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1 United States Patent [19] Jones et al. lllllllllllllllllillllllllllllllllllllllllllllllllllllllllllllllllllllllll US A [11] Patent Number: [45] Date of Patent: 5,297,284 Mar. 22, 1994 [54] METHOD AND SYSTEM FOR IMPLEMENTING VIRTUAL FUNCTIONS AND VIRTUAL BASE CLASSES AND SETTING A THIS POINTER FOR AN OBJECT-ORIENTED PROGRAMMING LANGUAGE [75] Inventors: David T. Jones, Preston; Martin J. O Riordan, Redmond; Mark J. Zbikowski, Woodinville, all of Wash. [73] Assignee: Microsoft Corporation, Redmond, Wash. [21] Appl. No.: 682,537 [22] Filed: Apr. 9, 1991 [51] Int. Cl G06F 9/42 [52] US. Cl /700; 364/DIG. 1; 364/255.l; 364/2558; 364/2614; 364/DIG. 2; 364/9383; 364/955; 364/955.6 [58] Field of Search /700; 364/280.4, 364/973; 394/900 [56] References Cited U.S. PATENT DOCUMENTS Re. 33,706 10/1991 MOhri /900 5,093,914 3/1992 Coplien et al /700 OTHER PUBLICATIONS Borland International, Turbo C+ + Programes Guide pp , Smith, Paul, OOP with Pascal, EXE, Sep v. 4 No. 4 p. 18(3). Margaret A. Ellis and Bjarne Stroustrup, The Annotated C++ Reference Manual, Addison-Wesley Publishing Company, New York, 1990, Ch. 10, Derived Classes, pp Primary Examiner-Kevin A. Kriess Assistant Examiner-Jon H. T. Backenstose Attorney, Agent, or Firm-Seed and Berry [57] ABSTRACT A method for a computer compiler for an object-ori ented programming language for implementing virtual functions and virtual base classes is provided. In pre ferred embodiments of the present invention, the data structure layout of an object includes a virtual function table pointer, a virtual base table pointer, occurrences of each non-virtual base class, the data members of the class, and occurrences of each virtual base class. If a class introduces a virtual function member and the class has a non-virtual base class with a virtual function table pointer, then the class shares the virtual function table pointer of the non-virtual base class that is?rst visited in a depth-?rst, left-to-right traversal of the inheritance tree. In preferred embodiments of the present invention, each instance of a given class shares a set of virtual function tables and virtual base tables for that class. In preferred embodiments, adjusters are used when a func tion member in a derived class overrides a function member that is de?ned in more than one base class, and when a derived class has a base class that overrides a function member in a virtual base class of that class and the derived class itself does not override the function member. 19 Claims, 17 Drawing Sheets

2 US. Patent Mar. 22, 1994 Sheet 1 of 17 5,297,284 Figure 1

3 US. Patent Mar. 22, 1994 Sheet 2 of 17 5,297,284 Figuré 2 Generate Data Structures bosl class with u vfph'? /-\21o Allocate and selnct 5'1. n 0 vfptr for m h :umnt class ban ) 206 baa dun with a vbptr? /'\209 Allocah and aloct Salad 0 a vbptr for vbptr to curnnt dos: sham

4 A US. Patent ' 22, 1994 Sheet 3 of 17 5,297,284 Figure 2 (Cont'd) Allocato nan-virtual bass classes / 210 Allocate data members cf current class Allocate virtual bass classes / ' 212 addnss I &vfptr /'\ 214 addrass - &vbpfr /\ 216 f 218 ls than address I addrss a hon-virtual b0 dc non- dual Of class This I address of 1st data member of cumnt class / 219 Q

5 US. Patent Mar. 22, 1994 Sheet 4 of 17 5,297,284 Figure 3 f... lmtlallzl \ virtual functicn table: Copy virtual function tabla: from baa classes Cmcta ccnstructor / 302 / 303 Select next virtual function In current class Add ontry to vtabll / 306 p 307 Placaoddnss ofaluct functlonhvtablc V

6 US. Patent Mar. 22, 1994 Sheet 5 of 17 5,297,284 Figure 3 (Cont d) Select next virtual base class / 312 more virtual base classes Select next /\ 4 virtual function in 31 selected class function overridew another functi ~ Create an adiustor / 319 Place adjuster address in vtable

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10 US. Patent Mar. 22, 1994 Sheet 9 of 17 5,297,284 Figure 7

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12 US. Patent Mar. 22, 1994 Sheet 11 of 17 5,297,284 Figure 9

