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Caché Objects

This chapter discusses objects in Caché. It discusses the following topics:

Introduction to Caché Object Classes

Caché provides object technology by means of the following object classes: %Library.RegisteredObjectOpens in a new tab, %Library.PersistentOpens in a new tab, and %Library.SerialObjectOpens in a new tab.

The following figure shows the inheritance relationships among these classes, as well as some of their parameters and methods. The names of classes of the %Library package can be abbreviated, so that (for example) %PersistentOpens in a new tab is an abbreviation for %Library.PersistentOpens in a new tab. Here, the items in all capitals are parameters and the items that start with percent signs are methods.

generated description: object classes

In a typical class-based application, you define classes based on these classes (and on specialized system subclasses). All objects inherit directly or indirectly from one of these classes, and every object is one of the following types:

  • A registered object is an instance of %RegisteredObjectOpens in a new tab or a subclass. You can create these objects but you cannot save them. The other two classes inherit from %RegisteredObjectOpens in a new tab and thus include all the parameters, methods, and so on of that class.

  • A persistent object is an instance of %PersistentOpens in a new tab or a subclass. You can create, save, open, and delete these objects.

    A persistent class is automatically projected to a table that you can access via Caché SQL. The following chapter discusses persistent classes in detail.

  • A serial object is an instance of %SerialObjectOpens in a new tab or a subclass. A serial class is meant for use as a property of another object. You can create and delete these objects, but you cannot save them or open them independently of the object that contains them.

    When contained in persistent objects, these objects have an automatic projection to SQL.


Via the classes %DynamicObjectOpens in a new tab and %DynamicArrayOpens in a new tab, Caché also provides the ability to work with objects and arrays that have no schema. These classes are not discussed in this book. For details, see Using JSON in Caché.

Basic Features of Object Classes

With the object classes, you can perform the following tasks, among others:

  • You can create an object (an instance of a class). To do so, you use the %New() method of that class, which it inherits from %RegisteredObjectOpens in a new tab.

    For example:

     set myobj=##class(Sample.Person).%New()
  • You can use properties.

    You can define properties in any class, but they are useful only in object classes, because only these classes enable you to create instances.

    Any property contains a single literal value, an object (possibly a collection object), or a multidimensional array (rare). The following example shows the definition of an object-valued property:

    Property Home As Sample.Address;

    Sample.Address is another class. The following shows one way to set the value of the Home property:

     Set myaddress=##class(Sample.Address).%New()
     Set myaddress.City="Louisville"
     Set myaddress.Street="15 Winding Way"
     Set myaddress.State="Georgia"
     Set myperson=##class(Sample.Person).%New()
     Set myperson.Home=myaddress
  • You can invoke methods of an instance of the class, if the class or its superclasses define instance methods. For example:

    Method PrintPerson()
     Write !, "Name: ", ..Name

    If myobj is an instance of the class that defines this method, you can invoke this method as follows:

     Do myobj.PrintPerson()
  • You can validate that the property values comply with the rules given in the property definitions.

    • All objects inherit the instance method %NormalizeObject(), which normalizes all the object's property values. Many data types allow different representations of the same value. Normalization converts a value to its canonical, or normalized, form. %NormalizeObject() returns true or false depending on the success of this operation.

    • All objects inherit the instance method %ValidateObject(), which returns true or false depending on whether the property values comply with the property definitions.

    • All persistent objects inherit the instance method %Save(). When you use the %Save() instance method, the system automatically calls %ValidateObject() first.

    In contrast, when you work at the routine level and do not use classes, your code must include logic to check the type and other input requirements.

  • You can define callback methods to add additional custom behavior when objects are created, modified, and so on.

    For example, to create an instance of a class, you invoke the %New() method of that class. If that class defines the %OnNew() method (a callback method), then Caché automatically also calls that method. The following shows a simple example:

    Method %OnNew() As %Status 
        Write "hi there"
        Quit $$$OK

    In realistic scenarios, this callback might perform some required initialization. It could also perform logging by writing to a file or perhaps to a global.


The %New() method of an object class creates an internal, in-memory structure to contain the object’s data and returns an OREF (object reference) that points to that structure. An OREF is a special kind of value in ObjectScript. You should remember the following points:

  • In the Terminal, when you display an OREF, you see a string that consists of a number, followed by an at sign, followed by the name of the class. For example:

    SAMPLES>set myobj=##class(Sample.Person).%New()
    SAMPLES>w myobj
  • Caché returns an error if you do not use an OREF where an OREF is expected:

    SAMPLES>set myobj.Name="Fred Parker"
    SET myobj.Name="Fred Parker"


    SAMPLES>do myobj.PrintPerson()
    DO myobj.PrintPerson()

    It is helpful to be able to recognize this error. It means that the variable is not an OREF but should be.

  • There is only one way to create an OREF: Use a method that returns an OREF. The methods that return OREFs are defined in the object classes or their subclasses.

    The following does not create an OREF, but rather a string that looks like an OREF:

    SAMPLES>set testthis="4@Sample.Person"
  • You can determine programmatically whether a variable contains an OREF. The function $IsObject returns 1 (true) if the variable contains an OREF; and it returns 0 (false) otherwise.


