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-% Copyright 2018 by Till Tantau
-%
-% This file may be distributed and/or modified
-%
-% 1. under the LaTeX Project Public License and/or
-% 2. under the GNU Free Documentation License.
-%
-% See the file doc/generic/pgf/licenses/LICENSE for more details.
-
-
-\section{Object-Oriented Programming}
-\label{section-oop}
-
-This section describes the |oo| module.
-
-\begin{pgfmodule}{oo}
- This module defines a relatively small set of \TeX\ commands for defining
- classes, methods, attributes and objects in the sense of object-oriented
- programming.
-\end{pgfmodule}
-
-In this chapter it is assumed that you are familiar with the basics of a
-typical object-oriented programming language like Java, C++ or Eiffel.
-
-
-\subsection{Overview}
-
-\TeX\ does not support object-oriented programming, presumably because it was
-written at a time when this style of programming was not yet ``en vogue''. When
-one is used to the object-oriented style of thinking, some programming
-constructs in \TeX\ often seem overly complicated. The object-oriented
-programming module of \pgfname\ may help here. It is written completely using
-simple \TeX\ macros and is, thus, perfectly portable. This also means, however,
-that it is not particularly fast (but not too slow either), so you should use
-it only for non-time-critical things.
-
-Basically, the oo-system supports \emph{classes} (in the object-oriented sense,
-this has nothing to do with \LaTeX-classes), \emph{methods},
-\emph{constructors}, \emph{attributes}, \emph{objects}, \emph{object
-identities}, and (thanks to Sa\v so \v Zivanovi\'c) \emph{inheritance} and
-\emph{overloading.}
-
-The first step is to define a class, using the macro |\pgfooclass| (all normal
-macros in \pgfname's object-oriented system start with |\pgfoo|). This macro
-gets the name of a class and in its body a number of \emph{methods} are
-defined. These are defined using the |\method| macro (which is defined only
-inside such a class definition) and they look a bit like method definitions in,
-say, Java. Object attributes are declared using the |\attribute| command, which
-is also defined only inside a class definition.
-
-Once a class has been defined, you can create objects of this class. Objects
-are created using |\pgfoonew|. Such an object has many characteristics of
-objects in a normal object-oriented programming language: Each object has a
-\emph{unique identity}, so when you create another object, this object is
-completely distinct from all other objects. Each object also has a set of
-private attributes, which may change over time. Suppose, for instance, that we
-have a |point| class. Then creating a new object (called an instance) of this
-class would typically have an |x|-attribute and a |y|-attribute. These can be
-changed over time. Creating another instance of the |point| class creates
-another object with its own |x|- and |y|-attributes.
-
-Given an object, you can call a method for this object. Inside the method the
-attributes of the object for which the method is being called can be accessed.
-
-The life of an object always ends with the end of the \TeX\ scope in which it
-was created. However, changes to attribute values are not local to scopes, so
-when you change an attribute anywhere, this change persists till the end of the
-life of the object or until the attribute is changed again.
-
-
-\subsection{A Running Example: The Stamp Class}
-
-As a running example we will develop a |stamp| class and |stamp| objects. The
-idea is that a stamp object is able to ``stamp something'' on a picture. This
-means that a stamp object has an attribute storing the ``stamp text'' and there
-is a method that asks the object to place this text somewhere on a canvas. The
-method can be called repeatedly and there can be several different stamp
-objects, each producing a different text. Stamp objects can either be created
-dynamically when needed or a library might define many such objects in an outer
-scope.
-
-Such stamps are similar to many things present in \pgfname\ such as arrow tips,
-patterns, or shadings and, indeed, these could all have been implemented in
-this object-oriented fashion (which might have been better, but the
-object-oriented subsystem is a fairly new addition to \pgfname).
-
-
-\subsection{Classes}
-
-We start with the definition of the |stamp| class. This is done using the
-|\pgfooclass| macro:
-
-\begin{command}{\pgfooclass\opt{|(|\meta{list of superclasses}|)|}\marg{class name}\marg{body}}
- This command defines a class named \meta{class name}. The name of the class
- can contain spaces and most other characters, but no periods. So, valid
- class names are |MyClass| or |my class| or |Class_C++_emulation??1|. The
- \meta{list of superclasses} is optional just like the parenthesis around
- it.
