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__TOC__
==Overview==
==Overview==
The ability to define classes in <var class="product">SOUL</var> is a powerful facility. However, some classes are likely to be extremely heavily accessed, require access to facilities not directly accessible in <var class="product">SOUL</var>, or both. It is for such cases that '''system classes''' were created.
   
   
The ability to define classes in [[User Language]] is a powerful facility. However, some classes are likely to be extremely heavily accessed, require access to facilities not directly accessible in User Language, or both.
A system class is a class that is built into the <var class="product">[[Model 204]]</var> nucleus and is typically written in Assembler language. As such, these classes are very efficient and provide an object-oriented interface to the <var class="product">Model 204</var> kernel. Because these classes are not written in <var class="product">SOUL</var>, however, the class definitions are not available in <var class="product">SOUL</var>, so the documentation must be consulted to determine the methods available in each class.
It is for such cases that '''system classes''' were created.
A system class is a class that is built into the [[Model 204]] nucleus and is typically written in Assembler language. As such, these classes are very efficient and provide an object-oriented interface to the Model 204 kernel. Because these classes are not written in User Language, however, the class definitions are not available in User Language, so the documentation must be consulted to determine the methods available in each class.
   
   
Some system classes are available to all useres license for any Sirius Software product, and most system classes are available to all users licensed for [[Janus SOAP]]. However, some system classes are tied to another product and so only available to users licensed for that product. For example, the system Socket class is only available to users licensed for [[Janus Sockets]].
Some system classes are available to all users licensed for version 7.5 or higher of <var class="product">Model&nbsp;204</var>, and some system classes are tied to another product and so only available to users licensed for that product. For example, the system <var>Socket</var> class is only available to users licensed for <var class="product">[[Janus Sockets]]</var>.
   
   
All the system classes for which a user is authorized are available in any User Language request and do not need to be declared. To use a system class, simply declare an object of that class. For example, the [[Stringlist class]] implements what is effectively an unbounded array of strings. To use this class, just declare a Stringlist object:
All the system classes for which a user is authorized are available in any <var class="product">SOUL</var> request and do not need to be declared. To use a system class, simply declare an object of that class. For example, the <var>[[Stringlist class|Stringlist]]</var> class implements what is effectively an unbounded array of strings. To use this class, just declare a <var>Stringlist</var> object:
<pre>
<p class="code">%list  is object stringlist
%list  is object stringlist
   ...
   ...
%list = new
%list = new
%list:add('First item')
%list:add('First item')
%list:add('Second item')
%list:add('Second item')
</pre>
</p>
Some system classes have shared methods and, like user classes,
Some system classes have shared methods and, like user classes,
these can be accessed with any object of the class or with the
these can be accessed with any object of the class or with the class name in parentheses.
class name in parentheses.
For example, the <var>[[MaxItemLength (Stringlist property)|MaxItemLength]]</var> property in the <var>Stringlist</var> class
For example, the [[MaxItemLength (Stringlist property)|MaxItemLength poperty]] in the Stringlist class
returns the maximum length of any <var>Stringlist</var> item.
returns the maximum length of any Stringlist item.
Since this length does not depend on a particular instance, it is a shared method and can be accessed as such:
Since this length does not depend on a particular instance, it is a shared method and can be accessed as such:
<pre>
<p class="code">print %(Stringlist):maxItemLength
print %(Stringlist):maxItemLength
  ...
  ...
%list is object stringList
%list is object stringList
print %list:maxItemLength
print %list:maxItemLength
</pre>
</p>
   
