Object Pascal For The Macintosh

Object Pascal For. the Macintosh Larry Tesler February 14, 1985 lntroduction The specification of the. language Object Pascal appears in theObject Pascal Report, ~ttached. The present memo specifies differences between the current Macintosh imp_lementation änd the specification. In cases noted as "temporary deviations", we intend to bring the implementation into conformance shortly. Multi-Level "inherited" According to the standard, the statement "inherited Draw" activates a method of the immediate ancestor of the type whose method contains the statement. In the Macintosh versicin~ if a more remote ancestral type, T, has a different implementation of the same method, then that method can be invoked by T. Draw. This feature is rarely used. Object Type Declaratlons According to the standard. an obj~ct type declaration may appear in a main program, in the interfäce of ä unit, or in_ the implementation of a unit. In the Macintosh version, it can_.on,fy appear.in fhe interface of a unit. Method declarations can only appear in the i~plementation of ttie same untt. These d~viations are temporary. · ··. According·to the standard; it i·s an error if a method is declared override in the type ·dectaration and then not implemented, or if an override method is implemented but not decfared override. In the Macintosh Version, the compiler does not check for these errors. This is a temporary deviation. Aqcording to the standard, the oame of a method in a method header is qualified by · the type name when the method body is introduced but not in the type declaration. In the Macintosh version, qualification is permitted in the type declaration. Range Checklng Accordi.ng·to the standard, it is an error to coerce a value to an object type if it is not in the domain ofthat type.· In the Macintosh version, if and only if the compiler switch {$R+} is in effect, the compiler generates code to test the validity of object-type coercions. Object Pascal for the Macintosh Page 2 Unsafe Use of Handles According to the standard, the implementation of a reference is not specified, and may be machine dependent. In the Macintosh version, an object reference is implemented as a handle, i.e., as a pointer to a pointer to an object. The object itself can change its memory address during execution as part of a compaction process that is invoked to prevent heap fragmentation. Because an object can change its location, it is unsafe to save a pointer to it in a variable or register and then later to access the object through the saved pointer. lf a procedure call should intervene, then the heap might compact to make room for a newly allocated object or to swap the procedure itself into memory, invalidating the pointer. Where possible, the compiler takes care to access objects only through handles and not through pointers. Examples: type T = object F: Integer; S: String[lO]; end; var Y: T; Y.F := Func; Proc(Y.F); with Y do begin F := Func; Proc(F); F := F + 1; DrawString(S); end…