-
- The \meta{body} is actually just executed, so any normal \TeX-code is
- permissible here. However, while the \meta{body} is being executed, the
- macros |\method| and |\attribute| are set up so that they can be used to
- define methods and attributes for this class (the original meanings are
- restored afterward).
-
- The definition of a class is local to the scope where the class has been
- defined.
- %
-\begin{codeexample}[code only]
-\pgfooclass{stamp}{
- % This is the class stamp
-
- \attribute text;
- \attribute rotation angle=20;
-
- \method stamp(#1) { % The constructor
- ...
- }
-
- \method apply(#1,#2) { % Causes the stamp to be shown at coordinate (#1,#2)
- ...
- }
-}
-
-% We can now create objects of type "stamp"
-\end{codeexample}
-
- Concerning the list of base classes, the Method Resolution Order
- (\textsc{mro}) is computed using the C3 algorithm also used in Python, v2.3
- and higher. The linearization computed by the algorithm respects both
- local precedence ordering and monotonicity. Resolution of both methods and
- attributes depends on the \textsc{mro}: when a method method name is called
- on an object of class $C$, the system invokes method method name from the
- first class in the \textsc{mro} of $C$ which defines method method name;
- when an object is created, each attribute |attr| is initialized to the
- value specified in the first class in the \textsc{mro} of $C$ which
- declares attribute |attr|.
-\end{command}
-
-The \meta{body} of a class usually just consists of calls to the macros
-|\attribute| and |\method|, which will be discussed in more detail in later
-sections.
-
-
-\subsection{Objects}
-
-Once a class has been declared, we can start creating objects for this class.
-For this the |\pgfoonew| command can be used, which has a peculiar syntax:
-
-\begin{command}{\pgfoonew\opt{\meta{object handle or attribute}|=|}|new |\meta{class name}|(|\meta{constructor arguments}|)|}
- Causes a new object to be created. The class of the object will be
- \meta{class name}, which must previously have been declared using
- |\pgfooclass|. Once the object has been created, the constructor method of
- the object will be called with the parameter list set to \meta{constructor
- arguments}.
-
- The resulting object is stored internally and its lifetime will end exactly
- at the end of the current scope.
-
- Here is an example in which three stamp objects are created.
- %
-\begin{codeexample}[code only]
-\pgfoonew \firststamp=new stamp()
-\pgfoonew \secondstamp=new stamp()
-{
- \pgfoonew \thirdstamp=new stamp()
- ...
-}
-% \thirdstamp no longer exists, but \firststamp and \secondstamp do
-% even if you try to store \thirdstamp in a global variable, trying
-% to access it will result in an error.
-\end{codeexample}
-
- The optional \meta{object handle or attribute} can either be an
- \meta{object handle} or an \meta{attribute}. When an \meta{object handle}
- is given, it must be a normal \TeX\ macro name that will ``point'' to the
- object (handles are discussed in more detail in
- Section~\ref{section-identities}). You can use this macro to call methods
- of the object as discussed in the following section. When an
- \meta{attribute} is given, it must be given in curly braces (the curly
- braces are used to detect the presence of an attribute). In this case, a
- handle to the newly created object is stored in this attribute.
- %
-\begin{codeexample}[code only]
-\pgfooclass{foo}
-{
- \attribute stamp obj;
- \attribute another object;
-
- \method foo() {
- \pgfoonew{stamp obj}=new stamp()
- \pgfoonew{another object}=new bar()
- }
- ...
-}
-\end{codeexample}
- %
-\end{command}
-
-\begin{command}{\pgfoogc}
- This command causes the ``garbage collector'' to be invoked. The job of
- this garbage collector is to free the global \TeX-macros that are used by
- ``dead'' objects (objects whose life-time has ended). This macro is called
- automatically after every scope in which an object has been created, so you
- normally do not need to call this macro yourself.
-\end{command}
-
-
-\subsection{Methods}
-
-Methods are defined inside the body of classes using the following command:
-
-\begin{command}{\method \meta{method name}|(|\meta{parameter list}|)|\marg{method body}}
- This macro, which is only defined inside a class definition, defines a new
- method named \meta{method name}. Just like class names, method names can
- contain spaces and other characters, so \meta{method names} like
- |put_stamp_here| or |put stamp here| are both legal.
-
- Three method names are special: First, a method having either the same name
- as the class or having the name |init| is called the \emph{constructor} of
- the class. There are (currently) no destructors; objects simply become
- ``undefined'' at the end of the scope in which they have been created. The
- other two methods are called |get id| and |get handle|, which are always
- automatically defined and which you cannot redefine. They are discussed in
- Section~\ref{section-identities}.
