Class Modifiers

The Class statement can have the following modifiers preceding the Class keyword:

• Final: The class cannot act as a base class.
• Stripped: The class does not contain an RTTI pointer.  Static RTTI is still stored.
• Exported: The class needs to be compatible with dynamically-linked QDL code.
• Abstract: The class cannot be instantiated.  This keyword must be present if one or more of the functions in the class are Abstract, including inherited functions from base classes that have not been given definitions.
• Published: The entire class interface is published in the RTTI.

The following restrictions are placed on the use of these modifiers:

• Final and Abstract are mutually exclusive.
• Stripped cannot be used with Abstract.

Final

Final specifies that the class cannot act as a base class.  All functions in a Final class are implicitly Final as well.  The Final keyword should be used when you want to ensure that the implementation for the class is not changed.

Certain optimizations are possible for a Final class. In particular:

• Early binding is possible in calls to member functions, which lowers the overhead of a function call.
• Member functions can know at compile-time the locations of all superclasses within the data structure that corresponds to an instance of the class.  Thus, access to members of superclasses is sped up. 

Stripped

A stripped class is a class that does not have a pointer to run-time type information.  The Stripped keyword is provided purely to save memory and the small overhead of setting the RTTI pointer for new instances of the class.

Whether to apply the Stripped modifier to a class is a judgment call, but in general you should only use the Stripped modifier if:

• You do not need to derive from the class (although derivation is possible, there are crippling restrictions on derived classes).
• Instances of the class need to be very small (under 16 bytes), and you may be making many thousands of instances of the class.
• You are creating a data structure that needs to have binary compatibility with a C struct. 

The built-in numerical types, Boolean, and String are all Stripped.

A Stripped class A with an instance a or reference thereto has the following properties:

• The Class instance returned from Class(a) is always for A, not a derived class.  If a is a reference, and A is not Final, it cannot be determined whether a is truly an A or an instance of a class derived from A.
• SizeOf (a) == SizeOf (A).  This is not always true for a non-Stripped class.
• a cannot be casted in a potentially unsafe manner (i.e. to a derived or sibling class).
• All members of A are implicitly Final.
• All base classes of A are implicitly Inline.
• The address of a cannot be stored in a "plain" reference (e.g. @).

When it says members of a stripped class are Final, that includes the destructor.  Thus, you generally cannot have a destructor in a class derived from a Stripped class--a very serious limitation indeed!  There is one exception, and that is if the Stripped class is also Abstract and no destructor is provided.

Exported

The Exported keyword currently has no meaning.

It is intended for use in QDL platforms that support closed world analysis.  It would mean that the class is shared with external programs and therefore must not have certain closed-world optimizations applied to it.

Abstract

An abstract class is a class that contains Abstract Functions. You must declare a class as Abstract to have Abstract functions, or to declare a class that inherits from a class that has Abstract functions, but which does not redeclare and define all of those functions. An Abstract function is a non-Final function that does not have a function body and therefore cannot be called. An Abstract class cannot be instantiated.

Like the Override keyword, the Abstract keyword is redundant (well, usually): the compiler knows whether there are abstract functions in the class; the Abstract keyword forces the programmer to acknowledge it.  However, the Abstract keyword can be applied even if there are no Abstract functions.  This should be done when the class should not be instantiated directly.

In an abstract class, a default constructor and destructor is not automatically created.  You must explicitly create both.  If no destructor is provided, then you cannot call Delete with a reference to the abstract class, since there is no destructor to call.

Published

The Published keyword indicates to the compiler that you want the contents of the class published at run-time.  In an indirect way, the members of all superclasses are also published, even if they do not have the Published keyword.

The contents of the class can be accessed at run-time using the PublishedData member of the Class instance associated with the class.  All members of the class are published, even private members.