This is not necessarily the current version of this TIP.This is not necessarily the current version of this TIP.
| TIP: | 257 |
| Title: | Object Orientation for Tcl |
| Version: | $Revision: 1.5 $ |
| Authors: |
Donal K. Fellows <donal dot k dot fellows at manchester dot ac dot uk> Will Duquette <will at wjduquette dot com> Steve Landers <steve at digitalsmarties dot com> Jeff Hobbs <jeffh at activestate dot com> Kevin Kenny <kennykb at users dot sourceforge dot net> Miguel Sofer <mig at utdt dot edu> |
| State: | Draft |
| Type: | Project |
| Tcl-Version: | 8.5 |
| Vote: | Pending |
| Created: | Monday, 26 September 2005 |
| Obsoletes: | TIP #50 |
This TIP proposes adding OO support to the Tcl core, semantically based on XOTcl. The commands it defines will be in the ::oo namespace, which is not used by any current mainstream OO system, and it will be designed specifically to allow classic XOTcl to be built on top.
Tcl has a long history of being comparatively agnostic about object-oriented programming, not favouring one OO system over another while promoting a wealth of OO extensions such as [incr Tcl][1], OTcl[2], XOTcl[3], stooop[4], Snit[5], etc. because in general, one size fits nobody.
However, many application domains require OO systems and having a common such base system will help prevent application and library authors from reinventing the wheel each time through because they cannot rely on an OO framework being present with each and every Tcl installation. For example, the http package supplied with Tcl has its own internal object model, and a similar mechanism is reinvented multiple times within tcllib. Other parts of tcllib do their own thing (to say nothing of the fact that both stooop and Snit are in tcllib themselves). This does not promote efficient reuse of each others code, and ensures that each of these packages has a poor object system. The request for an OO system is also one of the biggest feature requests for Tcl, and would make it far easier to implement megawidgets. It also leaves Tcl open to the ill-informed criticism that it doesn't support OO, despite being spoilt for choice in reality through the extensions listed above.
Given all this, the time has come for the core to provide OO support. The aim of the core OO system shall be that it is simple to get started with, flexible so that it can take you a long way, fast (we all know that we're going to get compared on this front!), and suitable for use as a foundation of many other things, including the re-implementation of various existing OO extensions, including those that are currently compiled and also those that are pure Tcl extensions.
Another requirement is that programmers should not have to alter all of their existing code in order to get started with the new system; rather, they should be able to adopt it progressively, over time, because it supports betters ways of working (e.g., faster and more flexible libraries).
This TIP proposes that the foundation of the OO system should be based on XOTcl as that is fast, semantically rich, well-supported, and relatively compatible with with the existing Tcl build system.
Some changes will be necessary. Certain aspects of XOTcl syntax are peculiar from a conventional OO point-of-view, and it is deeply unfortunate that a very large number of methods are predefined in the XOTcl base class. XOTcl's approach to object creation options is also highly idiosyncratic, and doesn't support the typical Tcl idioms. However, the changes must be made in such that classic XOTcl can be built on the new framework; as a result, the classic XOTcl base class will be derived from something more fundamental.
Class-based object system. This is what most programmers expect from OO, and it is very useful for many tasks.
Allows per-object customization and dynamic redefinition of classes.
Supports advanced OO features, such as:
These are subclasses of class, which permit more advanced customization of class behaviour.
These are constraints (implemented in Tcl code, naturally) on whether a method may be called.
These allow functionality to be brought into an object from other objects if necessary, enabling better separation of concerns.
Object and class names in the core extension to be all lower-case, in line with best common practice.
Methods have to be capable of being non-exported, by which we mean that they are not (simply) callable from contexts outside the object.
The majority of the API for updating an object or class's definition is to be moved to a separate utility command, oo::define.
More "conventional" naming of operations is to be used.
Many of the more advanced features of XOTcl are removed, especially when it is possible to implement them on top of other features. This particularly applies to:
Filter- and mixin-guards
Invariants
Pre- and post-conditions
Note that this TIP does not propose to actually include any XOTcl (or Itcl or Snit or ...) compatability packages in the core; it it about forming a foundation on which they can be built (which happens to also be a comparatively lightweight OO system in itself). Such compatability packages can either remain separate code, or be the subject of future TIPs.
