Abbreviated form of incr Tcl, iTcl is the term used for referring to the object- oriented extensions meant for Tcl programming language. Let’s learn more about . The name is often written as “itcl” as that contains no metacharacters (i.e., space or brackets). BOOK Tcl/Tk Tools: contains an out of date tutorial for itcl. A more tutorial introduction is presented in the paper included with this distribution. See the class command for more details on creating and using classes.
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In addition to supporting object-oriented programming. Tcl also provides the foundation for building extended OO systems on top. This chapter provides an introduction to these facilities. This chapter describes Tcl features that support object oriented programming. You can always refer there.
Incr Tcl – Wikipedia
Also, the author is hardly an expert in object oriented design and it would really be a case of the one-eyed leading the blind. Nevertheless, as we go along we will briefly describe some basic concepts for the benefit of the reader who really is completely unexposed to OO programming.
One of the knocks against Tcl in its early days was that it did not support object oriented programming. This criticism was both incorrect and unfair because Tcl did in fact support not one, but several, OO implementations.
This misconception was at least partly due to the fact that these OO systems did not come as part of the core language, but rather were tutoriial as extensions or packages. In fact, writing an OO system in Tcl became a rite of passage for many Tcl programmers. It was intended to make programmers used to that language feel at home. It was one of the earliest Tcl-based OO extensions to be widely used.
This became part of the Tcl 8. TclOO can be used as a standalone OO system by itself. However, one of its goals was also to provide the base facilities required for layering other OO systems on top. Most of the example code in this chapter is based on a framework for modeling banks. Our bank has accounts of different types, such as savings and checking, and which allow operations such as deposits and withdrawals. Some of these are common to all account types while others are unique.
We have certain privileged customers who get special treatment and we have to tutrial follow certain directives from Big Brother. No, Citibank cannot run its operations based on our framework but it suffices for our illustrative purposes. The core of OO programming is, no surprise, an object.
In most languages, ihcl of these messages involves calling methods which are just function calls with a context associated with the object. For example, the state contained in utcl object representing a bank account would include items such as the current balance and account number.
The object would respond to messages to deposit or withdraw funds.
itcl (n) – Linux Man Pages
A class is loosely speaking a template that defines the data items and methods collectively called members encapsulated by objects of a specific type. More often than not, creating a object of the class, often known igcl instantiating an object, is one of the duties of the class. Not every OO system has, or needs, the notion of a class.
TclOO provides facilities to support both the classy and the classless [ 1 ] models. Classes are created in TclOO using the oo:: Let us create a class, Accountthat models a banking account. This creates a new class Account that can be used to create objects representing bank accounts. The class Account is actually just another Tcl command and could have been created in any namespace we choose, not necessarily the global ihcl. There is however no class definition associated with our Account class and therefore, there is as yet no state or behaviour defined for objects of the class.
That is done through one or more class definition scripts. We will look at the contents of these definition scripts throughout this chapter, but for now, simply note that class definitions can be built up in incremental fashion.
A definition script can be passed as an additional argument to the oo:: We will see both forms of class definition as we go along. As is generally the case, Tcl has the flexibility to fit your programming style. Not commonly used in operational code, this ability to destroy classes is sometimes useful during interactive development and debugging to reset to a known clean state.
We will be using the Account class throughout the chapter so let us recreate it before we move on. In our simple example, the state for an account object includes an account number that uniquely identifies it and the current balance. We will need data members to hold this information and we define them through a class definition script passed to oo:: This defines the data tutirial for the class as per-object variables. AccountNumber and Balance are then visible within all methods of the class and can be referenced there without any qualifiers or declarations.
Data members do not have to be declared using variable in a class definition script. They can also be declared within a method using the my variable command which we show later.
Having defined the data members, let us move on to defining the methods that comprise the behaviour of an Account object. An Account object responds to requests to get the current balance and to requests for depositing and withdrawing funds. Methods are defined through the method command which, like variableis executed as part of a class definition script. As you see, a method is defined in exactly the same manner as proc defines a Tcl procedure.
Just like in that case a method takes an arbitrary number of arguments including a variable number of trailing tutoorial collected as the args variable.
The difference from a procedure lies in how it is invoked and the context in which the method executes. This means the object data members such as Balancedefined through variableare in the scope of the method and can directly be referenced without any qualifiers as seen in the method definition above. The method context also makes available several commands – such as selfnext and my – which can tutorail be called from within a method.
We will see these as we proceed with our example but for itco note the use of my to refer to a method of the object in whose context the current method is running. Another point about method definitions concerns itvl visibility. A private method on the other hand, can only be invoked from within another method in the object context.
Methods that begin with a lower case letter are exported by default. Thus in our example, deposit and withdraw are exported methods while UpdateBalance is not.
Method visibility can be changed by using the export and unexport commands inside a oo:: Method definitions can be deleted at any time with the deletemethod command inside turorial class definition script. Deletion of methods from classes is rarely used. However, deletion of methods from objects is sometimes useful in specialization of objects.
There is one final thing we need to do before we can start banking operations and that is to provide some means to initialize an Account object when it is created and perform any required clean up when it is destroyed. These tasks are performed through the special methods named constructor and destructor. These differ from normal methods in only two respects:. They are not explicitly invoked by name. Rather, the constructor method is automatically run when an object is created.
Conversely, the destructor method is run when the object is destroyed. The destructor method definition differs tutoral other methods in that it only has a single parameter – the script to be run. It does not have a parameter corresponding to arguments. Both constructors and destructors are optional.
They do not have to be defined in which case TclOO will simply generate empty methods for them. Every object has a method named unknown which is run when no method of that name is defined for that object actually in the method chain for that object as we see later. The unknown method is passed the name of the invoked method as its first argument followed by the arguments from the invocation call.
The default implementation of this method, which is inherited by all objects from the root oo:: Classes and objects can override the default implementation method to take some other action instead. The properties and methods exported from a COM component are not always known beforehand and in fact can be dynamically modified.
[incr Tcl] – Object-Oriented Programming in Tcl/Tk
If found, the lookup returns an index into a function table that can then be invoked through the ComCall method. The implementation of unknown looks like. This is a greatly simplified, not entirely accurate or tutroial, description for illustrative purposes. As we have seen in previous sections, you can incrementally modify a class using oo:: Practically nothing about a class tutorail sacred – you can add or delete methods, data members, change superclasses or mixins, and so on.
The question then arises as to what happens to objects that have already been created if a class is modified. Or if you add a mixin or a superclassthe method lookup sequence for the object will be appropriately modified.
However some care should be taken when modifying a class since existing objects may not hold all state expected by the new class. For example, the new constructors are obviously not run for the existing objects and thus some data members may be uninitialized. The modified class code has to account for such cases.
Having defined our model, we can now begin operation of our bank to illustrate how objects are used. An object of a class is created by invoking one of two built-in methods on the class itself.
The create method creates an object with a specific name. The new method generates a name for the created object. Creating an object also initializes the object by invoking a its constructor.
Objects in Tcl are not garbage collected as in some other languages and have to be explicitly destroyed by calling their built-in destroy method. As discussed earlierwhen calling a method from another method in the same object contextthe alias my is used to refer to the current object.
So the deposit method we saw earlier calls the UpdateBalance method as:. Data members are not directly accessible from outside the object. Methods, such as balance in our example, have to be defined to allow callers to read and modify their values.