For persistent classes, described in the next chapter, methods such as %OpenId() also return OREFs.

Stream Interface Classes

As noted earlier, Caché allocates a fixed amount of space to hold the results of string operations. If a string expression exceeds the amount of space allocated, a <MAXSTRING> error results. Unless long strings are enabled, this limit is 32 KB and no string property can be larger than about 32 KB. (No other property can exceed this limit, either, but other restrictions take effect before the long string limit applies.)

If you need to pass a long string value, or you need a property to contain a long string value, you use a stream. A stream is an object that can contain a single value whose size is larger than the string size limit. (Internally Caché creates and uses a temporary global to avoid the memory limitation.)

You can use stream fields with Caché SQL, with some restrictions. For details and a more complete introduction, see Using Caché Objects; also see the InterSystems Class Reference for these classes.

Stream Classes

The main Caché stream classes use a common stream interface defined by the %Stream.ObjectOpens in a new tab class. You typically use streams as properties of other objects, and you save those objects. Stream data may be stored in either an external file or a Caché global, depending on the class you choose:

To work with a stream object, you use its methods. For example, you use the Write() method of these classes to add data to a stream, and you use Read() to read data from it. The stream interface includes other methods such as Rewind() and MoveTo().


For example, the following code creates a global character stream and writes some data into it:

 Set mystream=##class(%Stream.GlobalCharacter).%New()
 Do mystream.Write("here is some text to store in the stream ")
 Do mystream.Write("here is some more text")
 Write "this stream has this many characters: ",mystream.Size,!
 Write "this stream has the following contents: ",!
 Write mystream.Read()

Collection Classes

When you need a container for sets of related values, you can use $LIST format lists and multidimensional arrays, as described earlier in this book.

If you prefer to work with classes, Caché provides list classes and array classes; these are called collections.

List and Array Classes for Use As Standalone Objects

To create list objects, you can use the following classes:

To manipulate a list object, use its methods. For example:

 Set Colors=##class(%Library.ListOfDataTypes).%New()
 Do Colors.Insert("Red")
 Do Colors.Insert("Green")
 Do Colors.Insert("Blue") 
 Write "Number of items in this list: ", Colors.Count()
 Write !, "Second item in the list: ", Colors.GetAt(2)

Similarly, to create array objects, you can use the following classes:

To manipulate an array object, use its methods. For example:

 Set ItemArray=##class(%Library.ArrayOfDataTypes).%New()
 Do ItemArray.SetAt("example item","alpha")
 Do ItemArray.SetAt("another item","beta")
 Do ItemArray.SetAt("yet another item","gamma")
 Do ItemArray.SetAt("still another item","omega")
 Write "Number of items in this array: ", ItemArray.Count()
 Write !, "Item that has the key gamma: ", ItemArray.GetAt("gamma")

List and Arrays as Properties

You can also define a property as a list or array.

To define a property as a list, use the following form:

Property MyProperty As list of Classname;

If Classname is a data type class, then Caché uses the interface provided by %Collection.ListOfDTOpens in a new tab. If Classname is an object class, then it uses the interface provided by %Collection.ListOfObjOpens in a new tab.

To define a property as an array, use the following form:

Property MyProperty As Array of Classname;

If Classname is a data type class, then Caché uses the interface provided by %Collection.ArrayOfDTOpens in a new tab. If Classname is an object class, then it uses the interface provided by %Collection.ArrayOfObjOpens in a new tab.

Useful ObjectScript Functions

ObjectScript provides the following functions for use with object classes:

  • $CLASSMETHOD enables you to run a class method, given as class name and method name. For example:

    SAMPLES>set class="Sample.Person"
    SAMPLES>set obj=$CLASSMETHOD(class,"%OpenId",1)
    SAMPLES>w obj.Name
    Van De Griek,Charlotte M.

    This function is useful when you need to write generic code that executes a class method, but the class name (or even the method name) is not known in advance. For example:

     //read name of class from imported document
     Set class=$list(headerElement,1) 
     // create header object
     Set headerObj=$classmethod(class,"%New")

    The other functions are useful in similar scenarios.

  • $METHOD enables you to run an instance method, given an instance and a method name. For example:

    SAMPLES>set obj=##class(Sample.Person).%OpenId(1)
    SAMPLES>do $METHOD(obj,"PrintPerson")
    Name: Van De Griek,Charlotte M.
  • $PROPERTY gets or sets the value of the given property for the given instance. For example:

    SAMPLES>set obj=##class(Sample.Person).%OpenId(2)
    SAMPLES>write $property(obj,"Name")
    Edison,Patrick J.
  • $PARAMETER gets the value of the given class parameter, given an instance. For example:

    SAMPLES>set obj=##class(Sample.Person).%OpenId(2)
    SAMPLES>write $parameter(obj,"EXTENTQUERYSPEC")
  • $CLASSNAME returns the class name for a given instance. For example:

    SAMPLES>set obj=##class(Sample.Person).%OpenId(1)
    SAMPLES>write $CLASSNAME(obj)

    With no argument, this function returns the class name of the current context. This can be useful in instance methods.

For More Information

For more information on the topics covered in this chapter, see the following books:

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