Many key semantic features of XOTcl are adopted with little or no change.
Like XOTcl, we shall support multiple inheritance (MI).
The main problem with MI in languages like C++ was always confusion caused by the fact that methods were resolved using integer offsets into method tables. By contrast, single inheritance is far too restrictive. XOTcl uses MI, and adds in mixins and filters, so we should have those capabilities too. As these are less well-known terms than normal inheritance, we define them here:
An auxiliary class whose behaviour is "mixed into" the current object or class, adding the mixin's methods to the target object's/class's methods. Often used to support cross-cutting functionality.
A nominated method that is permitted to control whether all calls to any other method of a class or object occur. This control is achieved by the nominated filter method being chained on the front of the sequence of methods in the "implementation list" for the actual target method. Often used to support transparent orthogonal functionality, such as access control or result cacheing.
The method name resolution and chaining scheme of XOTcl shall be used. The following gives the method-interception search order; any particular method implementation belongs in the last position in this order that it can possibly be in.
filters defined on the object, in the order that they are described in the object's filter list
methods declared by mixins added to the object, with mixins being processed in the order that they are described in the object's mixin list (the set of methods declared by the mixin are determined by recursively applying this algorithm)
methods declared by the object itself
filters defined on the object's class, in the order that they are described in the class's filter list
methods declared by mixins added to the object's class, with mixins being processed in the order that they are described in the class's mixin list
methods declared by the object's class itself
methods declared by the object's class's superclasses, with superclasses being processed in the order that they are described in the class's superclass list
Given the above ordering, for each method on an object there is an ordered list of implementations. We dispatch the method by executing the first implementation on the list, which can then hand off to subsequent methods in the list in order using the next command (described below).
This section describes the essential changes to XOTcl behavior required to meet the above goals, and the rationale for them. The paragraphs which describe the specific changes begin with the word Therefore, in bold type. Note that whereever possible, the semantics of XOTcl are to be used even where the syntax is not; deviations will be explicitly listed.
In XOTcl, every class and every object has an associated namespace. The namespace associated with a class ::myclass is ::xotcl::classes::myclass; the namespace associated with object ::myobject is simply ::myobject. XOTcl "instprocs" are simply procs defined in a class (or superclass) namespace; XOTcl per-object "procs" are simply procs defined in an object's namespace. Every such proc becomes an object subcommand.
This is part of the reason why XOTcl objects have such cluttered interfaces. Every method which is of use to the object appears in the object's interface - and there's no way to prevent this.
Therefore, in the new oo system "procs" and "instprocs" can be exported or non-exported. Exported procs appear as object subcommands; non-exported procs do not, but remain available as subcommands of the my command. In this way, the object itself can still use them, but they need appear in the object's interface only if desired.
Additionally, the standard info method will need to be extended to allow introspection of which methods are exported and which are not.
In XOTcl, the commands to define per-class methods, filters, and so on are subcommands of the class object; the commands to define per-object methods, filters, and so on are subcommands of the individual object. This is a problem, as it confuses the implementation-time interface with the run-time interface. The design is logical, given XOTcl's extreme dynamism; any implementation-time activity, such as defining a method or adding a filter can indeed be done at run-time. But again, this makes it difficult to define clean run-time interfaces for reuseable library code.
The solution described in the previous section, of making some methods private by declaring them non-exported, does not give us a full solution; having the instproc subcommand available only from instance code isn't all that useful.
Therefore, we add a new command, oo::define, which is used to define methods, filters, and so on. It can be called in two ways. The first is as follows:
oo::define objectOrClass subcommand args...
For example, the following XOTcl code defines a class with two methods:
xotcl::Class myclass
myclass instproc dothis {args} { # body }
myclass instproc dothat {args} { # body }
In the new oo core, the matching code would be this:
oo::class create myclass
oo::define myclass instproc dothis {args} { # body }
oo::define myclass instproc dothat {args} { # body }
oo::define can also be called with a script whose commands are aliased to the subcommands of oo::define. Thus, the above code could also be written as follows:
oo::class create myclass
oo::define myclass {
instproc dothis {args} { # body }
instproc dothat {args} { # body }
}
Finally, the class "create" method could be extended so that it could be called with such a script:
oo::class create myclass {
instproc dothis {args} { # body }
instproc dothat {args} { # body }
}
This allows a class to be defined cleanly and concisely, while guaranteeing that all class details can still be modified later on using oo::define.