   
There are already a large number of system classes, and their number is
There are already a large number of system classes, and their number is expected to grow over time.
expected to grow over time.
This raises the question of what happens when a new system class with the same name as a [[Classes and Objects|user class]] becomes available. For example, suppose there was already a user-written <var>Stringlist</var> class when the system class of the same name became available. What would be made of the following code?
This raises the question of what happens when a new system class with the same name as a [[Classes and Objects|User Language class]] becomes available. For example, suppose there was already a User Language Stringlist class when the system class of the same name became available. What would be made of the following code?
<p class="code">class stringList
<pre>
class stringList
   ...
   ...
end class
end class
...
  ...
%list is object stringList
%list is object stringList
</pre>
</p>
The answer is that the code would continue to work as it had before. This is because it is perfectly valid to declare a User Language class with the same name as a system class, and in such situations, all references to the class name are assumed to refer to the User Language class. To access the system Stringlist class in this situation, you precede the class name with "system:" wherever it appears:
The answer is that the code would continue to work as it had before. This is because it is perfectly valid to declare a user class with the same name as a system class, and in such situations, all references to the class name are assumed to refer to the user class. To access the system <var>Stringlist</var> class in this situation, you precede the class name with <code>system:</code> wherever it appears:
<pre>
<p class="code">%systemList  is object system:stringList
%systemList  is object system:stringList
  ...
  ...
print %(system:Stringlist):maxItemLength
print %(system:Stringlist):maxItemLength
</pre>
</p>
The "system:" prefix can be used, of course, regardless of whether a User Language class by the same name exists. In fact, some sites may wish to adopt a local standard that all system class references must be preceded by "system:" to make clear what they are and to avoid confusion with User Language classes. That said, it is probably best to avoid the names of system classes when declaring User Language classes.
The <code>system:</code> prefix can be used, of course, regardless of whether a user class by the same name exists. In fact, some sites may wish to adopt a local standard that all system class references must be preceded by <code>system:</code> to make clear what they are and to avoid confusion with user-written classes. That said, it is probably best to avoid the names of system classes when declaring user classes.
   
   
Except for the way they are defined, system classes behave exactly
Except for the way they are defined, system classes behave exactly
like User Language classes:
like user-written classes:
<ul>
<ul>
<li>Object variables are declared, constructed (New), and discarded in the same way.
<li>Object variables are declared, constructed (New), and discarded in the same way. </li>
<li>Object variables of a system class can be declared inside a User Language class or structure.
 
<li>Null tests and comparisons on system objects work the same way as for User Language objects.
<li>Object variables of a system class can be declared inside a user class or structure. </li>
<li>System object variables can be passed as parameters, and methods on system objects can be strung together.
 
<li>Null tests and comparisons on system objects work the same way as for user objects. </li>
 
<li>System object variables can be passed as parameters, and methods on system objects can be strung together. </li>
</ul>
</ul>
One distinction between User Language classes and system classes is that the system
One distinction between user classes and system classes is that the system class methods do not have explicit declarations.
class methods do not have explicit declarations.
So the only way to understand the return values and parameters to a system method is through the documentation.
So the only way to understand the return values and parameters to a system
method is through the documentation.


For documentation purposes, all system method parameters and return values
For documentation purposes, all system method parameters and return values
(or inputs and outputs) can be thought of as having one of four basic types:
(or inputs and outputs) can be thought of as having one of four basic types:
<table class="syntaxTable">
 
<tr><th>Object</th>
<table>
<tr><th><var>Object</var></th>
<td>Some method outputs or inputs are objects of a particular class. The input and output classes will be specified in the documentation.</td></tr>
<td>Some method outputs or inputs are objects of a particular class. The input and output classes will be specified in the documentation.</td></tr>
<tr><th>Float</th>
 
<td>All numeric input and outputs can be thought of as being declared as Float. Of course, Model 204's handling of Float values makes a Float input or output suitable even where only integral values are expected.</td></tr>
<tr><th><var>Float</var></th>
<tr><th>Longstring</th>
<td>All numeric input and outputs can be thought of as being declared as <var>Float</var>. Of course, <var class="product">Model 204</var>'s handling of <var>Float</var> values makes a <var>Float</var> input or output suitable even where only integral values are expected.</td></tr>
<td>Any input or output that is a string, even if it is guaranteed to be shorter than 256 bytes should be considered a longstring. Longstring values are treated differently in certain expressions and have different truncation behavior than other strings &mdash; they will cause request cancellation if assigned to something that requires the longstring to be truncated.</td></tr>
 
