# Object

Every object in a running program, including objects your program defines via "object" and "class" definitions and instances of intrinsic classes, ultimately derives from the intrinsic class Object.

You can never instantiate Object directly, since this is an "abstract" class. However, since every object is a subclass of Object, every object in the system inherits the methods defined by Object.

## Object methods

The Object class defines a number of methods. Since all objects are subclasses of Object, all objects inherit these methods. Most of these methods are related to the the relationships between objects and classes.

getSuperclassList()

Returns a list containing the immediate superclasses of the object. The list contains only the object's direct superclasses, which are the superclasses that were explicitly listed in the object's declaration for static objects, or the class used with the new operator for dynamic objects. This function returns an empty list for an object with no superclass. For an object with more than one direct superclass, the list contains the superclasses in the same order in which they were specified in the object's declaration.

For example, consider these definitions:

    class A: object;
    class B: object;
    class C: B;
    myObj: C, A;

The result of myObj.getSuperclassList() will be the list [C, A]. Note that class B is not included in the list, because it is not a direct superclass of myObj, but is a superclass only indirectly through class C.

getPropList()

Returns a list of the properties directly defined by this object. Each entry in the list is a property pointer value. The returned list contains only properties directly defined by the object; inherited properties are not included, but may be obtained by explicitly traversing the superclass list and calling this method on each superclass.

getPropParams(prop)

Returns information on the parameters taken by the given property or method of this object. The return value is a list with three elements:

- [1] is the minimum number of arguments taken by the method;
- [2] is the number of additional optional arguments taken by the method;
- [3] is true if the method accepts any number of additional arguments, nil if not.

The second element gives the number of optional arguments; only intrinsic methods will ever yield a non-zero value for this element, because regular methods cannot specify optional arguments. For example, the substring() method of the String intrinsic class can take either one or two arguments, so its return list is [1, 1, nil], indicating that the function takes a minimum of one argument, can take one additional optional argument, but does not take an unlimited varying argument list after those arguments.

If the third element is true, it indicates that the method was defined with the "..." varying argument list notation.

If the specified property is not defined for the object, the method returns [0, 0, nil], because it's valid to invoke the property with no arguments in this case.

isClass()

Returns true if the object was declared as a "class", nil otherwise.

isTransient()

Returns true if the object is transient, nil otherwise. A transient object is one that was created with new transient classname, or with a class-specific method that creates transient instances (such as TadsObject.createTransientInstance() or TadsObject.createTransientInstanceOf()).

ofKind(cls)

Determines if the object is an instance of the class cls, or an instance of any subclass of cls. Returns true if so, nil if not. This method always returns true if cls is Object, since every object ultimately derives from the Object intrinsic class.

propDefined(prop, flags?)

Determines if the object defines or inherits the property prop (a property pointer value - specify this by applying an ampersand prefix (&) to a property name), according to the flags value. If flags is not specified, a default value of PropDefAny is used. The valid flags values are:

- PropDefAny - the function returns true if the object defines or inherits the property.
- PropDefDirectly - the function returns true only if the object directly defines the property; if it inherits the property from a superclass, the function returns nil.
- PropDefInherits - the function returns true only if the object inherits the property from a superclass; if it defines the property directly, or doesn't define or inherit the property at all, the function returns nil.
- PropDefGetClass - the function returns the superclass object from which the property is inherited, or this object if the object defines the property directly. If the object doesn't define or inherit the property, the function returns nil.

propInherited(prop, origTargetObj, definingObj, flags?)

Determines if the object inherits the property prop (a property pointer value). origTargetObj is the "original target object," which is the object on which the method was originally invoked; that is, it's the object on the left-hand side of the . operator in the expression that originally invoked the method. definingObj is the "defining object," which is the object defining the method which will be inheriting the superclass implementation.

The return value depends on the value of the flags argument:

- PropDefAny - the function returns true if the object inherits the property, nil otherwise.
- PropDefGetClass - the function returns the class object from which the property is inherited, or nil if the property is not inherited.

This method is most useful for determining if the currently active method will invoke an inherited version of the method if it uses the inherited operator; this is done by passing targetprop for the prop parameter, targetobj for the origTargetObj parameter, and definingobj for the definingObj parameter. When a class is designed as a "mix in" (which means that the class is designed to be used with multiple inheritance as one of several base classes, and adds some isolated functionality that is "mixed" with the functionality of the other base classes), it sometimes useful to be able to check to see if the method is inherited from any other base classes involved in multiple inheritance. This method allows the caller to determine exactly what inherited will do.

Note that the inheritance order is deterministic (i.e., it will always be the same for a given situation), and that it depends on the full class tree of the original target object. For example, suppose we have a set of class definitions like this:

    class A: object  x() { "A.x\n"; inherited(); }
    class B: object  x() { "B.x\n"; inherited(); }

    class C: B, A    x() { "C.x\n"; inherited(); }

Now suppose we run some code like so:

    new B().x();
    new C().x();

The first line will simply display "B.x". B inherits directly from TadsObject, so when B.x() calls inherited(), it will find no definition of x() in any base class (since TadsObject doesn't define it), so inherited() will do nothing.

The second line, however, will display this:

    C.x
    B.x
    A.x

So, even though the call to inherited() in B.x() went straight to TadsObject when B.x() was invoked from the first line above, the same call to inherited() in B.x() proceeds to A.x() when invoked from the second line above. The difference is that C inherits from both B and A. B is the first superclass, so the call to inherited() in C.x() proceeds to B.x(). But C also inherits from A, and the superclass order is defined so that A comes after B in C's superclass list. So, the call to inherited() in B.x() proceeds to A.x() this time, since that's the next superclass in inheritance order for the original target object.

propType(prop)

Returns the datatype of the given property of the given object, or nil if the object does not define or inherit the property. This function does not evaluate the property, but merely determines its type. The return value is one of the TYPE_xxx values (see the reflection section for the list).

TADS 3 System Manual
Table of Contents | The Intrinsics > Object
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