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Automated Validation of Class Invariants in C++ Applications

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Automated Validation of Class Invariants in C++ Applications Automated Validation of Class Invariants In C++ Applications Tanton H. Gibbs, Brian A Malloy James F. Power Computer Science Department Computer Science Department Clemson University National University of Ireland Clemson, SC 29634 Maynooth, Co. Kildare USA. Ireland {thgibbs,malloy}@cs.clemson.edu [email protected] Abstract cated by Alan Turing [15], further developed by Hoare [13, 14], Floyd [8] and Dijkstra [7] and made popular in In this paper, we describe a non-invasive approach the Eiffel language [22]. However, Eiffel has not gar- for validation of class invariants in C++ applications. nered the wide acceptance of other object-oriented lan- ++ Our approach is fully automated so that the user need guages such as C and Java. Nevertheless, the impor- only supply the class invariants for each class hierar- tance of the Design by Contract approach is noteworthy, chy to be checked and our validator constructs an In- and one of the hypotheses of our work is that the Design variantVisitor, a variation of the Visitor Pattern, and an by Contract approach may complement software testing InvariantFacilitator. Instantiations of the InvariantVisi- by exposing errors that are not found by implementation- tor and InvariantFacilitator classes encapsulate the in- based or specification-based testing. variants in C++ statements and facilitate the validation In this paper, we describe a non-invasive approach for of the invariants. We describe both our approach and validation of class invariants in C++ applications. We use our results of validating invariants in keystone, a well the Object Constraint Language (OCL) to specify the in- tested parser front-end for C++. variants but our approach is amenable to other specifi- cation languages such as Z [28], Object-Z [27] or VDM [17]. Our approach is fully automated so that the user need only supply the class invariants for each class hier- 1 Introduction archy to be checked and our validator constructs an In- variantVisitor, a variation of the Visitor Pattern [2, 21], The current focus in the software industry is to im- and an InvariantFacilitator. Instantiations of the Invari- prove the quality rather than the speed or size of an ap- antVisitor and InvariantFacilitator classes encapsulate plication. A report by the Workshop on Strategic Di- the invariants in C++ statements and facilitate the vali- rections in Software Quality asserts that software quality dation of the invariants. We describe both our approach will become the dominant success criterion in the soft- and our results of validating invariants in keystone, a well ware industry [23]. One process that supports software tested parser front-end for C++ [20, 24, 25]. quality is testing, which exercises the software to gather In the next section we provide background about the information about the software. However, studies indi- Visitor pattern, class invariants, OCL and keystone. In cate that testing consumes more than fifty percent of the Section 3, we overview our approach including a de- cost of software development, with some estimates plac- scription of the InvariantVisitor and the InvariantFacili- ing the cost even higher [12]. tator classes. In Section 4 we describe our incorporation An alternative to testing that promises to improve of invariant validation into keystone, and in Section 5 we software quality is Design by Contract, which advocates benchmark our validator and provide interesting insights establishing and checking assertions about the attributes into the validation of keystone. In Section 6 we review of a class. Design by Contract was originally advo- related work and in Section 7 we draw conclusions. 1 Accepted for the 17th IEEE International Conference on Automated Software Engineering Edinburgh, UK, September 23-27, 2002, pp. 205-214 1 £ ¡ ¢£ ¡ ¢ Shape ¡ Visitor 1 accepts visits 0..* virtual void draw() = 0 virtual void visit(Shape *) = 0 virtual void moveTO(int, int) = 0 virtual void visit(Circle *) = 0 virtual int area() = 0 virtual void visit(Rectangle*) = 0 virtual void accept(Visitor *) = 0 virtual ~Visitor() = 0 virtual ~Shape() = 0 Circle Rectangle PSVisitor PDFVisitor XMLVisitor int x, y, radius int x, y, width, height void visit(Circle *c) { // make XML version // of circle } void accept(Visitor *v) void accept(Visitor *v) { v−>visit(this); } { v−>visit(this); } Figure 1. The Visitor Pattern. The Visitor pattern consists of two inheritance hierarchies, an element hierarchy, illustrated on the left by the Shape hierarchy, and a Visitor hierarchy, illustrated on the right. 