Object Tcl - Example

Introduction

This page provides a simple example of how to:

Export C++ classes to the Object Tcl system
Describe new classes in Object Tcl
Inherit Object Tcl classes from C++ classes
Build an augmented Tcl shell
Manipulate Object Tcl objects

C++ Class

Class A is a simple C++ class that maintains a reference (pointer) to another class A object and provides three methods. The first method, doIt, is a virtual method that prints a simple message identifying itself as class A's version of doIt. The second method, doItNext, invokes the doIt method upon the other class A object referenced by the private attribute next. The final method, setNext, can be used to set the next class A object reference.

Class A is described and implemented in C++ in the following two files:

File A.H

#ifndef A_H
#define A_H

// File A.H

class A
{
public:
   A (A *next);			// Constructor member function
   virtual ~A ();		// Destructor member function
   virtual void doIt (void);	// Virtual member function
   void doItNext (void);	// Member function
   void setNext (A *obj);	// Member function
private:
   A *next;			// Member variable
};

#endif // A_H

File A.C

#include <iostream.h>
#include "A.H"

// File A.C

A::A (A *n)
{
   next = n;
   cout << "A::constructed" << endl;
}

A::~A ()
{
   cout << "A::destructed" << endl;
}

void A::doIt (void)
{
   cout << "A::doIt" << endl;
}

void A::setNext (A *n)
{
   next = n;
}

void A::doItNext (void)
{
   if (next != NULL)
   { 
      next->doIt();	// "doIt" is dynamically bound so it could be the
			// "doIt" of class A or a subclass of A.
   }
}

Exporting C++ Class A to Object Tcl

The following file contains the CDL for class A. This file may be processed by the CDL processor and the resulting C++ files linked into the final application.

# File A_cdl.cdl

pass {
// Generated from A_cdl.cdl
#include "A.H"
#include "A_cdl.H"
}

class A {
   constructor {obref A}
   method doIt -dynamic {void} {void}	# "doIt" required dynamic binding
   method doItNext -static {void} {void}
   method setNext -static {obptr A} {void}
   # The "A" after the obref type indicates the actual type of the
   # parameter expected by the "setNext" method.
}

An Object Tcl Class

Class B is a new class described in Object Tcl, below, that inherits from class A.

Class B provides its own version of the doIt method, that is also defined in class A. Class B's version of the doIt method prints a simple message identifying itself and then calls the doIt method of its parent class (A).

# File B.tcl

otclInterface B -isA A {
   constructor {next value}
   method doIt {}	# Redefines the "doIt" method available in class A
}

otclImplementation B {
   # The constructor method passes on its first argument up to
   # the constructor of the "A" parent class.
   constructor {n v} {{A $n}} {
      set value $v
      puts "B::constructed with value $value"
   }
   destructor {
      puts "B::destructed"
   }
   # The new version of the "doIt" method.
   method doIt {} {
      puts "B::doIt, value is $value, calling A::doIt"

      # Invoke the "doIt" method but make sure it is the version
      # of the "A" parent class and this version that would be
      # chosen by default using dynamic binding.
      $this -A doIt
   }
   attribute value
}

Building

To build a version of the Tcl shell (tclsh) that includes the Object Tcl package, with class A and class A exported to Object Tcl, perform the following:

Compile class A:
```
system% CC -c A.C
```

Process the CDL for class A:

system% cdl -h A_cdl.cdl A_cdl.H
system% cdl -s A_cdl.cdl A_cdl.C

Compile the C++ code generated by the processing of A's CDL description:
```
system% CC -I$(OTCL) -I$(TCL) -c A_cdl.C
```

Link the object files together using a C++ linker:

system% CC -o examplesh -L$(OTCL) -L$(TCL) A.o A_cdl.o \
$(OTCL)/tclAppInit.o -lotcl -ltcl -lm

Note: the files tclAppInit.o and tclMain.o are modified version of the files from the tcl7.3 distribution. These files have been modified to initialize the Object Tcl package and compile under C++.

Testing

To test the augmented Tcl shell:

system% ./examplesh
% source TestB.tcl
About to construct an instance of class A
A::constructed

About to call method doIt on instance of class A
A::doIt

About to construct an instance of class B
A::constructed
B::constructed with value 5

About to call method doIt on instance of class B
B::doIt, value is 5, calling A::doIt
A::doIt

Setting instance of class A to point at instance of class B
About to call method doItNext on instance of class A
B::doIt, value is 5, calling A::doIt
A::doIt

About to destruct instance of class A
A::destructed

About to destruct instance of class B
B::destructed
A::destructed
%

Where TestB.tcl is:

# TestB.tcl

puts "About to construct an instance of class A"
set a [otclNew A ""]

puts "\nAbout to call method doIt on instance of class A"
$a doIt

source B.tcl
puts "\nAbout to construct an instance of class B"
set b [otclNew B "" 5]

puts "\nAbout to call method doIt on instance of class B"
$b doIt

puts "\nSetting instance of class A to point at instance of class B"
$a setNext $b
puts "About to call method doItNext on instance of class A"
$a doItNext

puts "\nAbout to destruct instance of class A"
otclDelete $a

puts "\nAbout to destruct instance of class B"
otclDelete $b

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