如何在抽象 class 中声明 operator++(int)？

Question

I have an abstract class that is basically meant to serve as an "iterator interface". That is, it is an abstract iterator that a few concrete classes will later implement. To make the abstract class an iterator, I need to overload T operator++(int) , where T is the class for which the operator is overloaded.

class AbstractClass {
    virtual AbstractClass operator++(int) = 0; // compile time error
    virtual AbstractClass& operator++() = 0;   // but this is fine
}

However, I cannot write AbstractClass operator++(int) as only pointers and references to an abstract class are permitted as return values.

Is there a way to require the subclasses to overload operator++(int) in plain C++11?

Answer 1

Abstract classes with virtual members are usually accessed through base pointers, so you teh caller doesn't know the derived type, but the return value from operator++(int) must be known at compile time by the caller, so there's no way to do this directly. I would simply not provide this method, and have your class be iterator-like, but not fully iterator-conforming.

However, there is a complex workaround, and that's to make a non-abstract iterator that itself can handle a virtual iterator as a member. This gets super complicated.

//abstract interface for iterators
template<class value_type>
struct virtual_iterator_interface {
    virtual ~virtual_iterator_interface(){};
    virtual std::unique_ptr<virtual_iterator_interface> clone()const=0;
    virtual value_type& deref()=0;
    virtual void increment()=0;
};
//wrapper implementation for iterators
template<class value_type, class It>
struct virtual_iterator : virtual_iterator_interface<value_type> {
    It it;
    virtual_iterator(It it_) : it(it_) {}
    std::unique_ptr<virtual_iterator_interface> clone()const
    {return std::make_unique<virtual_iterator>(it);}
    value_type& deref()
    {return *it;}
    void increment()
    {return ++it;}
};

static const struct from_iterator_t {} from_iterator;
// The iterator that holds a pointer to an abstracted iterator
template<class value_type>
class erased_iterator {
    std::unique_ptr<virtual_iterator_interface<value_type>> ptr;
public:
    template<class It>
    erased_iterator(from_iterator_t, It it)
    :ptr(std::make_unique<virtual_iterator<value_type,It>>(it)) {}
    erased_iterator(std::unique_ptr<virtual_iterator_interface<value_type>> ptr_)
    :ptr(std::move(ptr_)) {}
    erased_iterator(const erased_iterator& rhs)
    :ptr(rhs.ptr->clone()) {}
    erased_iterator(erased_iterator&& rhs) = default;
    erased_iterator& operator=(const erased_iterator& rhs)
    {ptr=rhs.ptr->clone();}
    erased_iterator& operator=(erased_iterator&& rhs) = default;

    //TADA! Iterator things are now possible!
    value_type& operator*(){return ptr->deref();}
    erased_iterator& operator++(){ptr->increment(); return *this;}
    erased_iterator operator++(){erased_iterator t(it->clone()); ptr->increment(); return t;}
};

In addition to complexity, this allocates iterators on the heap, which makes them very slow.

Also note: virtual members prevent inlining and optimization, so makes the code slower. For things like iterators, which are supposed to be lightweight, this can actually make the code a lot slower.