<tr><th>Unicode</th>
<tr><th><var>Longstring</var></th>
<td>Identical to longstring except the character set is considered to be the double-byte unicode character set. Unicode variables and values are passed as unicode parameters without any conversion while all other values are converted to an EBCDIC string as needed, and then converted to unicode using the Online's EBCDIC to unicode translation tables. Any untranslatable EBCDIC string results in a request canceling error.  
<td>Any input or output that is a string, even if it is guaranteed to be shorter than 256 bytes should be considered a <var>[[Longstrings|Longstring]]</var>. <var>Longstring</var> values are treated differently in certain expressions and have different truncation behavior than other <var>String</var> values &mdash; they will cause request cancellation if assigned to something that requires the <var>Longstring</var> to be truncated.</td></tr>
 
<tr><th><var>Unicode</var></th>
<td>Identical to <var>Longstring</var> except the character set is considered to be the double-byte Unicode character set. <var>Unicode</var> variables and values are passed as Unicode parameters without any conversion, while all other values are converted to an EBCDIC string as needed, and then converted to <var>Unicode</var> using the Online's EBCDIC to Unicode translation tables. Any untranslatable EBCDIC string results in a request canceling error.  
</table>  
</table>  
The one exception to this simple picture is the ''Item'' input
 
and output from [[Collections|collection classes]].
The one exception to this simple picture is the <var>Item</var> input and output from [[Collections|collection classes]].
Methods in collection classes behave as if any item input or output
Methods in collection classes behave as if any item input or output
was an assignment to or from the item class type.
was an assignment to or from the item class type.
To illustrate the importance of this distinction, consider the following
To illustrate the importance of this distinction, consider the following code:
code:
<p class="code">%list    is object stringList
<pre>
%list    is object stringList
%string  is string len 10
%string  is string len 10
   
   
Line 85: Line 82:
%list:add('The bear went over the mountain')
%list:add('The bear went over the mountain')
%string = %list:item(1)
%string = %list:item(1)
</pre>
</p>
   
   
In this example, the last assignment would result in a request cancellation
In this example, the last assignment would result in a request cancellation
because, even though the first Stringlist item is only 31 bytes long, it
because, even though the first <var>Stringlist</var> item is only 31 bytes long, it
is a longstring value so causes request cancellation when truncated on
is a <var>Longstring</var> value so causes request cancellation when truncated on assignment.
assignment.
   
   
On the other hand consider the following:
On the other hand consider the following:
<pre>
<p class="code">%list    is collection arrayList of string len 255
%list    is collection arrayList of string len 255
%string  is string len 10
%string  is string len 10
   
   
Line 100: Line 95:
%list:add('The bear went over the mountain')
%list:add('The bear went over the mountain')
%string = %list:item(1)
%string = %list:item(1)
</pre>
</p>
   
   
In this case the last assignment does not cause request cancellation, because
In this case the last assignment does not cause request cancellation, because
it acts like an assignment from a String Len 255 variable to a String Len 10
it acts like an assignment from a <code>String Len 255</code> variable to a <code>String Len 10</code>,
which results in silent truncation of the input.
which results in silent truncation of the input.
==Using system classes instead of $functions==
==Using system classes instead of $functions==
There are several suites of $functions that are heavily used at many
There are several suites of $functions that are heavily used at many
sites that "behave badly" in an object-oriented environment, especially inside
sites that "behave badly" in an object-oriented environment, especially inside non-shared methods.
non-shared methods.
For example, the [[$lists|$list]] suite of $functions do not work well inside of methods.
For example, the $list suite of $functions do not work well inside of methods.
You might want a $list for each instance of a class, and
You might want a $list for each instance of a class, and
it might seem logical to do something like the following:
it might seem logical to do something like the following:
<pre>
<p class="code">class section
class section
   public
   public
       constructor new
       constructor new
Line 123: Line 116:
   end constructor
   end constructor
end class
end class
</pre>
</p>
The preceding code would compile without errors and actually work, more or less
The preceding code would compile without errors and actually work, more or less
correctly, as long as only a single instance of the class is created.
correctly, as long as only a single instance of the class is created.
Unfortunately, if a second instance of the class were created,
Unfortunately, if a second instance of the class were created,
the <code>$listnew</code> in the New constructor for the object would return the same $list identifier as
the <code>$listnew</code> in the <code>New</code> constructor for the object would return the same $list identifier as
for the first instance, and it would cause any data in the $list for the first
for the first instance, and it would cause any data in the $list for the first instance to be deleted.
instance to be deleted.
   