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14 US. Patent Mar. 22, 1994 Sheet 13 of 17 5,297,284 Figure 1 1 Km

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17 US. Patent Mar. 22, 1994 Sheet 16 of 17 5,297,284 Figure 14 r1401 [My / B3zrvfpfr O- - ) &(B3::?:31) O 53::vbp kr " ).' 83::mb3l 1403 Bubaz /14 6 Alavfptr. Alamo" mm \) K1404 &(B3::h1l))'" /

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19 1 METHOD AND SYSTEM FOR IMPLEMENTING VIRTUAL FUNCTIONS AND VIRTUAL BASE CLASSES AND SETTING A THIS POINTER FOR AN OBJECT -ORIENTED PROGRAMMING LANGUAGE DESCRIPTION 1. Technical Field This invention relates generally to the?eld of compil ers for computer programming languages and more speci?cally to compilers for languages having object oriented features.. 2. Background of the Invention The use of object-oriented programming techniques can facilitate the development of complex computer programs. Programming languages that support object oriented techniques have been developed. One such programming language is C+ +. Two common characteristics of object>oriented pro gramming languages are support for data encapsulation and data type inheritance. Data encapsulation refers to the binding of functions and data. Inheritance refers to the ability to declare a data type in terms of other data types. In the GC+ + language, object-oriented techniques are supported through the use classes. A class is a user de?ned type. A class declaration describes the data members and function members of the class. For exam ple, the following declaration de?nes data members and function member of a class named CIRCLE. class CIRCLE { int x, y; int radius; } void draw( ); Variables x and y specify the center location of a circle and variable radius speci?es the radius of the circle. These variables are referred to as data members of the class CIRCLE. The function draw is a user-de?ned function that draws the circle of the speci?ed radius at the speci?ed location. The function draw is referred to as a function member of class CIRCLE. The data mem bers and function members of a class are bound together in that the function operates an instance of the class. An instance of a class is also called an object of the class. In the syntax of C+ +, the following statement de clares the objects a and b to be of type class CIRCLE. CIRCLE a, b; This declaration causes the allocation of memory for the objects a and b, such an allocation is called an in stance of the class. The following statements assign data to the data members of objects a and b. a.radius== l; 5,297,284 l CLE..FILL inherits the characteristics of the base class CIRCLE. class CIRCLE_FILL : CIRCLE { int pattern; void?ll( ); This declaration speci?es that class CIRCLE_.FILL includes all the data and function members that are in class CIRCLE in addition to the those data and function members introduced in the declaration of class CIR CLE_FILL, that is, data member pattern and function member?ll. In this example, class CIRCLE_FILL would have data members it, y, radius, and pattern and function members draw and fill. Class CIRCLEJILL is said to inherit the characteristics of class CIRCLE. A class that inherits the characteristics of another class is a derived class (e.g., CIRCLE_..FILL). A class that does not inherit the characteristics of another class is a primary class (e. g., CIRCLE). A class whose character istics are inherited by another class is a base class (e. g., CIRCLE is a base class of CIRCLE_FILL). A derived class may inherit the characteristics of several classes, that is, a derived class may have several base classes. This is referred to as multiple inheritance. A derived class may specify that a base class is to be inherited virtually. Virtual inheritance of a base class means that only one instance of the virtual base class exists in the derived class. For example, the following is an example of a derived class with two non-virtual base classes. class PATTERN : CIRCLE, CIRCLE{... }; In this declaration class PATTERN inherits class CIR CLE twice non-virtually. There are two instances of class CIRCLE in class PATTERN. The following is an example of a derived class with two virtual base classes. class PATTERN: virtual CIRCLE, virtual CIR CLE{... }; The derived class PATTERN inherits class CIRCLE twice virtually. Since the class CIRCLE is virtually inherited twice, there is only one object of class CIR CLE in the derived class PATTERN. This is the sim plest use of virtual inheritance and is not particularly useful. One skilled in the art would appreciate virtual inheritance can be very useful when the class derivation is more complex. A class may also specify whether its function mem bers are to be virtually inherited. Declaring that a func tion member is virtual means that the function can be overridden by a function of the same name and type in a derived class. In the following example, the function draw is declared to be virtual in classes CIRCLE and CIRCLE_F ILL. b.radius=2; The following statements are used to draw the circles de?ned by objects a and b. a.draw(); b.draw(); A derived class is a class that inherits the characteris tics data members and function members of its base classes. For example, the following derived class CIR 65 class CIRCLE l int x, y; int radius; virtual void draw( ); l; class CIRCLE_FILL : CIRCLE { int pattern; virtual void draw( );

