In C++, type erasure is a powerful technique that allows you to write generic code without sacrificing performance or losing type safety. One popular way to achieve type erasure is through the use of functors.

What is Type Erasure?

Type erasure refers to the process of hiding the specifics of a specific type and treating it as an abstract concept. This enables you to operate on different types in a unified manner, providing flexibility and modularity in your code.

Functors in C++

Functors in C++ are objects that can be called like functions. They are typically implemented as classes with an operator() overload. Functors offer a convenient way to encapsulate behavior and can be used for type erasure.

Creating a Functor for Type Erasure

To create a functor for type erasure, you can define a base class that serves as an interface and derive specific implementations from it. These derived implementations can be used to store different types while presenting a consistent interface.

Here’s an example of a base functor class:

class BaseFunctor {
public:
    virtual void operator()() const = 0;
    virtual ~BaseFunctor() = default;
};

Implementing Functors for Type Erasure

Let’s say we want to create a functor that can store and invoke any callable object, regardless of its type. We can create a derived functor class AnyCallable that takes advantage of type erasure.

template<typename Callable>
class AnyCallable : public BaseFunctor {
public:
    AnyCallable(Callable callable) : m_callable(callable) {}

    void operator()() const override {
        m_callable();
    }

private:
    Callable m_callable;
};

Using Functors for Type Erasure

Now that we have a functor that supports type erasure, we can use it to store and execute various callable objects.

void function() {
    std::cout << "Hello, World!" << std::endl;
}

struct Functor {
    void operator()() const {
        std::cout << "This is a functor!" << std::endl;
    }
};

int main() {
    AnyCallable<void(*)()> erasureFunction(function);
    AnyCallable<Functor> erasureFunctor(Functor());

    erasureFunction();
    erasureFunctor();

    return 0;
}

The AnyCallable functor can store a function pointer (void(*)()) or a Functor object. The type erasure allows calling them interchangeably through the () operator.

Conclusion

Functors provide a flexible and efficient way to achieve type erasure in C++. By encapsulating behavior within functors, you can handle different types uniformly. This approach allows for writing generic code that remains performant and type-safe.

#C++ #TypeErasure