-
- Overloading of methods by differing numbers of parameters is not possible,
- that is, it is illegal to have two methods inside a single class with the
- same name (despite possibly different parameter lists). However, two
- different classes may contain a method with the same name, that is, classes
- form namespaces for methods. Also, a class can (re)implement a method from
- a superclass.
-
- The \meta{method name} must be followed by a \meta{parameter list} in
- parentheses, which must be present even when the \meta{parameter list} is
- empty. The \meta{parameter list} is actually a normal \TeX\ parameter list
- that will be matched against the parameters inside the parentheses upon
- method invocation and, thus, could be something like |#1#2 foo #3 bar.|,
- but a list like |#1,#2,#3| is more customary. By setting the parameter list
- to just |#1| and then calling, say, |\pgfkeys{#1}| at the beginning of a
- method, you can implement Objective-C-like named parameters.
-
- When a method is called, the \meta{body} of the method will be executed.
- The main difference to a normal macro is that while the \meta{body} is
- executed, a special macro called |\pgfoothis| is set up in such a way that
- it references the object for which the method is executed.
-\end{command}
-
-In order to call a method for an object, you first need to create the object
-and you need a handle for this object. In order to invoke a method for this
-object, a special syntax is used that is similar to Java or C++ syntax:
-
-\begin{pgfmanualentry}
- \pgfmanualentryheadline{\meta{object handle}\opt{|.|\meta{super class}}|.|\meta{method name}|(|\meta{parameters}|)|}%
- \pgfmanualbody
- This causes the method \meta{method name} to be called for the object
- referenced by the \meta{object handle}. The method is the one defined in
- the class of the object or, if it is not defined there, the method defined
- in the superclasses of the object's class (if there are several
- superclasses that define the same method, the method resolution order is
- used to determine which one gets called). If the optional \meta{super
- class} is specified, the method implementation of that class will be used
- rather than the implementation in the object's class. The \meta{parameters}
- are matched against the parameters of the method and, then, the method body
- is executed. The execution of the method body is \emph{not} done inside a
- scope, so the effects of a method body persist.
- %
-\begin{codeexample}[code only]
-\pgfooclass{stamp}{
- % This is the class stamp
-
- \method stamp() { % The constructor
- }
-
- \method apply(#1,#2) { % Causes the stamp to be shown at coordinate (#1,#2)
- % Draw the stamp:
- \node [rotate=20,font=\huge] at (#1,#2) {Passed};
- }
-}
-
-\pgfoonew \mystamp=new stamp()
-
-\begin{tikzpicture}
- \mystamp.apply(1,2)
- \mystamp.apply(3,4)
-\end{tikzpicture}
-\end{codeexample}
-
- Inside a method, you can call other methods. If you have a handle for
- another object, you can simply call it in the manner described above. In
- order to call a method of the current object, you can use the special
- object handle |\pgfoothis|.
-
- \begin{command}{\pgfoothis}
- This object handle is well-defined only when a method is being executed.
- There, it is then set to point to the object for which the method is
- being called, which allows you to call another method for the same
- object.
- %
-\begin{codeexample}[code only]
-\pgfooclass{stamp}{
- % This is the class stamp
-
- \method stamp() {}
-
- \method apply(#1,#2) {
- \pgfoothis.shift origin(#1,#2)
-
- % Draw the stamp:
- \node [rotate=20,font=\huge] {Passed};
- }
-
- % Private method:
- \method shift origin(#1,#2) {
- \tikzset{xshift=#1,yshift=#2}
- }
-}
-\end{codeexample}
- \end{command}
-\end{pgfmanualentry}
-
-\begin{command}{\pgfoosuper|(|\meta{class},\meta{object handle}|).|\meta{method name}|(|\meta{arguments}|)|}
- This macro gives you finer control over which method gets invoked in case
- of multiple inheritance. This macro calls \meta{method name} of the object
- specified by \meta{object handle}, but which implementation of the method
- is called is determined as follows: it will be the implementation in the
- first class (in the method resolution order) after \meta{class} that
- defines \meta{method name}.
-\end{command}
-
-
-\subsection{Attributes}
-
-Every object has a set of attributes, which may change over time. Attributes
-are declared using the |\attribute| command, which, like the |\method| command,
-is defined only inside the scope of |\pgfooclass|. Attributes can be modified
-(only) by methods. To take the |stamp| example, an attribute of a |stamp|
-object might be the text that should be stamped when the |apply| method is
-called.