Note that we do not lose any object-oriented flexibility by this scheme. An oo::xotcl package can use the "forward" feature to forward "instproc" and its partners to oo::define, thus defining them all as methods; and once they are methods, all of the usual techniques of method chaining, mix-ins, and filters apply.
oo::define will need two subcommands XOTcl doesn't currently provide: export and unexport. export takes as arguments a list of method names; all named methods are exported and become visible in the object or class's interface. unexport does the opposite. Each can include wildcards in its argument list, just as namespace export does.
XOTcl defines two standard Metaclasses, xotcl::Object and xotcl::Class. xotcl::Object is the root of the class hierarchy; all XOTcl classes implicitly inherit from xotcl::Object. XOTcl classes are themselves objects, and are instances of xotcl::Class. xotcl::Class can itself be subclassed to produce different families of classes with different standard behaviours.
The new core object system will use the same basic mechanism, based on the metaclasses oo::object and oo::class. However, one of the problems with XOTcl is that XOTcl objects have too much standard behavior; the new core object system must provide a simpler foundation, with the XOTcl behavior optionally available.
Therefore, we will decompose the features of xotcl::Object and xotcl::Class into a number of simpler metaclasses.
oo::object will be the root of the class hierarchy. However, instances of oo::object will have a minimal set of standard methods, so that clean interfaces can be built on top of it, as can be done with Snit types and instances.
Core object system classes will be instances of oo::class or its subclasses. Likewise, oo::class will define only minimal behaviour.
The standard XOTcl class and object methods will be provided by a number of standard classes, all of which will be subclasses of oo::object. A user-defined class can include some or all of the standard XOTcl behavior by multiply inheriting from some or all of these standard classes. Each such standard class will provide a subset of the standard XOTcl methods. The following is an incomplete list of the necessary classes:
oo::definer will define one method for each subcommand of oo::define; the methods will be "forward"ed to oo::define.
oo::struct will define all of the data access methods, e.g., set, unset, lappend, incr, and so forth.
Thus, oo::class is the mechanism for defining classes with clean interfaces and maximum data hiding and encapsulation; oo::struct is the mechanism for defining classes for maximal public access.
The above classes and metaclasses will be implemented such that they can be used as a foundation for the ::xotcl::Class and ::xotcl::Object metaclasses (see below for a discussion).
A class may wish to make use of the capabilities of oo::struct internally without exporting its methods.
Therefore, the inheritance mechanism should be extended such that the newly defined class can declare whether a parent class's methods should be exported or not.
XOTcl has a unique creation syntax. The object name can be followed by what look like Tk or Snit options - but aren't. Instead, any token in the argument list that begins with a hyphen is assumed to be the name of one of the object's methods; it must be followed by the method's own arguments. For example, a standard XOTcl class will have a "set" method, which has the same syntax as the standard Tcl "set" command. Thus, the following code:
myclass myobj -set a 1 -set b 2
creates an instance of "myclass" called "myobj" whose instance variables "a" and "b" and set to 1 and 2 respectively. This is an intriguing and innovative interface, and it is unlike any other Tcl object system. Additionally, it makes it difficult to implement standard Tk-like options.
Therefore, standard core object system classes will not use this mechanism (though it might be available on demand by inheriting from some other standard metaclass). Instead, standard core object system classes will have no creation behavior other than that implemented by their designers in their constructors.
Constructors may have any argument list the user pleases, including default arguments, the "args" argument (as in the proc command), and XOTcl-style non-positional arguments. It is up to the developer to handle the arguments appropriately.
It is expected that one of the key responsibilities of any XOTcl compatability package would be to define a constructor that parses the arguments in the expected way and uses them to invoke methods on the newly created object.
In XOTcl, a class's constructor is implemented using its "init" instproc. This is troubling; constructors are intended to do things just once, and are often written to take advantage of that, whereas an "init" instproc can theoretically be called at any time. For any given class, then, one of two conditions will obtain: either "init" must be written so that it can be called at any time, or the class will have an inherent logic bug.
