The Visitor pattern is a useful design pattern in object-oriented programming that allows for adding new behaviors to a set of classes without modifying their code. The traditional implementation of the Visitor pattern in C++ involves using dynamic dispatch and runtime type information, which can lead to potential runtime errors and lack of type safety.

However, with the introduction of variadic templates in C++11, it became possible to implement a type-safe visitor pattern. In this blog post, we will explore how to implement a type-safe visitor using variadic templates in C++.

What is a Visitor Pattern?

The Visitor pattern allows adding new operations to a set of classes without modifying their code. It separates the operation logic from the classes themselves, making it easy to add new operations in the future.

The Traditional Visitor Pattern

In the traditional implementation of the Visitor pattern, we define an abstract base visitor class that declares a set of visit functions for each class we want to visit. Each visit function takes a reference to the visited class as a parameter. An example implementation would look like this:

class Visitor {
public:
    virtual void visit(Foo& foo) = 0;
    virtual void visit(Bar& bar) = 0;
    // Add visit functions for other classes
};

To add a new operation, we create a new derived visitor class and implement the visit functions for the classes we want to operate on. However, this approach relies on runtime type information and can lead to potential issues if a visit function is missing or called with the wrong type.

Implementing Type-Safe Visitor using Variadic Templates

To implement a type-safe visitor pattern using variadic templates, we can make use of a technique called double dispatch. Double dispatch allows resolving the specific visit function at compile-time based on both the visitor and visited types.

Here’s how we can implement a type-safe visitor using variadic templates:

class Visitor {
public:
    virtual ~Visitor() = default;
};

template <typename... Types>
class Visitable {
public:
    virtual ~Visitable() = default;

    virtual void accept(Visitor& visitor) = 0;
};

template <typename Type, typename... Rest>
class Visitable<Type, Rest...> : public Visitable<Rest...> {
public:
    using Visitable<Rest...>::accept;

    void accept(Visitor& visitor) override {
        if (auto typedVisitor = dynamic_cast<Type*>(&visitor)) {
            typedVisitor->visit(*this);
        } else {
            Visitable<Rest...>::accept(visitor);
        }
    }
};

class Foo : public Visitable<Foo> {
public:
    void doSomething() {
        // Do something specific to Foo
    }
};

class Bar : public Visitable<Bar> {
public:
    void doSomethingElse() {
        // Do something specific to Bar
    }
};

class MyVisitor : public Visitor {
public:
    void visit(Foo& foo) {
        foo.doSomething();
    }

    void visit(Bar& bar) {
        bar.doSomethingElse();
    }
};

int main() {
    Foo foo;
    Bar bar;

    MyVisitor visitor;
    foo.accept(visitor);
    bar.accept(visitor);

    return 0;
}

In this example, we define a templated Visitable class that acts as the base class for all visitable classes. Each visitable class derives from Visitable and overrides the accept function.

The accept function is responsible for dispatching the appropriate visit function based on the actual dynamic type of the visitor. It does this by attempting a dynamic_cast of the visitor to the specific visitor type. If the cast succeeds, it calls the corresponding visit function. Otherwise, it falls back to the accept of the remaining visitable types.

Finally, we define a MyVisitor class that derives from the base Visitor class and implements the visit functions for each visitable class.

Conclusion

Using variadic templates, we can implement a type-safe visitor pattern in C++. This approach improves type safety and eliminates the need for runtime type information, leading to more robust and maintainable code. Incorporating this pattern in your projects can provide flexibility and extensibility when adding new behaviors to a set of classes.

#visitorpattern #variadictemplates