-
-When an attribute is changed, this change is \emph{not} local to the current
-\TeX\ group. Changes will persist till the end of the object's life or until
-the attribute is changed once more.
-
-To declare an attribute you should use the |\attribute| command:
-%
-\begin{command}{\attribute \meta{attribute name}\opt{|=|\meta{initial value}}|;|}
- This command can only be given inside the body of an |\pgfooclass| command.
- It declares the attribute named \meta{attribute name}. This name, like
- method or class names, can be quite arbitrary, but should not contain
- periods. Valid names are |an_attribute?| or |my attribute|.
-
- You can optionally specify an \meta{initial value} for the attribute; if
- none is given, the empty string is used automatically. The initial value is
- the value that the attribute will have just after the object has been
- created and before the constructor is called.
- %
-\begin{codeexample}[code only]
-\pgfooclass{stamp}{
- % This is the class stamp
-
- \attribute text;
- \attribute rotation angle = 20;
-
- \method stamp(#1) {
- \pgfooset{text}{#1} % Set the text
- }
-
- \method apply(#1,#2) {
- \pgfoothis.shift origin(#1,#2)
-
- % Draw the stamp:
- \node [rotate=\pgfoovalueof{rotation angle},font=\huge]
- {\pgfoovalueof{text}};
- }
-
- \method shift origin(#1,#2) { ... }
-
- \method set rotation (#1) {
- \pgfooset{rotation angle}{#1}
- }
-}
-\end{codeexample}
- %
-\end{command}
-
-Attributes can be set and read only inside methods, it is not possible to do so
-using an object handle. Spoken in terms of traditional object-oriented
-programming, attributes are always private. You need to define getter and
-setter methods if you wish to read or modify attributes.
-
-Reading and writing attributes is not done using the ``dot-notation'' that is
-used for method calls. This is mostly due to efficiency reasons. Instead, a set
-of special macros is used, all of which can \emph{only be used inside methods}.
-
-\begin{command}{\pgfooset\marg{attribute}\marg{value}}
- Sets the \meta{attribute} of the current object to \meta{value}.
- %
-\begin{codeexample}[code only]
-\method set rotation (#1) {
- \pgfooset{rotation angle}{#1}
-}
-\end{codeexample}
- %
-\end{command}
-
-\begin{command}{\pgfooeset\marg{attribute}\marg{value}}
- Performs the same action as |\pgfooset| but in an |\edef| full expansion
- context.
-\end{command}
-
-\begin{command}{\pgfooappend\marg{attribute}\marg{value}}
- This method adds the given \meta{value} to the \meta{attribute} at the end.
-\end{command}
-
-\begin{command}{\pgfooprefix\marg{attribute}\marg{value}}
- This method adds the given \meta{value} to the \meta{attribute} at the
- beginning.
-\end{command}
-
-\begin{command}{\pgfoolet\marg{attribute}\marg{macro}}
- Sets the \meta{attribute} of the current value to the current value of
- \meta{macro} using \TeX's |\let| command.
- %
-\begin{codeexample}[code only]
-\method foo () {
- \pgfoolet{my func}\myfunc
- % Changing \myfunc now has no effect on the value of attribute my func
-}
-\end{codeexample}
- %
-\end{command}
-
-\begin{command}{\pgfoovalueof\marg{attribute}}
- Expands (eventually) to the current value of \meta{attribute} of the
- current object.
- %
-\begin{codeexample}[code only]
-\method apply(#1,#2) {
- \pgfoothis.shift origin(#1,#2)
-
- \node [rotate=\pgfoovalueof{rotation angle},font=\huge]
- {\pgfoovalueof{text}};
-}
-\end{codeexample}
- %
-\end{command}
-
-\begin{command}{\pgfooget\marg{attribute}\marg{macro}}
- Reads the current value of \meta{attribute} and stores the result in
- \meta{macro}.
- %
-\begin{codeexample}[code only]
-...
- \method get rotation (#1) {
- \pgfooget{rotation angle}{#1}
- }
-...
-
-\mystamp.get rotation(\therotation)
-``\therotation'' is now ``20'' (or whatever).
