Compiler Design Laboratory

5. The Visitor Design Pattern

5.1 Description

Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

5.2 Motivation

Consider a compiler that represents programs as abstract syntax trees. It will need to perform operations on abstract syntax trees for "static semantic" analyses like checking that all variables are defined. It will also need to generate code. So it might define operations for type-checking, code optimization, flow analysis, checking for variables being assigned values before they're used, and so on. Moreover, we could use the abstract syntax trees for pretty-printing, program restructuring, code instrumentation, and computing various metrics of a program.

Most of these operations will need to treat nodes that represent assignment statements differently from nodes that represent variables or arithmetic expressions. Hence there will be one class for assignment statements, another for variable accesses, another for arithmetic expressions, and so on. The set of node classes depends on the language being compiled, of course, but it doesn't change much for a given language.

This diagram shows part of the Node class hierarchy. The problem here is that distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change. It will be confusing to have type-checking code mixed with pretty-printing code or flow analysis code. Moreover, adding a new operation usually requires recompiling all of these classes. It would be better if each new operation could be added separately, and the node classes were independent of the operations that apply to them.

We can have both by packaging related operations from each class in a separate object, called a visitor, and passing it to elements of the abstract syntax tree as it's traversed. When an element "accepts" the visitor, it sends a request to the visitor that encodes the element's class. It also includes the element as an argument. The visitor will then execute the operation for that element-the operation that used to be in the class of the element.

For example, a compiler that didn't use visitors might type-check a procedure by calling the TypeCheck operation on its abstract syntax tree. Each of the nodes would implement TypeCheck by calling TypeCheck on its components (see the preceding class diagram). If the compiler type-checked a procedure using visitors, then it would create a TypeCheckingVisitor object and call the Accept operation on the abstract syntax tree with that object as an argument. Each of the nodes would implement Accept by calling back on the visitor: an assignment node calls VisitAssignment operation on the visitor, while a variable reference calls VisitVariableReference. What used to be the TypeCheck operation in class AssignmentNode is now the VisitAssignment operation on TypeCheckingVisitor.

To make visitors work for more than just type-checking, we need an abstract parent class NodeVisitor for all visitors of an abstract syntax tree. NodeVisitor must declare an operation for each node class. An application that needs to compute program metrics will define new subclasses of NodeVisitor and will no longer need to add application-specific code to the node classes. The Visitor pattern encapsulates the operations for each compilation phase in a Visitor associated with that phase.

With the Visitor pattern, you define two class hierarchies: one for the elements being operated on (the Node hierarchy) and one for the visitors that define operations on the elements (the NodeVisitor hierarchy). You create a new operation by adding a new subclass to the visitor class hierarchy. As long as the grammar that the compiler accepts doesn't change (that is, we don't have to add new Node subclasses), we can add new functionality simply by defining new NodeVisitor subclasses.

5.3 Application

Use the Visitor pattern when:

• an object structure contains many classes of objects with differing interfaces, and you want to perform operations on these objects that depend on their concrete classes.
• many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid "polluting" their classes with these operations. Visitor lets you keep related operations together by defining them in one class. When the object structure is shared by many applications, use Visitor to put operations in just those applications that need them.
• the classes defining the object structure rarely change, but you often want to define new operations over the structure. Changing the object structure classes requires redefining the interface to all visitors, which is potentially costly. If the object structure classes change often, then it's probably better to define the operations in those classes.

5.4 Structure

5.5 Participants

• Visitor (NodeVisitor)
  • declares a Visit operation for each class of ConcreteElement in the object structure. The operation's name and signature identifies the class that sends the Visit request to the visitor. That lets the visitor determine the concrete class of the element being visited. Then the visitor can access the element directly through its particular interface.
• ConcreteVisitor (TypeCheckingVisitor)
  • implements each operation declared by Visitor. Each operation implements a fragment of the algorithm defined for the corresponding class of object in the structure. ConcreteVisitor provides the context for the algorithm and stores its local state. This state often accumulates results during the traversal of the structure.
• Element (Node)
  • defines an Accept operation that takes a visitor as an argument.
• ConcreteElement (AssignmentNode,VariableRefNode)
  • implements an Accept operation that takes a visitor as an argument.
• ObjectStructure (Program)
  • can enumerate its elements.
  • may provide a high-level interface to allow the visitor to visit its elements.
  • may either be a composite (see Composite design pattern) or a collection such as a list or a set.

    5.6 Collaborations

    A client that uses the Visitor pattern must create a ConcreteVisitor object and then traverse the object structure, visiting each element with the visitor.

    When an element is visited, it calls the Visitor operation that corresponds to its class. The element supplies itself as an argument to this operation to let the visitor access its state, if necessary.