Therefore, the class constructor will not be implemented as a standard instproc. Instead, the oo::define command will have a new subcommand, constructor, which will be used as follows:
oo::define myclass constructor {} {
# body
}
The constructor so defined will act almost exactly like an instproc; it may call superclass constructors using the "super" command, etc. However, it may never be called explicitly, but only via the class's "create" and "new" methods.
In XOTcl, a class's destructor is defined by overriding the the "destroy" instproc. This is problematic for two reasons: first, a destructor doesn't need an argument list. An instproc is too powerful for the task. Second, successful destruction should not depend on the destructor's chaining to its superclass destructors properly.
Therefore, the class destructor will be defined by a new subcommand of oo::define, destructor, as follows:
oo::define myclass destructor {
# Body
}
The destructor has no argument list.
The destructor cannot be called explicitly. Instead, the destructors are invoked in the proper order by the standard destroy method (defined in oo::object), which need never be overridden.
The changes described in this section are not absolutely essential to meeting the goals described earlier. However, they are desirable in that they lead to cleaner, more maintainable code.
XOTcl has many features which can be applied to a class for use by all class instances, or to a single object. For example, a "filter" can be defined for a single object, while an "instfilter" can be defined for a class and applied to all instances of that class.
This is exactly backward. Most behavior will be defined for classes; additional per-object behavior is the special case, and consequently should have the less convenient name.
Therefore, all XOTcl subcommands that begin with "inst" will lose their "inst" prefix; the matching per-object subcommands will gain a "self." prefix, to indicate that it is operating on the object itself and not the members of the class. Thus, a filter is defined on a class for its instances using the "filter" subcommand; a filter is defined on a particular object using the "self.filter" subcommand.
The word "proc" conveys a standalone function; an object's subcommands are more typically described as its "methods".
Therefore, the XOTcl "instproc" and "proc" subcommands should be renamed as "instmethod" and "method" - or, if the new naming convention described in the previous section is adopted, method and self.method.
In XOTcl, the main objects are xotcl::Class and xotcl::Object. However, the Tcl Style Guide dictates that public command names begin with a lower-case letter.
Therefore, all public names in the oo:: namespace will begin with a lower case letter, e.g., the standard core object system equivalents of xotcl::Class and xotcl::Object will be oo::class and oo::object.
The names in any oo::xotcl compatibility module would naturally follow the existing XOTcl conventions.
This section documents the core object system API in detail, based on the essential and desirable changes discussed in the previous sections.
The namespace(s) that define the following three commands are not defined in this specification; all that is defined is that they will be on the object's namespace path during the execution of any method and should always be used without qualification.
The my command allows methods of the current object to be called during the execution of a method, just as if they were invoked using the object's command. Unlike the object's command, the my command may also invoke non-exported methods.
my methodName ?arg arg ...?
The next command allows methods to invoke the implementation of the method with the same name in their superclass (as determined by the normal inheritance rules; if a per-object method overrides a method defined by the object's class, then the next command inside the object's method implementation will invoke the class's implementation of the method). The arguments to the next command are the arguments to be passed to the superclass method; this is in contrast to the XOTcl next command, but other features in Tcl 8.5 make this approach viable and much easier to control. The current stack level is temporarily bypassed for the duration of the processing of the next command; this is in contrast to the XOTcl version of the next command, but it allows a method to always execute identically with respect to the main calling context.
next ?arg arg ...?
It is an error to invoke the next command when there is no superclass definition of the current method.
The self command allows executing methods to discover information about the object which they are currently executing in. Without arguments, the self command returns the current fully-qualified name of the object (to promote backward compatability). Otherwise, it is a command in the form of an ensemble (though it is not defined whether it is manipulable with namespace ensemble).
The following subcommands of self are defined. None of these subcommands take additional arguments.
Returns a three-item list describing the class, object and method that invoked the current method, respectively. Syntax:
self caller
Returns the name of the class that defines the currently executing method. If the method was declared in the object instead of in the class, this returns the class of the object containing the method definition. Syntax:
self class
When invoked inside a filter, returns a three-item list describing the object or class for which the filter has been registered. The first element is the name of the class or object, the second element is either method (for a method defined in a class for its instances) or self.method (for a method defined by a single object), and the third element is the name of the method.