   
Furthermore, since both objects would have the same $list identifier, they would
Furthermore, since both objects would have the same $list identifier, they would
be sharing the same $list, which is not likely to be what was intended.
be sharing the same $list, which is not likely to be what was intended.
The Stringlist class avoids these problems:
The <var>Stringlist</var> class avoids these problems:
<pre>
<p class="code">class section
class section
   public
   public
       constructor new
       constructor new
Line 144: Line 135:
   end constructor
   end constructor
end class
end class
</pre>
</p>
Clearly, the code is no more complicated than the $list version, but by
Clearly, the code is no more complicated than the $list version, but by
using a system object, the statement <code>%list = new</code> creates a
using a system object, the statement <code>%list = new</code> creates a
new Stringlist instance for each instance of the Section class, thereby
new <var>Stringlist</var> instance for each instance of the <code>Section</code> class, thereby
solving the problem with the $list version.
solving the problem with the $list version.
In addition, the Stringlist object is strongly datatyped, so users
In addition, the <var>Stringlist</var> object is strongly datatyped, so users
or maintainers of the class have a clearer sense of the function of
or maintainers of the class have a clearer sense of the function of the <code>List</code> variable.
the List variable.
Also, errors such as assigning a number to that <code>List</code>
Also, errors such as assigning a number to that List
variable are caught at compile-time.
variable are caught at compile-time.
Finally, use of an object in this context makes it possible to
Finally, use of an object in this context makes it possible to
take advantage of the better object-oriented syntax for stringing methods:
take advantage of the better object-oriented syntax for "stringing" methods:
<pre>
<p class="code">%calculus  is object section
%calculus  is object section
   ...
   ...
%calculus:list:add(%student)
%calculus:list:add(%student)
</pre>
</p>
   
   
The system classes that replace existing handle-based suites
The system classes that replace existing handle-based suites of $functions are:
of $functions are:
<table>
<table class="syntaxTable">
<tr><th><var>[[Recordset class|Recordset]]</var>, <var>[[Record class|Record]]</var>, <var>[[SortedRecordset class|SortedRecordset]]</var>, <var>[[RecordsetCursor class|RecordsetCursor]]</var></th>
<tr><th><var>[[Recordset class|Recordset]]</var>, <var>[[Record class|Record]]</var>, <var>[[SortedRecordset class|SortedRecordset]]</var>, <var>[[RecordsetCursor class|RecordsetCursor]]</var></th>
<td>Replace traditional User Language file-oriented facilities.</td></tr>
<td>Replace traditional <var class="product">SOUL</var> file-oriented facilities.</td></tr>
 
<tr><th><var>Socket</var></th>
<tr><th><var>Socket</var></th>
<td>Replaces $Sockets. Only available to licensees of both [[Janus Sockets]] and [[Janus SOAP]].</td></tr>
<td>Replaces $Sockets. Only available to licensees of [[Janus Sockets]].</td></tr>
 
<tr><th><var>[[Stringlist class|Stringlist]]</var></th>
<tr><th><var>[[Stringlist class|Stringlist]]</var></th>
<td>Replaces $Lists.</td></tr>
<td>Replaces $lists.</td></tr>
 
<tr><th><var>[[XmlDoc class|XmlDoc]]</var>, <var>[[XmlNode class|XmlNode]]</var>, <var>[[XmlNodelist class|XmlNodelist]]</var></th>
<tr><th><var>[[XmlDoc class|XmlDoc]]</var>, <var>[[XmlNode class|XmlNode]]</var>, <var>[[XmlNodelist class|XmlNodelist]]</var></th>
<td>Replace $XML_ $functions. Only available to licensees of [[Janus SOAP]].</td></tr>
<td>Replace $XML_ $functions. Only available to licensees of [[Janus SOAP]].</td></tr>
</table>  
</table>  
Other system classes are documented in the manuals
Other system classes are included in the [[Lists of classes and methods]].
for the products to which they apply.
 