2 Background An instantiation of a particular visitor encapsulates additional functionality to augment the functionality al- To facilitate automation of invariant validation, we ready in the Shape hierarchy. When an instance of a exploit a variation of the Visitor pattern, the Acyclic Vis- class derived from Shape calls visit on the visitor object, itor [2, 21], which we describe in Section 3. In this sec- the instance effectively identifies its type to the visitor. tion we first describe the Visitor pattern [9, 30], and then The visitor operation that is called can perform what- provide background about invariants and the object con- ever action is appropriate for the particular instance. For straint language, OCL. We conclude this section with example, an instance of XMLVisitor can build an XML an overview of keystone, an application that provides a representation of a Circle, as illustrated by the XMLVis- itor parser front-end for the ISO C++ language; we use key- class shown on the right side of Figure 1. stone as our case study in Sections 4 and 5. 2.2 Class invariants 2.1 The Visitor Pattern An invariant on a class C is a set of Boolean con- The Visitor pattern permits the designer of a class hi- ditions or predicates that every instance of C will sat- erarchy to add functionality without “polluting” the hier- isfy after instantiation (i.e., after constructor invocation) archy with unrelated operations [9]. The Visitor pattern and before and after every method invocation by an- can also be used to obviate down casting by enabling op- other object [22]. A class invariant is a property of a erations that depend on the concrete classes [30]. These class instance that must be preserved by all methods of class dependent operations, as well as additional opera- the class. In spite of its name, an invariant is not re- tions, can be added to an existing class hierarchy without quired to hold at all execution points. For example, a modifying the classes in the hierarchy. method might violate the invariant while working toward The Visitor pattern consists of two inheritance hierar- its goal; however, the invariant must be re-established chies, an element hierarchy and a Visitor hierarchy. We before the method terminates execution. use the familiar Shape example [2, 9, 22, 29] as our el- Class invariants are used to ensure that the operations ement hierarchy, illustrated on the left of Figure 1, and performed on instances of the class maintain the integrity a Visitor hierarchy illustrated on the right of the figure. constraints of the class. These constraints are described The Shape hierarchy consists of an abstract base class in terms of the member functions and data attributes of with five methods and two derived classes for special- the class. izing Shape. Four of the five methods in class Shape represent typical operations that one might perform on 2.3 The Object Constraint Language (OCL) all shapes, such as drawing, moving, finding the area or deleting a Shape. The fifth method is accept(Visitor *), which permits a visitor object to visit a particular el- A Unified Modeling Language (UML) diagram, such ement. as a class diagram, provides a high level of program Accepted for the 17th IEEE International Conference on Automated Software Engineering Edinburgh, UK, September 23-27, 2002, pp. 205-214 1 <<enumeration>> OccurSpecifier convFunction, destructor, destructorQualifier, elabEnum, elabClass, label, namespace, pseudoDestructor, qualifier, typename <<use>> NameOccurrence uses in builds name : String <<subsystem>> hasQualifier : Boolean search Program Processor specifiedAs: OccurSpecifier <<subsystem>> ignoreUsings : Boolean Symbol Table Scanner looks for returns Scope NameDeclaration Parser name : String Type kind : Kind containedIn : Scope <<use>> <<enumeration>> Kind class, enum, function, label, namespace, variable, typedef Figure 2. Keystone summary. The Program Processor subsystem, illustrated on the left, is respon- sible for directing symbol table construction and name lookup. The Program Processor marshals information about the name in a NameOccurrence object and directs the search for a correspond- ing NameDeclaration in the Symbol Table subsystem, illustrated on the right. abstraction [4]. These UML diagrams are not refined ing and directing symbol table construction and name enough to describe low-level aspects of a specification, lookup. This responsibility includes two phases: (1) such as invariant conditions that must hold for instances assembling the necessary information for creation of a of objects in the system. The Object Constraint language NameOccurrence object, and (2) directing the search (OCL) is a formal language used to describe expressions for a corresponding NameDeclaration object in the on models specified in the Unified Modeling Language Symbol Table subsystem. The Symbol Table subsys- (UML) [3, 34]. These expressions typically specify in- tem is the symbol table in the parser, including class hi- variant conditions that must hold for the system being erarchies for type information, Type, and scope informa- modeled, or queries over objects described in a model. tion, Scope, shown on the right of Figure 2. The Sym- OCL expressions, when evaluated, do not have side ef- bol Table subsystem, the target of our validation effort, fects, so their evaluation cannot alter the state of the cor- is discussed further in Section 4. responding executing system even though an OCL ex- pression can be used to specify a state change. OCL 3 Overview of the Technique expressions allow the modeler to express invariants in a language independent manner.
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