-\end{codeexample}
- %
-\end{command}
-
-
-\subsection{Identities}
-\label{section-identities}
-
-Every object has a unique identity, which is simply an integer. It is possible
-to retrieve the object id using the |get id| method (discussed below), but
-normally you will not need to do so because the id itself cannot be used to
-access an object. Rather, you access objects via their methods and these, in
-turn, can only be called via object handles.
-
-Object handles can be created in four ways:
-%
-\begin{enumerate}
- \item Calling |\pgfoonew|\meta{object handle}|=...| will cause \meta{object
- handle} to be a handle to the newly created object.
- \item Using |\let| to create an alias of an existing object handle: If
- |\mystamp| is a handle, saying |\let\myotherstamp=\mystamp| creates a
- second handle to the same object.
- \item |\pgfooobj|\marg{id} can be used as an object handle to the object
- with the given \meta{id}.
- \item Using the |get handle| method to create a handle to a given object.
-\end{enumerate}
-
-Let us have a look at the last two methods.
-
-\begin{command}{\pgfooobj\marg{id}}
- Provided that \meta{id} is the id of an existing object (an object whose
- life-time has not expired), calling this command yields a handle to this
- object. The handle can then be used to call methods:
- %
-\begin{codeexample}[code only]
-% Create a new object:
-\pgfoonew \mystamp=new stamp()
-
-% Get the object's id and store it in \myid:
-\mystamp.get id(\myid)
-
-% The following two calls have the same effect:
-\mystamp.apply(1,1)
-\pgfooobj{\myid}.apply(1,1)
-\end{codeexample}
- %
-\end{command}
-
-The |get id| method can be used to retrieve the id of an object. This method is
-predefined for every class and you should not try to define a method of this
-name yourself.
-
-\begin{predefinedmethod}{get id(\meta{macro})}
- Calling \meta{obj}|.get id(|\meta{macro}|)| stores the id \meta{obj} in
- \meta{macro}. This is mainly useful when you wish to store an object for a
- longer time and you cannot guarantee that any handle that you happen to
- have for this object will be available later on.
-
- The only way to use the retrieved id later on is to call |\pgfooobj|.
-
- Different object that are alive (that are still within the scope in which
- they were created) will always have different ids, so you can use the id to
- test for equality of objects. However, after an object has been destroyed
- because its scope has ended, the same id may be used again for newly
- created objects.
-
- Here is a typical application where you need to call this method: You wish
- to collect a list of objects for which you wish to call a specific method
- from time to time. For the collection process you wish to offer a macro
- called |\addtoobjectlist|, which takes an object handle as parameter. It is
- quite easy to store this handle somewhere, but a handle is, well, just a
- handle. Typically, shortly after the call to |\addtoobjectlist| the handle
- will no longer be valid or even exist, even though the object still exists.
- In this case, you wish to store the object id somewhere instead of the
- handle. Thus, for the object passed to |\addtoobjectlist| you call the
- |get id| method and store the resulting id, rather than the handle.
-\end{predefinedmethod}
-
-There is a second predefined method, called |get handle|, which is also used to
-create object handles.
-
-\begin{predefinedmethod}{get handle(\marg{macro name})}
- Calling this method for an object will cause \meta{macro name} to become a
- handle to the given object. For any object handle |\obj| -- other than
- |\pgfoothis| -- the following two have the same effect:
- %
- \begin{enumerate}
- \item |\let|\meta{macro name}|=\obj|
- \item |\obj.get handle(|\meta{macro name}|)|
- \end{enumerate}
-
- The first method is simpler and faster. However, for |\pgfoothis| there is
- a difference: The call |\pgfoothis.get handle(|\meta{macro name}|)| will
- cause \meta{macro name} to be an object handle to the current object and
- will persist to be so even after the method is done. By comparison,
- |\let|\meta{macro name}|=\pgfoothis| causes |\obj| to be the same as the
- very special macro |\pgfoothis|, so |\obj| will always refer to the current
- object, which may change over time.
-\end{predefinedmethod}
-
-
-\subsection{The Object Class}
-\label{section-object}
-
-The object-oriented module predefines a basic class |object| that can be used
-as a base class in different context.
-
-\begin{ooclass}{object}
- This class current only implements one method:
-
- \begin{method}{copy(\meta{handle})}
- Creates a new object and initializes the values of its (declared)
- attributes to the values of the original. The method takes one
- argument: a control sequence which receives the handle of the copy.