==Constant methods==
==Constant methods==
Constant methods are methods that belong to system classes but are unique
Constant methods are methods that belong to system classes but are unique
because they are evaluated at compilation time.
because they are evaluated at compilation time.
Line 210: Line 200:
Thus, <code>'0d25':x</code> is basically a hex constant.
Thus, <code>'0d25':x</code> is basically a hex constant.
   
   
The individual Constant methods are described in the following subsections.
The following are the available Constant methods:
===X function===
<div class="showVisit">
<table class="wikitable">
This function returns the unencoded value of a hex-encoded string.
<tr class="head"><th>Method</th><th>Description</th></tr>
It treats each input character as an input hex digit.
 
<tr><td>[[U (String function)|U function]]</td><td>Converts an EBCDIC string to a Unicode string </td></tr>
The X method is available as of [[Sirius Mods]] version 7.3.
 
====X syntax====
<tr><td>[[X (String function)|X function]]</td><td>Returns the unencoded value of a hex-encoded string</td></tr>
<pre>
%out = string:X
</pre>
====Syntax Terms====
<table class="syntaxTable">
<tr><th>%out</th>
<td>A string variable to receive the unencoded value of the method object string.</td></tr>
<tr><th>string</th>
<td>A hex-encoded string value.</td></tr>
</table>
</table>
====Usage Notes====
</div>
<ul>
 
<li>As in all [[Janus SOAP]] system methods with string outputs,
the ''%out'' string variable provides [[Longstrings|longstring behavior]].
This adds 28 bytes of VTBL usage to every X constant, however.
<li>Although its utility is dubious, the following is valid and sets %x to X'01':
<pre>
%x = 'f0f1':x:x
</pre>
<li>The X function requires a method object string with an even
number of bytes.
<li>The X method is a compile-time-only equivalent
of the [[HexToString (String function)|HexToString method]] of the intrinsic String class.
</ul>
====Example====
The following statement would display <code>'1234'</code>:
[[Intrinsic classes#printtext|printText]] {'f1f2f3f4':x}
===U function===
This function converts an EBCDIC method object string to a Unicode string.
The function also converts XML style hexadecimal character references,
XHTML entity references, and '''&amp;''' references to the represented Unicode character.


The U method is available as of [[Sirius Mods]] version 7.3.
[[Category:SOUL object-oriented programming topics]]
====U syntax====
%unicode = string:U
====Syntax Terms====
<table class="syntaxTable">
<tr><th>%unicode</th>
<td>A Unicode string variable to receive the Unicode encoding of the method object string.</td></tr>
<tr><th>string</th>
<td>A constant character string value, which may include an XML-style hexadecimal character reference or an XHTML entity reference. That is, ''string'' may contain an ampersand (&) in the following cases:
<ul>
<li>As the substring '&amp;amp;'.
This substring is converted to a single ampersand character.
<li>At the start of a hexadecimal character reference (for example, the eight characters
'&amp;#x201C;' for the Unicode "Left double quotation mark").
The character reference is converted to the referenced character.
<li>As of [[Sirius Mods]] version 7.6, an XHTML entity reference (for example,
the six characters '&amp;nbsp;' for the "non-breaking-space" character).
The entity reference is converted to the referenced character.
</ul>
A decimal character reference (for example, '&amp;#172;') is ''not'' allowed.</td></tr>
</table>
====Usage Notes====
<ul>
<li>The U method is a compile-time-only equivalent
of the [[EbcdicToUnicode (String function)|EbcdicToUnicode method]] of the intrinsic String class (with its CharacterDecode argument implicitly set to <code>True</code>).
<li>Using the U method (or EbcdicToUnicode) is necessary for converting to type Unicode if the string you want to convert may contain a hexadecimal character reference.
Such a reference cannot be meaningfully assigned to a Unicode variable otherwise,
whereas keyboard-available characters can simply be assigned
directly to a Unicode variable without character reference and without
conversion by U.
<li>You can find the list of XHTML entities on the Internet at [http://www.w3.org/TR/xhtml1/dtds.html#h-A2 http://www.w3.org/TR/xhtml1/dtds.html#h-A2]
</ul>
====Example====
The first Print statement below displays a plus sign (''''+''''); the second Print displays a copyright sign (&#169;); the third displays '2122':
<pre>
%p Unicode Initial('+')
print %p
* Entity for copyright symbol:
%copy Unicode Initial('&amp;copy;':U)
print %copy
* Constant for trademark symbol:
%tm Unicode Initial('&amp;#x2122;':U)
print %tm:UnicodeToUtf16:StringToHex
</pre>
Simply specifying <code>print %tm</code>  in the the third Print statement above
(or its equivalent <code>print %tm:UnicodeToEbcdic</code>) would
attempt to translate to EBCDIC and fail because
the Unicode trademark character does not translate to a valid EBCDIC character.
But the UnicodeToUtf16 method can convert the Unicode variable
to a byte-stream Longstring, which the StringToHex method converts to its
hex representation.

