong() which describes the room.  Inheritance thus keeps
you from having to worry about what the function GetLong() should
look like.

Naturally, this textbook tries to go beyond this fundamental knowledge
of inheritance to give the coder a better undertstanding of how
inheritance works in LPC programming.  Without getting into detail that
the advanced domain coder/beginner mudlib coder simply does not yet
need, this chapter will try to explain exactly what happens when you
inherit an object.

6.2 Cloning and Inheritance
Whenever a file is referenced for the first time as an object (as opposed
to reading the contents of the file), the game tries to load the file into
memory and create an object.  If the object is successfully loaded into
memory, it becomes as master copy.  Master copies of objects may be
cloned but not used as actual game objects.  The master copy is used to
support any clone objects in the game.

The master copy is the source of one of the controversies of mud LPC
coding, that is whether to clone or inherit.  With rooms, there is no
question of what you wish to do, since there should only be one instance
of each room object in the game.  So you generally use inheritance in
creating rooms.  Many mud administrators, including myself, however
encourage creators to clone the standard monster object and configure it
from inside room objects instead of keeping monsters in separate files
which inherit the standard monster object.

As I stated above, each time a file is referenced to create an object, a
master copy is loaded into memory.  When you do something like:
void reset() {
    object ob;
    ob = new("/std/monster");
      /* clone_object("/std/monster") some places */
    ob->SetKeyName("foo monster");
    ...  rest of monster config code followed by moving
it to the room ...
}
the driver searches to see if their is a master object called "/std/monster". 
If not, it creates one.  If it does exist, or after it has been created, the
driver then creates a clone object called "/std/monster#<number>".  If
this is the first time "/std/monster" is being referenced, in effect, two
objects are being created: the master object and the cloned instance.

On the other hand, let's say you did all your configuring in the create()
of a special monster file which inherits "/std/monster".  Instead of
cloning the standard monster object from your room, you clone your
monster file.  If the standard monster has not been loaded, it gets loaded
since your monster inherits it.  In addition, a master copy of your file
gets loaded into memory.  Finally, a clone of your monster is created
and moved into the room, for a total of three objects added to the game. 
Note that you cannot make use of the master copy easily to get around
this.  If, for example, you were to do:
    "/realms/descartes/my_monster"->eventMove(this_object());
instead of
    new("/realms/descartes/my_monster")->eventMove(this_object());
you would not be able to modify the file "my_monster.c" and update it,
since the update command destroys the current master version of an
object.  On some mudlibs it also loads the new version into memory. 
Imagine the look on a player's face when their monster disappears in
mid-combat cause you updated the file!

Cloning is therefore a useful too when you plan on doing just that-
cloning.  If you are doing nothing special to a monster which cannot be
done through a few call others, then you will save the mud from getting
loaded with useless master copies.  Inheritance, however, is useful if
you plan to add functionality to an object (write your own functions) or
if you have a single configuration that gets used over and over again
(you have an army of orc guards all the same, so you write a special orc
file and clone it).

6.3 Inside Inheritance
When objects A and B inherit object C, all three objects have their own
set of data sharing one set of function definitions from object C.  In
addition, A and B will have separate functions definitions which were
entered separately into their code.  For the sake of example throughout
the rest of the chapter, we will use the following code.  Do not be
disturbed if, at this point, some of the code makes no sense:

STD_ITEM C
private string name, cap_name, short, long;
private int setup;

void SetKeyName(string str)
nomask string GetKeyName();
private int query_setup();
static void unsetup();
void SetShort(string str);
string GetShort();
void SetLong(string str);
string GetLong();


void SetKeyName(string str) { 
    if(!query_setup()) {
        name = str;
    setup = 1;
}

nomask string GetKeyName() { return name; }

private query_setup() { return setup; }

static void unsetup() { setup = 0; }

string GetName() {
    return (name ? capitalize(name) : ""); }
}

void SetShort(string str) { short = str; }

string GetShort() { return short; }

void SetLong(string str) { long = str; }

string GetLong() { return str; }

void create() { seteuid(getuid()); }

STD_ITEM B
inherit "/std/objectc";

private int wc;

void set_wc(int wc);
int query_wc();
int wieldweapon(string str);

void create() { ::create(); }

void init() {
    if(environment(this_object()) == this_player())
      add_action("wieldweapon", "wield");
}

void set_wc(int x) { wc = x; }

int query_wc() { return wc; }

int wieldweapon(string str) {
    ... code for wielding the weapon ...
}

STD_ITEM A
inherit "/std/objectc";

int ghost;

void create() { ::create(); }

void change_name(string str) {
    if(!((int)this_object()->is_player())) unsetup();
    SetKeyName(str);
}

string GetName() {
    if(ghost) return "A ghost";
    else return ::GetName();
}

