Design patterns Visitor Pattern Visitor Example in C++


Example

// A simple class hierarchy that uses the visitor to add functionality.
//
class VehicleVisitor;
class Vehicle
{
    public:
        // To implement the visitor pattern
        // The class simply needs to implement the accept method 
        // That takes a reference to a visitor object that provides
        // new functionality.
        virtual void accept(VehicleVisitor& visitor) = 0
};
class Plane: public Vehicle
{
    public:
        // Each concrete representation simply calls the visit()
        // method on the visitor object passing itself as the parameter.
        virtual void accept(VehicleVisitor& visitor) {visitor.visit(*this);}

        void fly(std::string const& destination);
};
class Train: public Vehicle
{
    public:
        virtual void accept(VehicleVisitor& visitor) {visitor.visit(*this);}

        void locomote(std::string const& destination);
};
class Automobile: public Vehicle
{
    public:
        virtual void accept(VehicleVisitor& visitor) {visitor.visit(*this);}

        void drive(std::string const& destination);
};

class VehicleVisitor
{
    public:
        // The visitor interface implements one method for each class in the
        // hierarchy. When implementing new functionality you just create the
        // functionality required for each type in the appropriate method.

        virtual void visit(Plane& object)      = 0;
        virtual void visit(Train& object)      = 0;
        virtual void visit(Automobile& object) = 0;

    // Note: because each class in the hierarchy needs a virtual method
    // in visitor base class this makes extending the hierarchy ones defined
    // hard.
};

An example usage:

// Add the functionality `Move` to an object via a visitor.
class MoveVehicleVisitor
{
    std::string const& destination;
    public:
        MoveVehicleVisitor(std::string const& destination)
            : destination(destination)
    {}
    virtual void visit(Plane& object)      {object.fly(destination);}
    virtual void visit(Train& object)      {object.locomote(destination);}
    virtual void visit(Automobile& object) {object.drive(destination);}
};

int main()
{
    MoveVehicleVisitor  moveToDenver("Denver");
    Vehicle&            object = getObjectToMove();
    object.accept(moveToDenver);
}