Latest revision as of 20:16, 16 May 2016

Overview

The ability to define classes in SOUL is a powerful facility. However, some classes are likely to be extremely heavily accessed, require access to facilities not directly accessible in SOUL, or both. It is for such cases that system classes were created.

A system class is a class that is built into the Model 204 nucleus and is typically written in Assembler language. As such, these classes are very efficient and provide an object-oriented interface to the Model 204 kernel. Because these classes are not written in SOUL, however, the class definitions are not available in SOUL, so the documentation must be consulted to determine the methods available in each class.

Some system classes are available to all users licensed for version 7.5 or higher of Model 204, and some system classes are tied to another product and so only available to users licensed for that product. For example, the system Socket class is only available to users licensed for Janus Sockets.

All the system classes for which a user is authorized are available in any SOUL request and do not need to be declared. To use a system class, simply declare an object of that class. For example, the Stringlist class implements what is effectively an unbounded array of strings. To use this class, just declare a Stringlist object:

%list is object stringlist ... %list = new %list:add('First item') %list:add('Second item')

Some system classes have shared methods and, like user classes, these can be accessed with any object of the class or with the class name in parentheses. For example, the MaxItemLength property in the Stringlist class returns the maximum length of any Stringlist item. Since this length does not depend on a particular instance, it is a shared method and can be accessed as such:

print %(Stringlist):maxItemLength ... %list is object stringList print %list:maxItemLength

There are already a large number of system classes, and their number is expected to grow over time. This raises the question of what happens when a new system class with the same name as a user class becomes available. For example, suppose there was already a user-written Stringlist class when the system class of the same name became available. What would be made of the following code?

class stringList ... end class ... %list is object stringList

The answer is that the code would continue to work as it had before. This is because it is perfectly valid to declare a user class with the same name as a system class, and in such situations, all references to the class name are assumed to refer to the user class. To access the system Stringlist class in this situation, you precede the class name with system: wherever it appears:

%systemList is object system:stringList ... print %(system:Stringlist):maxItemLength

The system: prefix can be used, of course, regardless of whether a user class by the same name exists. In fact, some sites may wish to adopt a local standard that all system class references must be preceded by system: to make clear what they are and to avoid confusion with user-written classes. That said, it is probably best to avoid the names of system classes when declaring user classes.

Except for the way they are defined, system classes behave exactly like user-written classes:

  • Object variables are declared, constructed (New), and discarded in the same way.
  • Object variables of a system class can be declared inside a user class or structure.
  • Null tests and comparisons on system objects work the same way as for user objects.
  • System object variables can be passed as parameters, and methods on system objects can be strung together.

One distinction between user classes and system classes is that the system class methods do not have explicit declarations. So the only way to understand the return values and parameters to a system method is through the documentation.

For documentation purposes, all system method parameters and return values (or inputs and outputs) can be thought of as having one of four basic types:

Object Some method outputs or inputs are objects of a particular class. The input and output classes will be specified in the documentation.
Float All numeric input and outputs can be thought of as being declared as Float. Of course, Model 204's handling of Float values makes a Float input or output suitable even where only integral values are expected.
Longstring Any input or output that is a string, even if it is guaranteed to be shorter than 256 bytes should be considered a Longstring. Longstring values are treated differently in certain expressions and have different truncation behavior than other String values — they will cause request cancellation if assigned to something that requires the Longstring to be truncated.
Unicode Identical to Longstring except the character set is considered to be the double-byte Unicode character set. Unicode variables and values are passed as Unicode parameters without any conversion, while all other values are converted to an EBCDIC string as needed, and then converted to Unicode using the Online's EBCDIC to Unicode translation tables. Any untranslatable EBCDIC string results in a request canceling error.