    The following interaction diagram illustrates the collaborations between an object structure, a visitor, and two elements:

    

    5.7 Consequences

    Some of the benefits and liabilities of the Visitor pattern are as follows:
    1. Visitor makes adding new operations easy. Visitors make it easy to add operations that depend on the components of complex objects. You can define a new operation over an object structure simply by adding a new visitor. In contrast, if you spread functionality over many classes, then you must change each class to define a new operation.
    2. A visitor gathers related operations and separates unrelated ones. Related behavior isn't spread over the classes defining the object structure; it's localized in a visitor. Unrelated sets of behavior are partitioned in their own visitor subclasses. That simplifies both the classes defining the elements and the algorithms defined in the visitors. Any algorithm-specific data structures can be hidden in the visitor.
    3. Adding new ConcreteElement classes is hard. The Visitor pattern makes it hard to add new subclasses of Element. Each new ConcreteElement gives rise to a new abstract operation on Visitor and a corresponding implementation in every ConcreteVisitor class. Sometimes a default implementation can be provided in Visitor that can be inherited by most of the ConcreteVisitors, but this is the exception rather than the rule.

       So the key consideration in applying the Visitor pattern is whether you are mostly likely to change the algorithm applied over an object structure or the classes of objects that make up the structure. The Visitor class hierarchy can be difficult to maintain when new ConcreteElement classes are added frequently. In such cases, it's probably easier just to define operations on the classes that make up the structure. If the Element class hierarchy is stable, but you are continually adding operations or changing algorithms, then the Visitor pattern will help you manage the changes.

    4. Visiting across class hierarchies. An iterator (see Iterator (257)) can visit the objects in a structure as it traverses them by calling their operations. But an iterator can't work across object structures with different types of elements. For example, the Iterator interface defined on page 263 can access only objects of type Item:

       template <class Item>
       class Iterator 
       {
        // ...
        Item CurrentItem() const;
       };
       

       This implies that all elements the iterator can visit have a common parent class Item. Visitor does not have this restriction. It can visit objects that don't have a common parent class. You can add any type of object to a Visitor interface. For example, in

       class Visitor 
       {
        public:
         // ...
         void VisitMyType(MyType*);
         void VisitYourType(YourType*);
       };
       

       MyType and YourType do not have to be related through inheritance at all.
    5. Accumulating state. Visitors can accumulate state as they visit each element in the object structure. Without a visitor, this state would be passed as extra arguments to the operations that perform the traversal, or they might appear as global variables.
    6. Breaking encapsulation. Visitor's approach assumes that the ConcreteElement interface is powerful enough to let visitors do their job. As a result, the pattern often forces you to provide public operations that access an element's internal state, which may compromise its encapsulation.

    5.8 Implementation

    Each object structure will have an associated Visitor class. This abstract visitor class declares a VisitConcreteElement operation for each class of ConcreteElement defining the object structure. Each Visit operation on the Visitor declares its argument to be a particular ConcreteElement, allowing the Visitor to access the interface of the ConcreteElement directly. ConcreteVisitor classes override each Visit operation to implement visitor-specific behavior for the corresponding ConcreteElement class.

    class Visitor 
    {
     public:
      virtual void VisitElementA(ElementA*);
      virtual void VisitElementB(ElementB*);
     
     // and so on for other concrete elements
     protected:
      Visitor();
    };
    

    Each class of ConcreteElement implements an Accept operation that calls the matching Visit... operation on the visitor for that ConcreteElement. Thus the operation that ends up getting called depends on both the class of the element and the class of the visitor

    The concrete elements are declared as:

    class Element
    {
     public:
     virtual
      ~Element();
     virtual
      void Accept(Visitor&) = 0;
     protected:
    
      Element();
    };
     
    class ElementA : public Element 
    {
     public:
      ElementA();
     virtual void Accept(Visitor& v) { v.VisitElementA(this); }
    };
     
    class ElementB : public Element 
    {
     public:
      ElementB();
     virtual void Accept(Visitor& v) { v.VisitElementB(this); }
    };
    

    A class CompositeElement can implement the following accept method:

    class CompositeElement : public Element 
    {
     public: 
      virtual void Accept(Visitor&);
     private: 
      List* _children;
    };
     
    void CompositeElement::Accept (Visitor& v) 
    {
     ListIteratori(_children);
     
     for(i.First(); !i.IsDone(); i.Next()) 
     {
      i.CurrentItem()->Accept(v);
     }
    
     v.VisitCompositeElement(this);
    }
    

    5.9 Assignments

    1. For the following simple rules is required:

    Expression:: = Expression '+' Term | Term
    Term:: = Term '*' Factor | Factor
    Factor:: = '(' Expression ')' | IntegerConstant
    

    - to shape each terminal and nonterminal in a Java class;
    - to implement methods to accept these classes node;
    - to write a class that displays the AST visitor's Polish shaped infix;
    - to write a class that displays the AST visitor's Polish prefixed shape;
    - to write a class that evaluates the AST visitor's;
    - to build an expression tree example on which to execute the three visitors;
    

    Assignments time: 1 week.

    https://www.digitalocean.com/community/tutorials/visitor-design-pattern-java https://www.cs.unc.edu/~stotts/GOF/hires/pat5kfso.htm