As you can see, object C is inherited both by object A and object B. 
Object C is a representation of a much oversimplified base object, with B
being an equally oversimplified weapon and A being an equally
simplified living object.  Only one copy of each function is retained in
memory, even though we have here three objects using the functions. 
There are of course, three instances of the variables from Object C in
memory, with one instance of the variables of Object A and Object B in
memory.  Each object thus gets its own data.  

6.4 Function and Variable Labels
Notice that many of the functions above are proceeded with labels which
have not yet appeared in either this text or the beginner text, the labels
static, private, and nomask.  These labels define special priveledges
which an object may have to its data and member functions.  Functions
you have used up to this point have the default label public.  This is
default to such a degree, some drivers do not support the labeling.

A public variable is available to any object down the inheritance tree
from the object in which the variable is declared.  Public variables in
object C may be accessed by both objects A and B.  Similarly, public
functions may be called by any object down the inheritance tree from the
object in which they are declared. 

The opposite of public is of course private.  A private variable or
function may only be referenced from inside the object which declares it. 
If object A or B tried to make any reference to any of the variables in
object C, an error would result, since the variables are said to be out of
scope, or not available to inheriting classes due to their private labels. 
Functions, however, provide a unique challenge which variables do not. 
External objects in LPC have the ability to call functions in other objects
through call others.  The private label does not protect against call
others.

To protect against call others, functions use the label static.  A function
which is static may only be called from inside the complete object or
from the game driver.  By complete object, I mean object A can call
static functions in the object C it inherits.  The static only protects against
external call others.  In addition, this_object()->foo() is considered an
internal call as far as the static label goes.

Since variables cannot be referenced externally, there is no need for an
equivalent label for them.  Somewhere along the line, someone decided
to muddy up the waters and use the static label with variables to have a
completely separate meaning.  What is even more maddening is that this
label has nothing to do with what it means in the C programming
language.  A static variable is simply a variable that does not get saved to
file through the efun save_object() and does not get restored through
restore_object().  Go figure.

In general, it is good practice to have private variables with public
functions, using query_*() functions to access the values of inherited
variables, and set_*(), add_*(), and other such functions to change
those values.  In realm coding this is not something one really has to
worry a lot about.  As a matter of fact, in realm coding you do not have
to know much of anything which is in this chapter.  To be come a really
good realm coder, however, you have to be able to read the mudlib
code.  And mudlib code is full of these labels.  So you should work
around with these labels until you can read code and understand why it
is written that way and what it means to objects which inherit the code.

The final label is nomask, and it deals with a property of inheritance
which allows you to rewrite functions which have already been defined. 
For example, you can see above that object A rewrote the function
GetName().  A rewrite of  function is called overriding the
function.  The most common override of a function would be in a case
like this, where a condition peculiar to our object (object A) needs to
happen on a call ot the function under certain circumstances.  Putting test
code into object C just so object A can be a ghost is plain silly.  So
instead, we override GetName() in object A, testing to see if the
object is a ghost.  If so, we change what happens when another object
queries for the cap name.  If it is not a ghost, then we want the regular
object behaviour to happen.  We therefore use the scope resolution
operator (::) to call the inherited version of the GetName()
function and return its value.

A nomask function is one which cannot be overridden either through
inheritance or through shadowing.  Shadowing is a sort of backwards
inheritance which will be detailed in the advanced LPC textbook.  In the
example above, neither object A nor object B (nor any other object for
that matter) can override GetKeyName().  Since we want to use
GetKeyName() as a unique identifier of objects, we don't want people
faking us through shadowing or inheritance.  The function therefore gets
the nomask label.

6.5 Summary
Through inheritance, a coder may make user of functions defined in
other objects in order to reduce the tedium of producing masses of
similar objects and to increase the consistency of object behaviour across
mudlib objects.  LPC inheritance allows objects maximum priveledges in
defining how their data can be accessed by external objects as well as
objects inheriting them.  This data security is maintained through the
keywords, nomask, private, and static.

In addition, a coder is able to change the functionality of non-protected
functions by overriding them.  Even in the process of overriding a
function, however, an object may access the original function through
the scope resolution operator.

Copyright (c) George Reese 1993