The one exception to this simple picture is the Item input and output from collection classes. Methods in collection classes behave as if any item input or output was an assignment to or from the item class type. To illustrate the importance of this distinction, consider the following code:

%list is object stringList %string is string len 10 %list = new %list:add('The bear went over the mountain') %string = %list:item(1)

In this example, the last assignment would result in a request cancellation because, even though the first Stringlist item is only 31 bytes long, it is a Longstring value so causes request cancellation when truncated on assignment.

On the other hand consider the following:

%list is collection arrayList of string len 255 %string is string len 10 %list = new %list:add('The bear went over the mountain') %string = %list:item(1)

In this case the last assignment does not cause request cancellation, because it acts like an assignment from a String Len 255 variable to a String Len 10, which results in silent truncation of the input.

Using system classes instead of $functions

There are several suites of $functions that are heavily used at many sites that "behave badly" in an object-oriented environment, especially inside non-shared methods. For example, the $list suite of $functions do not work well inside of methods. You might want a $list for each instance of a class, and it might seem logical to do something like the following:

class section public constructor new variable list is float end public constructor new %list = $listnew end constructor end class

The preceding code would compile without errors and actually work, more or less correctly, as long as only a single instance of the class is created. Unfortunately, if a second instance of the class were created, the $listnew in the New constructor for the object would return the same $list identifier as for the first instance, and it would cause any data in the $list for the first instance to be deleted.

Furthermore, since both objects would have the same $list identifier, they would be sharing the same $list, which is not likely to be what was intended. The Stringlist class avoids these problems:

class section public constructor new variable list is object stringlist end public constructor new %list = new end constructor end class

Clearly, the code is no more complicated than the $list version, but by using a system object, the statement %list = new creates a new Stringlist instance for each instance of the Section class, thereby solving the problem with the $list version. In addition, the Stringlist object is strongly datatyped, so users or maintainers of the class have a clearer sense of the function of the List variable. Also, errors such as assigning a number to that List variable are caught at compile-time. Finally, use of an object in this context makes it possible to take advantage of the better object-oriented syntax for "stringing" methods:

%calculus is object section ... %calculus:list:add(%student)

The system classes that replace existing handle-based suites of $functions are:

Recordset, Record, SortedRecordset, RecordsetCursor Replace traditional SOUL file-oriented facilities.
Socket Replaces $Sockets. Only available to licensees of Janus Sockets.
Stringlist Replaces $lists.
XmlDoc, XmlNode, XmlNodelist Replace $XML_ $functions. Only available to licensees of Janus SOAP.

Other system classes are included in the Lists of classes and methods.

Constant methods

Constant methods are methods that belong to system classes but are unique because they are evaluated at compilation time. These methods require constant inputs and may have no parameters, but they have no run-time overhead.

For example, the following Constant method sets %x to a string with hex value x'0678':

%x = '0678':x

The hex conversion is done at compilation time. The method in this example (named X) is a compile-time-only equivalent of the intrinsic String class HexToString method.

As other intrinsic methods, the term "method object" is used for the constant value to which a Constant method is applied, even though the value is not really an object.

Unlike other intrinsic methods, Constant methods may be invoked only against constants. A variable as method object is not allowed:

%x = %y:x

Also, a statement like %x = 1234:x is not allowed, because a Constant method requires the constant to have the correct datatype (in this case, String).

Once a Constant method is evaluated, the expression compilation is replaced by the constant result of the evaluation, and all intermediate work quads/variables are deleted. Thus, '0d25':x is basically a hex constant.

The following are the available Constant methods:

MethodDescription
U functionConverts an EBCDIC string to a Unicode string
X functionReturns the unencoded value of a hex-encoded string