20 i 3 Continuing with the example, the following statement declares object a to be of type class CIRCLE and object b to be of type class CIRCLE_FILL. CIRCLE a; CIRCLE FILL b; The following statement refers to the function draw as de?ned in class CIRCLE. a.draw(); Whereas, the following statement refers to the function draw de?ned in class CIRCLE_FILL. b.draw(); Moreover, the following statements type cast object b to an object of type class CIRCLE and invoke the func tion draw that is de?ned in class CIRCLE_FILL. Thus, the type casting preserves the call to the overrid ing function ClRCLE_FILL::draw. Although object-oriented techniques facilitate the development of complex computer programs, the re sulting computer programs can be less ef?cient in exe cution speed and require more memory than a program developed without object-oriented techniques. It would be desirable to have method and system for implement ing the techniques of object-oriented programming to improve the execution speed and reduce the memory requirements of the computer program. SUMMARY OF THE INVENTION It is an object of the present invention to provide improved methods of implementing virtual functions in a compiler for an object-oriented programming lan guage. It is another object of the present invention to pro vide improved methods for implementing virtual base classes in a compiler for an object-oriented program ming language. It is another object of the present invention to pro vide an improved compiler for an object-oriented pro gramming language that reduces the run-time storage requirements for each instance of a class. It is another object of the present invention to pro vide a method in a compiler for an object-oriented pro gramming language in which virtual function table pointers can be shared between a derived class and a base class. It is another object of the present invention to pro vide a compiler for an object-oriented programming language that reduces the number of adjusters needed for the virtual functions. These and other objects of the invention, which will become more apparent as the invention is described more fully below, are obtained by providing an im proved compiler for a programming language wherein the object data structure layouts for a class include a virtual function table pointer, a virtual base table pointer, occurrences of object data structures for non virtual base classes, data members of the class, and ob ject data structures for virtual base classes of the class, wherein associated with each class is a set of virtual function tables and virtual base tables that are shared by each instance of the class, and wherein adjusters adjust the this pointer when invoking a virtual function. In preferred embodiments of the present invention, a de 5,297,284 l rived class that introduces a virtual function member and has a non-virtual base class with a virtual function table pointer, shares the virtual function table pointer of the?rst non-virtual base class encountered in a depth?rst, left-to-right traversal of the inheritance tree. In preferred embodiments, the this pointer is set equal to the address of the virtual function table pointer for the class when the class has a virtual function table pointer, else the address of the virtual base table pointer when the class has a virtual base table pointer, else the this pointer of a non-virtual base class when the class has a non-virtual base class, else the address of a data member of the class. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the inheritance tree for class F. FIG. 2 shows a flow diagram of a method for gener ating a class data structure in a preferred embodiment. FIG. 3 is a flow diagram of a method for initializing the virtual function-tables in a preferred embodiment. FIG. 4 is a schematic diagram of the data structure, the virtual function table, and the function members of class A1 that are generated in a preferred embodiment of the present invention. FIG. 5 is a schematic diagram of the run-time alloca tions of objects a and b of type class A1 in a preferred embodiment of the present invention. FIG. 6 is a schematic diagram of the data structure, the virtual function table, and the function members of class B1 that are generated in a preferred embodiment of the present invention. FIG. 7 is a schematic diagram of the data structure of class A2 that is generated in a preferred embodiment of the present invention. FIG. 8 is a schematic diagram of the data structure, the virtual function table, and the function members of class B2 that are generated in a preferred embodiment of the present invention. FIG. 9 is a schematic diagram of the data structure, the virtual function table, and the function members of class A3 that are generated in a preferred embodiment of the present invention. FIG. 10 is a schematic diagram of the data structure, the virtual function tables, and the function members of class C1 that are generated in a preferred embodiment of the present invention. FIG. 11 is a schematic diagram of the data structure, the virtual function table, and the function members of class A4 that are generated in a preferred embodiment of the present invention. FIG. 12 is a schematic diagram of the data structure, the virtual function tables, and the function members of class C0 that are generated in a preferred embodiment of the present invention. FIG. 13 is a schematic diagram of the data structure, the virtual function tables, and the function members of class C2 that are generated in a preferred embodiment of the present invention. FIG. 14 is a schematic diagram of the data structure, the virtual function tables, the virtual base table, and the function members of class B3 that are generated in a preferred embodiment of the present invention. FIG. 15 is a schematic diagram of the data structure, the virtual function tables, the virtual base table, and the virtual function members of class C3 that are generated in a preferred embodiment of the present invention.

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