- \end{method}
-\end{ooclass}
-
-
-\subsection{The Signal Class}
-\label{section-signals}
-
-In addition to the basic mechanism for defining and using classes and object,
-the class |signal| is predefined. It implements a so-called signal--slot
-mechanism.
-
-\begin{ooclass}{signal}
- This class is used to implement a simple signal--slot mechanism. The idea
- is the following: From time to time special things happen about which a
- number of objects need to be informed. Different things can happen and
- different object will be interested in these things. A |signal| object can
- be used to signal that such special things of a certain kind have happened.
- For example, one signal object might be used to signal the event that ``a
- page has been shipped out''. Another signal might be used to signal that
- ``a figure is about to be typeset'', and so on.
-
- Objects can ``tune in'' to signals. They do so by \emph{connecting} one of
- their methods (then called a \emph{slot}) to the signal. Then, whenever the
- signal is \emph{emitted}, the method of the connected object(s) get called.
- Different objects can connect different slots to the same signal as long as
- the argument lists will fit. For example, the object that is used to signal
- the ``end of page has been reached'' might emit signals that have, say, the
- box number in which the finished page can be found as a parameter
- (actually, the finished page is always in box 255). Then one object could
- connect a method |handle page(#1)| to this signal, another might connect
- the method |emergency action(#1)| to this signal, and so on.
-
- Currently, it is not possible to ``unregister'' or ``detach'' a slot from a
- signal, that is, once an object has been connect to a signal, it will
- continue to receive emissions of this signal till the end of the life-time
- of the signal. This is even true when the object no longer exists (but the
- signal does), so care must be taken that signal objects are always created
- after the objects that are listening to them.
-
- \begin{method}{signal()}
- The constructor does nothing.
- \end{method}
-
- \begin{method}{connect(\meta{object handle},\meta{method name})}
- This method gets an \meta{object handle} as parameter and a
- \meta{method name} of this object. It will queue the object-method pair
- in an internal list and each time the signal emits something, this
- object's method is called.
-
- Be careful not to pass |\pgfoothis| as \meta{object handle}. This would
- cause the signal object to connect to itself. Rather, if you wish to
- connect a signal to a method of the current object you first need to
- create an alias using the |get handle| method:
- %
-\begin{codeexample}[code only]
-\pgfooclass{some class}{
- \method some class() {
- \pgfoothis.get handle(\me)
- \somesignal.connect(\me,foo)
- \anothersignal.connect(\me,bar)
- }
- \method foo () {}
- \method bar (#1,#2) {}
-}
-\pgfoonew \objA=new some class()
-\pgfoonew \objB=new some class()
-\end{codeexample}
- \end{method}
-
- \begin{method}{emit(\meta{arguments})}
- This method emits a signal to all connected slots. This means that for
- all objects that have previously been connected via a call of
- |connect|, the method (slot) that was specified during the call of
- |connect| is invoked with given \meta{arguments}.
- %
-\begin{codeexample}[code only]
-\anothersignal.emit(1,2)
-% will call \objA.bar(1,2) and \objB.bar(1,2)
-\end{codeexample}
- \end{method}
-\end{ooclass}
-
-
-\subsection{Implementation Notes}
-
-For the curious, here are some notes on how the oo-system is implemented:
-%
-\begin{itemize}
- \item There is an object id counter that gets incremented each time an
- object is created. However, this counter is local to the current scope,
- which means that it is reset at the end of each scope, corresponding to
- the fact that at the end of a scope all objects created in this scope
- become invalid. Newly created objects will then have the same id as
- ``deleted'' objects.
- \item Attributes are stored globally. For each attribute of each object
- there is a macro whose name is composed of the object's id and the
- attribute name. Changes to object attributes are always global.
- \item A call to the garbage collector causes a loop to be executed that
- tries to find objects whose object number is larger than the current
- maximum alive objects. The global attributes of these objects are then
- freed (set to |\relax|) by calling a special internal method of these
- (dead) objects.
-
- The garbage collector is automatically called after each group in
- which an object was created using |\aftergroup|.
- \item When a method is called, before the method call some code is executed
- that sets a global counter storing the current object id to the object
- id of the object being called. After the method call some code is
- inserted that restores the global counter to its original value. This
- is done without scopes, so some tricky |\expandafter| magic is needed.
- Note that, because of this process, you cannot use commands like
- |\pgfutil@ifnextchar| at the end of a method.
- \item An object handle contains just the code to set up and restore the
- current object number to the number of the object being called.
-\end{itemize}