Reference 3
Contents
Object Creation and Message Syntax
'Instantiation', or the instantiation of a class, is used in this manual to mean the creation of an object instance without regard to how that creation is achieved. For instantiation to occur the instance must be associated with a class, must be sized if the class is indexed, and must be named unless created at runtime as dynamic and nameless. Objects range in complexity from simple BYTEs and INTs through the “toolbox objects” such as windows and views that participate in Mac Toolbox operations. Mops provides several language elements designed to cope with this range of complexity in an economical way.
Message syntax is very uniform unless you need to explicity control the binding of a message, that is, the association of a message with a specific method definition. In the simple case no uncertainty about the receiver's class exists and the binding occurs at compile time (early binding), but in other cases it must be achieved at runtime (late binding). The choice between the two, and the attendant tradeoffs, is dictated by overall program design as discussed in the following chapter, “More About Objects”. Complete syntax for early-bound messages is given below, and some differences are commented on.
Object Declarations vs. Creation Methods
Declaration Instantiation
All compiled – as opposed to dynamic – objects require only a simple declaration for their instantiation. Typically, the predefined classes described as basic data structures in Chapter 1 of Part III, as well as the string, file, floating-point, and vector objects (Chapters 2, 3, 10, 11, and 12) are instantiated in this way. (See below for toolbox classes.) A declared class instance is implicity created as a compiled object in the dictionary unless qualified as TEMP. The form of the declaration is simply
[array-size] class-name instance-name
(Brackets indicate optionality.) As can be seen, the declarations for scalar and array classes differ only in the array-size value that specifies the number of indexed elements the object is to have. Some examples are:
Var MAXTEMP
Uint MASK_1
Handle BUFFER4
32 wArray GAME_STATE
6 X-Array IOERR_VECTS
HandleList HEAP_OBJS
String+ LOGDATA
File INITIALVALS
Float BIGNUMB
16 fArray BIGNUMBS
SWord_vector VECTORVALUE
NOTE: Though constant and value definitions look much like array-object declarations, they are not objects but simply primitive data items, i.e., Mops words. (Unlike the analogous objects they cannot participate in complex objects as either ivars or array elements.)
The event-related classes Event and Mouse (Chapter 4, Part 3) would be instantiated as above but are seldom if ever subclassed as it is very unlikely that the need to do so would arise.
Special-case TEMP Declarations
The TEMP{ ... } declaration ˜envelops', or takes a list of data-object declarations and declares them to be temporary within the scope of a word definition or a local section (see œMiscellaneous Topics, Part 2). Its effect within a definition is to make the objects local in scope as well as temporary. An additional effect is to allow an elementary data object to be instantiated in a register, or in a vector register if a Vector object.
Otherwise the objects declared as TEMP are created in a memory area on the return stack. The TEMP declaration syntax is
TEMP{ [ REGISTER ] obj-declaration [ [ REGISTER ] obj-declaration ... ] }
The optional form TEMP { ... } (with white space characters after the TEMP) is equivalent to TEMP{ ... } . A syntax example:
Temp{ Var AVARIABLE
register Uint AUINTEGER
9 bArray ABARRAY
register Float AFPVARIABLE
register Word_vector AVECTVARIABLE
}
Creation-Method Instantiation
Nameless dynamic objects, that is, heap-based objects of variable lifetime, are accessed either individually by a handle or by index into a collection of such objects (mediated by implicit handles). You declare either an ObjHandle or HandleList object and then send the appropriate object-creation message to either. The name of the creation method in either case is NEWOBJ: and the class of the desired instance is passed on the stack (as an xt). Examples:
Note that the deferred tick of the class name gives the method the needed class information, and that in the case of a handleList the newObj: message may appear within a DO loop that instantiates some required number of same-class instances.
(1) objHandle SHORTLIFE_OBJ \ Declaration of handle for object
['] Fugit newObj: SHORTLIFE_OBJ \ Creation of Fugit-subclass
\ instance
(2) handleList BUNCH_OF_OBJS \ Declaration of a list of implicit
\ handles
['] Lotsa newObj: BUNCH_OF_OBJS \ Creation of one object with
\ implicit handle
. . .
['] Lotsa newObj: BUNCH_OF_OBJS \ Creation of another object in the
\ same list.
. . .
['] Oddball newObj: BUNCH_OF_OBJS \ Creation of an object of a
\ differing class
The Mops Toolbox Classes
Many interactions with the Mac Toolbox are quite complex and so are the Mops predefined classes and corresponding objects that deal with them. This is particularly true of objects of the classes (or subclasses) that facilitate windowing, e.g., WINDOW, WINDOW+, and VIEW plus a few others (Chapters 5, 6, and 9 of Part III). Window objects specifically might be said to lead two lives in that, while they are normal Mops object in most respects, some portion of their data area is shared between Mops and the toolbox. (Also, much of the object’s behavior is transparent to the user, which is after all the purpose of the toolbox classes, to hide lower-level detail of toolbox interactions.) Except for View they all have one or more methods that result in a toolbox call.
Like other objects a toolbox-class object can be created at compile time by declaration, as is shown in most examples. When necessary one or more or more can also be created dynamically using either of the newObj: messages discussed above. The one thing that is different about most classes of toolbox object is that they must be sent a NEW: or GETNEW: message to become effective. While these mesages may superficially look like object-creation messages they are not but serve to make the existent object known to, or ˜alive in', the Mac toolbox.
Much of the necessary NEW: messaging is performed by Mops automatically and transparently via the class hierarchy. To take a common example, when the user's code sends a NEW: to an object of class Window+ (or user-declared subclass thereof), the receiver¹s default action is to ˜call' NEW: on its contained view which in turn does the same on any child views, and so on down the line. (Similar default behavior is defined for other messages as well, such as DRAW:) Thus, not only is the user insulated from system calls and all the attendant details, but also much inter-object messaging is made as transparent as possible.
Message Syntax
The basic form of a message to an object is:
[ stack-item ... ] selector [ object-name | objPtr-name | SELF | SUPER ]
where:
- stack-item …
implies any sequence of required message parameters - selector
is the name of method to be executed - object-name
is the name of the receiver - objPtr-name
is the name of the object pointer to a receiver object created at runtime in non-relocatable memory - SELF
implies the sending object as receiver, with message lookup - SUPER
beginning in the sender's own class (i.e., last defined method) implies the sending object as receiver, but with message lookup beginning in the sender's superclass (bypassing any message override in the sender's class).
Although the syntax diagram above may look complex, it results in just a few simple forms. All messages conforming to that syntax are subject to early binding, i.e., complete resolution at compile time, the standard for Mops objects and messages. Some examples are:
" A Window Title" new: MyWind2
elemIdx at: stateArray
update: theScreen
winMsg Writer: self
clear: self
moved: super
where: theMouse
Sometimes late binding is necessary, however, typically for dynamic objects or program-design reasons as described in the next chapter. In this case square brackets, i.e., [ ], usually appear in the message surrounding the identity of the receiver, which might be given by a runtime expression. Also, the receiver may be given by an object handle. Forms such as œselector ** and œselector [] are possible, as well as œselector [SELF]
The possible forms of late-bound messages are described in detail in the œEarly Vs. Late Binding section of the following chapter.
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