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?
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.
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