Returns the name of the currently executing method. Syntax:
self method
Returns the namespace associated with the current object. Syntax:
self namespace
Returns the fully-qualified name of the method that will be executed when the next command is invoked, or an empty string if there is no superclass definition for the method. Syntax:
self next
Returns the name of the current object, the same as if the self command is invoked with no arguments. Syntax:
self object
When invoked from a filter or mixin, returns a two-item list consisting of the name of the class that holds the target method and the name of the target method. Syntax:
self target
oo::define objectOrClass subcommand ?arg ...?
oo::define objectOrClass script
The oo::define command is used to add behavior to objects or classes. In the second form, script is a Tcl script whose commands are the subcommands of oo::define; this is a notational convenience, as the two forms are semantically equivalent. (Note that the context in which script executes is otherwise not defined.)
The subcommands of oo::define (which may be unambiguously abbreviated when not in the script form) shall be:
constructor - this is valid only for classes, takes two arguments (a proc-style argument list, and a body script), and sets the constructor for the instances of the class to be executed as defined by the body script after binding the actual arguments to the call that creates an instance of the class to the formal arguments listed. The constructor is called after the object is created but before any instance variables are guaranteed to be set. If no constructor is specified, the constructor will accept exactly the same arguments as the constructor in the parent class, and will delegate all the arguments to that parent-class constructor.
oo::define class constructor argList body
copy - this creates an exact copy of an object with the given name. If name is the empty string, a new name will be generated automatically.
oo::define object copy name
destructor - this is valid only for classes. It defines the class destructor; a destructor is like a method but takes no arguments. It is called by the object's destroy method, which is defined automatically and which cannot be overridden. The syntax is as follows:
oo::define class destructor body
In classic XOTcl, the destructor is simply a method; it must explicitly call the parent destructor using XOTcl's next command. In the core OO system, the chain of destructors is called in the proper sequence automatically and independently of the content of any particular destructor.
Note that destructors are called whenever the object is deleted by any mechanism (except when the overall interpreter is deleted, when execution of Tcl scripts has ceased to be possible anyway).
export - this specifies that the named methods are exported, i.e. part of the public API of the class's instances. The syntax is as follows:
oo::define class export name ?name ...?
An exported method is accessible to clients of the object; an unexported method is accessible only to the object's own code through the my command.
filter - this subcommand (operating on the class if the object is a class, and on the object itself otherwise - see the self.filter subcommand for how to force it the other way) controls the list of filter methods for a class or object. Each filter method in the list is called when any method is invoked on the class's instances or the object, and it is up to the filter to decide whether to invoke the filtered method call (using the next command) or return a suitable replacement value.
oo::define objectOrClass filter filterList
forward - this subcommand (operating on the class if the object is a class, and on the object itself otherwise - see the self.forward subcommand for how to force it the other way) defines a class method which is automatically forwarded (i.e. delegated) to some other command, according to a simple pattern. Each arg is used literally.
oo::define objectOrClass forward name targetCmd ?arg ...?
method - this subcommand (only valid for classes) defines a class method (i.e. a method supported by every instance of the class). By default, methods are exported if they start with a lower-case letter (i.e. any character in \u0061 to \u007a inclusive) and are not exported otherwise.
oo::define class method name args body
mixin - This subcommand defines a mixin for a class or object (operating on the class if the object is a class, and on the object itself otherwise - see the self.mixin subcommand for how to force it the other way) which is a way of bringing in additional method implementations (which may add to or wrap existing methods) on an ad hoc basis. The list of mixins is traversed when searching for methods before the inheritance hierarchy, and mixed-in methods may chain to any methods they override using the next command.
oo::define objectOrClass mixin mixinList
parameter - this subcommand defines a parameter (or parameters), an instance variable with an identically named and automatically defined access method. If any name is a two-element list, the first element is the name of the variable and the second element is the default value to assign to the variable.
oo::define class parameter name ?name ...?
The access methods are always defined something like this, for a parameter named bar in a class named foo:
oo::define foo method bar {args} {
my variable {bar vbl}
if {[llength $args] == 0} {
return $vbl
} else if {[llength $args] == 1} {
return [set vbl [lindex $args 0]]
}
return -code error "wrong # args: ..."
}
superclass - This specifies the superclass (or classes) of a class. Inheritance will follow the XOTcl pattern (except with a somewhat different class hierarchy, of course). Syntax:
oo::define class superclass classList
unexport - This specifies that the named methods are unexported, i.e. private. The syntax is as follows:
oo::define class unexport name ?name ...?
