SFINAE：无法重新声明班级成员

Question

I am trying to create my own "smart iterator" and I'd like to use SFINAE to have some operators depending on the tag of the iterator :

Here is my code :

template<class Iterator, class Predicat, class Tag>
class RangeFilterIterator {
public:
    RangeFilterIterator(Iterator begin, Iterator end, Predicat predicat) :
        mBegin(begin), mEnd(end), mPredicat(predicat) {}

    bool operator !=(RangeFilterIterator const &r) {
        return mBegin != r.mBegin;
    }

    typename Iterator::value_type &operator*() {return *mBegin;}

    RangeFilterIterator &operator++() {
        while(mBegin != mEnd && mPredicat(*mBegin++));
        return *this;
    }

    template<class = std::enable_if_t<std::is_base_of<std::random_access_iterator_tag, Tag>::value>>
    RangeFilterIterator &operator+(std::size_t n) {
        while(n--)
            ++(*this);
        return *this;
    }

    template<class = std::enable_if_t<!std::is_base_of<std::random_access_iterator_tag, Tag>::value>>
    RangeFilterIterator &operator+(std::size_t n) = delete;

private:
    Iterator mBegin, mEnd;
    Predicat mPredicat;
};

template<typename Container, typename Predicate>
auto RangeFilter(Container const &c, Predicate p) {
    using Iterator = RangeFilterIterator<typename Container::iterator,
                                         Predicate,
                                         typename Container::iterator::iterator_category>;
    Iterator begin(const_cast<Container&>(c).begin(), const_cast<Container&>(c).end(), p);
    Iterator end(const_cast<Container&>(c).end(), const_cast<Container&>(c).end(), p);
    return Range(begin, end);
}

and at the line RangeFilterIterator &operator+(std::size_t n) = delete I got the error : class member cannot be redeclared .

I am not "good" with template, but I thought that with SFINAE only one of the two will be "declared". Am I missing something? It is possible to do otherwise?

Answer 1

Okay when I am using return type parameter instead of template parameter, it works.

template<class tag = Tag>
std::enable_if_t<std::is_base_of<std::random_access_iterator_tag, tag>::value, RangeFilterIterator>
&operator+(std::size_t n) {
    while(n--)
        ++(*this);
    return *this;
}

template<class tag = Tag>
std::enable_if_t<!std::is_base_of<std::random_access_iterator_tag, tag>::value, RangeFilterIterator>
&operator+(std::size_t n) = delete;