Type erasure techniques

Question

All type erasure techniques in C++ are done with function pointers (for behaviour) and void* (for data). The "different" methods simply differ in the way they add semantic sugar. Virtual functions, e.g., are just semantic sugar for

struct Class {
    struct vtable {
        void (*dtor)(Class*);
        void (*func)(Class*,double);
    } * vtbl
};

iow: function pointers.

That said, there's one technique I particularly like, though: It's shared_ptr<void>, simply because it blows the minds off of people who don't know you can do this: You can store any data in a shared_ptr<void>, and still have the correct destructor called at the end, because the shared_ptr constructor is a function template, and will use the type of the actual object passed for creating the deleter by default:

{
    const shared_ptr<void> sp( new A );
} // calls A::~A() here

Of course, this is just the usual void*/function-pointer type erasure, but very conveniently packaged.

Fundamentally, those are your options: virtual functions or function pointers.

How you store the data and associate it with the functions can vary. For example, you could store a pointer-to-base, and have the derived class contain the data and the virtual function implementations, or you could store the data elsewhere (e.g. in a separately allocated buffer), and just have the derived class provide the virtual function implementations, which take a void* that points to the data. If you store the data in a separate buffer, then you could use function pointers rather than virtual functions.

Storing a pointer-to-base works well in this context, even if the data is stored separately, if there are multiple operations that you wish to apply to your type-erased data. Otherwise you end up with multiple function pointers (one for each of the type-erased functions), or functions with a parameter that specifies the operation to perform.

I would also consider (similar to void*) the use of "raw storage": char buffer[N].

In C++0x you have std::aligned_storage<Size,Align>::type for this.

You can store anything you want in there, as long as it's small enough and you deal with the alignment properly.

Stroustrup, in The C++ programming language (4th edition) §25.3, states:

Variants of the technique of using a single runt-time representation for values of a number of types and relying on the (static) type system to ensure that they are used only according to their declared type has been called type erasure.

In particular, no use of virtual functions or function pointers is needed to perform type erasure if we use templates. The case, already mentioned in other answers, of the correct destructor call according to the type stored in a std::shared_ptr<void> is an example of that.

The example provided in Stroustrup's book is just as enjoyable.

Think about implementing template<class T> class Vector, a container along the lines of std::vector. When you will use your Vector with a lot of different pointers types, as it often happens, the compiler will supposedly generate different code for every pointer type.

This code bloat can be prevented by defining a specialization of Vector for void* pointers and then using this specialization as a common base implementation of Vector<T*> for all others types T:

template<typename T>
class Vector<T*> : private Vector<void*>{
// all the dirty work is done once in the base class only 
public:
    // ...
    // static type system ensures that a reference of right type is returned
    T*& operator[](size_t i) { return reinterpret_cast<T*&>(Vector<void*>::operator[](i)); }
};

As you can see, we have a strongly typed container but Vector<Animal*>, Vector<Dog*>, Vector<Cat*>, ..., will share the same (C++ and binary) code for the implementation, having their pointer type erased behind void*.

See this series of posts for a (fairly short) list of type erasure techniques and the discussion about the trade-offs: Part I, Part II, Part III, Part IV.

The one I haven't seen mentioned yet is Adobe.Poly, and Boost.Variant, which can be considered a type erasure to some extent.

As stated by Marc, one can use cast std::shared_ptr<void>. For example store the type in a function pointer, cast it and store in a functor of only one type:

#include <iostream>
#include <memory>
#include <functional>

using voidFun = void(*)(std::shared_ptr<void>);

template<typename T>
void fun(std::shared_ptr<T> t)
{
    std::cout << *t << std::endl;
}

int main()
{
    std::function<void(std::shared_ptr<void>)> call;

    call = reinterpret_cast<voidFun>(fun<std::string>);
    call(std::make_shared<std::string>("Hi there!"));

    call = reinterpret_cast<voidFun>(fun<int>);
    call(std::make_shared<int>(33));

    call = reinterpret_cast<voidFun>(fun<char>);
    call(std::make_shared<int>(33));


    // Output:,
    // Hi there!
    // 33
    // !
}