An exported method is accessible to clients of the object; an unexported method is accessible only to the object's own code, through the my command.
The following subcommands are all per-object versions of the class subcommands listed above. When they are applied to a class, they operate on the class instance itself as an object, and not on the instances (current and future) of that class (which is why the distinction is required).
self.class - This subcommand gets and sets the class of an object. Changing the class of an object can result in many methods getting added or removed.
self.export - This increases the set of commands exported by the object.
self.filter - This is a per-object version of the filter subcommand.
self.forward - This is a per-object version of the forward subcommand.
self.method - This is a per-object version of the method subcommand.
self.mixin - This is a per-object version of the mixin subcommand.
self.unexport - This decreases the set of commands exported by the object.
The following classes are defined, and are the only pre-constructed objects in the core system.
The root of the class hierarchy is oo::object. There are two ways to create a new instance of an object.
oo::object create name
set var [ oo::object new ]
The first constructs a new object called name of class oo::object; the object is represented as a command in the current scope. The second constructs a new object of class oo::object with a name guaranteed to be different from every existing command and returns the fully qualified of the command created (which it is naturally a good idea to save in a variable, perhaps called var).
The name of an object is also the name of a command in the form of an ensemble where the subcommands of the ensemble are the exported method names of the object. It is undefined whether the ensemble is manipulable with namespace ensemble.
The new object has one predefined exported method (destroy) and two predefined non-exported methods (eval and variable). Other subcommands and other behaviour can be added using oo::define.
The oo::object class (an instance of oo::class) serves as the base class for all other core OO system classes.
The constructor for the oo::object class takes no arguments and does nothing.
The destructor does nothing.
The instances of oo::object (i.e. all objects and classes) have the following methods:
This non-exported method concatenates its arguments according to the rules of concat, and evaluates the resulting script in the namespace associated with the object. The result of the script evaluation is the result of the object eval method.
object eval ?arg arg ...?
This exported method deletes the object; it takes no additional arguments and returns the empty string as its result. The basic implementation in the oo::object class shall just use the core Tcl rename command to perform the deletion.
object destroy
This non-exported method takes an arbitrary number of unqualified variable names and binds the variable with that name in the object's namespace to the same name in the current scope. If an argument consists of a two-element list, the first element is the name of the variable to bind in the object's namespace, and the second element is the name of the variable to bind in the current scope.
object variable ?varName varName ...?
When an attempt is made to invoke an unknown method on any object, the core then attempts to pass all the arguments (including the command name) to the public unknown method of the object. If no such method exists, an error message is generated. Instances of the core oo::object class do not have an unknown method by default.
This class is the class of all classes (i.e. its instances are objects that manufacture objects according to a standard pattern). Note that oo::object is an instance of oo::class, as is oo::class itself.
oo::class create name ?definition?
This creates a new class called name; the class is an object in its own right (of class oo::class), and hence is represented as a command in the current scope. oo::class returns the fully qualified command name.
The newly-created class command is used to define objects which belong to the class, just as oo::object is. By default, instances of the new class have no more behaviour than instances of oo::object do; new class behavior can be added to the class in two ways. First, a definition can be specified when creating the class; second, additional behaviour can be added to the class using oo::define.
The definition, if given, consists of a series of statements that map to the subcommands of oo::define. The following three code snippets are equivalent; each defines a class called ::dog whose instances will have two subcommands: bark and chase.
# Method 1
oo::class create dog
oo::define dog method bark {} {
puts "Woof, woof!"
}
oo::define dog method chase {thing} {
puts "Chase $thing!"
}
# Method 2
oo::class create dog
oo::define dog {
method bark {} {
puts "Woof, woof!"
}
method chase {thing} {
puts "Chase $thing!"
}
}
# Method 3
oo::class create dog {
method bark {} {
puts "Woof, woof!"
}
method chase {thing} {
puts "Chase $thing!"
}
}
The constructor for oo::class concatenates its arguments and passes the resulting script to oo::define (along with the fully-qualified name of the created class, of course).
The destructor for oo::class cuts the class out of the inheritance and mixin hierarchy of all instances of the class.
