In C++, functors are objects that act as function-like entities by overloading the function call operator. They provide a way to encapsulate logic and state, making them powerful tools for generic programming. However, sometimes we need to pass functors to functions or store them in containers without knowing their exact type. This is where type erasure comes in.

Type erasure is a technique that allows us to strip away the type information of an object while preserving its behavior. It provides a way to treat objects of different types as if they had the same type. In the context of functors, type erasure can be used to expand their functionality and make them more versatile.

One common way to implement type erasure for functors in C++ is by using a polymorphic class and inheritance. The idea is to create a base class that defines a common interface for all the different types of functors we want to store or pass around. We can then derive specific functor classes from this base class, each implementing the desired behavior.

Let’s take a look at an example:

#include <iostream>

class FunctorBase {
public:
    virtual void operator()() const = 0;
};

template <typename Functor>
class FunctorWrapper : public FunctorBase {
public:
    FunctorWrapper(Functor functor) : functor(functor) {}

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

private:
    Functor functor;
};

void doSomethingWithFunctor(const FunctorBase& functor) {
    functor();
}

int main() {
    auto functor1 = []() {
        std::cout << "Hello, Functor 1!" << std::endl;
    };

    auto functor2 = []() {
        std::cout << "Hello, Functor 2!" << std::endl;
    };

    FunctorWrapper<decltype(functor1)> wrappedFunctor1(functor1);
    FunctorWrapper<decltype(functor2)> wrappedFunctor2(functor2);

    doSomethingWithFunctor(wrappedFunctor1);
    doSomethingWithFunctor(wrappedFunctor2);

    return 0;
}

In this example, we define a base class FunctorBase that declares a pure virtual function operator(), which represents the function call operator of the functor. We then define a template class FunctorWrapper that derives from FunctorBase and implements the operator() by delegating the call to the stored functor object.

To use the type-erased functors, we create instances of FunctorWrapper by providing the specific functor types as template arguments. We can then pass these instances to the doSomethingWithFunctor function, which expects an object of type FunctorBase.

By using this type erasure technique, we can now store functors of different types in a container, pass them to functions, or treat them as generic objects without explicitly knowing their underlying types.

#C++ #Functors #TypeErasure