The instances of oo::class have the following methods:
Creates a new instance of the class with the given name. All subsequent arguments are given to the class's constructor. The result of the create method is always the fully-qualified name of the newly-created object. Syntax:
class create objName ?arg arg ...?
Creates a new instance of the class with an automatically chosen name. All subsequent arguments are given to the class's constructor. The result of the new method is always the fully-qualified name of the newly-created object. Syntax:
class new ?arg arg ...?
Note that the oo::class object itself does not export the new method; it is good practice for all classes to have names.
This metaclass (subclass of oo::class) is designed for easy support of an XOTcl-light style of OO. It arranges for its instances to have the following methods, each of which is delegated to the identically-named subcommand of the oo::define command described below, operating on the class instance that is an instance of oo::definer.
constructor, destructor, export, filter, forward, method, mixin, parameter, superclass, unexport
In addition, the classes that are instances of oo::definer are augmented with an unknown method such that unknown methods are treated as a call to the create method (if the unknown method name is a non-empty string) or new method (if the unknown method name is the empty stirng) of the class.
Thus, the following commands are equivalent:
# Method 1
oo::definer create dog
oo::define dog method bark {} {
puts "Woof, woof!"
}
dog create fido
fido bark
# Method 2
oo::definer dog
dog method bark {} {
puts "Woof, woof!"
}
dog fido
fido bark
The oo::definer constructor just passes all its arguments to its parent constructor (i.e. the oo::class constructor).
This class (subclass of oo::object) has no default constructor. It has the following exported methods:
This is the analog of the core Tcl append command except that the variable name is resolved in the context of the object's namespace.
struct append varName ?arg arg ...?
This is the analog of the core Tcl array command except that the array name is resolved in the context of the object's namespace.
struct array subcommand varName ?arg arg ...?
This is a public exposure of the eval method defined by the oo::object class.
This is the analog of the core Tcl info exists command except that the variable name is resolved in the context of the object's namespace.
struct exists varName
This is the analog of the core Tcl incr command except that the variable name is resolved in the context of the object's namespace.
struct incr varName ?increment?
This is the analog of the core Tcl lappend command except that the variable name is resolved in the context of the object's namespace.
struct lappend varName ?arg arg ...?
This is the analog of the core Tcl set command except that the variable name is resolved in the context of the object's namespace.
struct set varName ?value?
This is the analog of the core Tcl trace command operating on variables (no other types of tracable items are supported by this method) except that variable names are resolved in the context of the object's namespace.
struct trace subcommand ?arg arg ...?
This is the analog of the core Tcl unset command except that the variable name is resolved in the context of the object's namespace.
struct unset ?varName varName ...?
This is the analog of the core Tcl vwait command except that the variable name is resolved in the context of the object's namespace.
struct vwait varName
The following non-exported methods are defined:
This method takes one argument, the name of a variable to be resolved in the context of the object's namespace, and returns the fully qualified name of the variable such that it can be used with extensions such as Tk (e.g. for the label widget's -textvariable option). This method does not assign any value to the variable. Syntax:
struct var varName
It is expected that this convenience method will normally be used solely through the my command.
The core Tcl info command shall be extended in the following ways.
An object subcommand that shall provide information about a particular object. Its first argument shall be the name of an object to get information about, its second argument shall be a subsubcommand indicating the type of information to retrieve and all subsequent arguments shall be arguments, as appropriate. The following types of information shall be available:
Returns the list of arguments to a method supported by an object.
info object object args method
Returns the body of a method supported by an object.
info object object body method
Returns the current list of enabled assertion types for an object (see the documentation for oo::check for the list of acceptable assertion types).
info object object check
Returns the class of an object, or if className is specified, whether the object is (directly or indirectly through inheritance or mixin) an instance of the named class.
info object object class ?className?
Returns whether a particular argument to a method has a default value specified, much as info default does for a normal procedure argument.
info object object default method argName defaultValueVar
Returns the list of filters defined for an object.
info object object filters
Returns boolean information about how an object relates to the class hierarchy. Supports a range of subcommands to allow the specification of what sort of test is to be performed:
Returns whether the named object is a class.
info object object isa class
Returns whether the named object is a class that is not of immediate type oo::class but rather one of its subtypes instead.
info object object isa metaclass
Returns whether the named object has mixinClassName as one of its mixins.
info object object isa mixin mixinClassName
Returns whether object really names an object.
info object object isa object
Returns whether the objec is of type class (i.e. an instance of that class or an instance of a subclass of that class).
info object object isa typeof class
Returns the list of methods defined for an object.
info object object methods
Returns the list of mixins for an object.
info object object mixins
Returns the list of all variables defined within the object, or optionally just those that match pattern according to the rules of string match.
info object object vars ?pattern?
A class subcommand that shall provide information about a particular class. Its first argument shall be the name of a class to get information about, its second argument shall be a subsubcommand indicating the type of information to retrieve and all subsequent arguments shall be arguments, as appropriate. The following types of information shall be available:
Returns the list of arguments to a method supported by an object.
info class object args method
Returns the body of a method supported by an object.
info class object body method
Returns whether a particular argument to a method has a default value specified, much as info default does for a normal procedure argument.
info class object default method argName defaultValueVar
Returns the list of filters defined for an object.
info class object filters
Returns a list of all direct instances of the class (but not instances of any subclasses of the class).
info class class instances
Returns the list of methods defined for an object.
info class object methods
Returns the list of mixins for an object.
info class object mixins
Returns a list of all parameters defined by the class.
info class class parameters
Returns a list of all subclasses of the class, or optionally just those that match pattern.
info class class subclasses ?pattern?
Returns a list of all superclasses of the named class in the class hierarchy. The list will be ordered in inheritance-precedence order.
info class class superclasses
Object::autoname: This is trivially implemented in a small procedure, and core objects can pick names for themselves and are renameable.
Object::check: Preconditions and postconditions are not supported (they add a lot of complexity) and neither are invariants. Hence, there is no need to control whether they are executed.
Object::cleanup: This is not an especially well-defined method (what if the object happens to hold handles to complex resources such as network sockets; it is not generally possible for the state of the remote server to be reset) and can be added in any compatability layer.
Object::configure: This feature has been deliberately omitted from the core object system. This would be value added by any XOTcl extension.
Object::extractConfigureArg: This feature is part of configure.
Object::getExitHandler: This feature is not necessary for this version. If it existed, it would not need to be a part of the base object.
Object::info: The introspection features are moved into the core info command.
Object::invar: Invariants may be implemented using filters.
Object::move: This feature is equivalent to the use of the standard rename operation.
Object::noinit: This feature has been deliberately omitted from the core object system because its use is dependent on the use of other deliberately-omitted features (i.e., configure). This would be value added by any XOTcl extension.
Object::parametercmd: The core object system always handles parameters in the same simple way; customisation of this process should be done by subclasses of oo::class that override the parameter method.
Object::requireNamespace: It should be possible to do away with this feature through better integration with the core.
Object::setExitHandler: See the comments for getExitHandler above.
Class::__unknown: Auto-loading of unknown classes is handled by the standard core unknown command.
Class::abstract: Abstractness is relatively easy to implement on top of the proposed infrastructure and is not critical to getting an implementation.
Class::allinstances: This feature is trivially implemented in a small procedure.
Class::alloc: The core objects have no default behaviour, so the difference with the basic core class behaviour is moot.
Class::create: Core object creation is a much more sealed process, but the lack of configure-like behaviour means that the complexity of this method is not necessary. Instead, constructors are called automatically.
Class::parameterclass: Core object system parameters are not implemented by classes.
Class::volatile: This feature is omitted as it is believed that it is possible to implement automated lifecycle management as a mixin.
This document has been placed in the public domain.
The following sections are non-normative.
The XOTcl Object class should derive from the core oo::struct class. The XOTcl Class class must derive from the core oo::definer and the XOTcl Object classes. This gives the following diagram (core classes are in lower case with their namespace omitted, XOTcl classes are in upper case).
+--------+
,------->| object |
| +--------+
| ,^.
creates | ______|______
| | |
| +-------+ +--------+
`--| class | | struct |
+-------+ +--------+
,^. ,^.
| |
+---------+ +--------+
| definer | | Object |<-.
+---------+ +--------+ |
,^. ,^. |
|______ ______| | creates
| |
+-------+ |
| Class |---------'
+-------+
Note that class instances create objects (or subclasses thereof), but Class instances create Objects (or subclasses thereof).
This is not necessarily the current version of this TIP.