c++ - 为什么我的自定义迭代器不能使用 STL 拷贝？

Question

我为 answer to another question 写了一个 OutputIterator .在这里:

#include <queue>

using namespace std;

template< typename T, typename U >
class queue_inserter {
    queue<T, U> &qu;  
public:
    queue_inserter(queue<T,U> &q) : qu(q) { }
    queue_inserter<T,U> operator ++ (int) { return *this; }
    queue_inserter<T,U> operator * () { return *this; }
    void operator = (const T &val) { qu.push(val); }
};

template< typename T, typename U >
queue_inserter<T,U> make_queue_inserter(queue<T,U> &q) {
    return queue_inserter<T,U>(q);
}    

这对于这个小复制功能非常有用:

template<typename II, typename OI>
void mycopy(II b, II e, OI oi) {
    while (b != e) { *oi++ = *b++; }
}

但它对 algorithms 的 STL copy 根本不起作用。以下是我遇到的奇妙的 C++ 错误:

i.cpp:33: error: specialization of ‘template<class _Iterator> struct std::iterator_traits’ in different namespace
/usr/include/c++/4.0.0/bits/stl_iterator_base_types.h:127: error:   from definition of ‘template<class _Iterator> struct std::iterator_traits’
/usr/include/c++/4.0.0/bits/stl_algobase.h: In function ‘_OI std::__copy_aux(_II, _II, _OI) [with _II = int*, _OI = queue_inserter<int, std::deque<int, std::allocator<int> > >]’:
/usr/include/c++/4.0.0/bits/stl_algobase.h:335:   instantiated from ‘static _OI std::__copy_normal<true, false>::copy_n(_II, _II, _OI) [with _II = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, _OI = queue_inserter<int, std::deque<int, std::allocator<int> > >]’
/usr/include/c++/4.0.0/bits/stl_algobase.h:387:   instantiated from ‘_OutputIterator std::copy(_InputIterator, _InputIterator, _OutputIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, _OutputIterator = queue_inserter<int, std::deque<int, std::allocator<int> > >]’
i.cpp:53:   instantiated from here
/usr/include/c++/4.0.0/bits/stl_algobase.h:310: error: no type named ‘value_type’ in ‘struct std::iterator_traits<queue_inserter<int, std::deque<int, std::allocator<int> > > >’
/usr/include/c++/4.0.0/bits/stl_algobase.h:315: error: no type named ‘value_type’ in ‘struct std::iterator_traits<queue_inserter<int, std::deque<int, std::allocator<int> > > >’
/usr/include/c++/4.0.0/bits/stl_algobase.h:315: error: ‘__value’ is not a member of ‘<declaration error>’
/usr/include/c++/4.0.0/bits/stl_algobase.h:335:   instantiated from ‘static _OI std::__copy_normal<true, false>::copy_n(_II, _II, _OI) [with _II = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, _OI = queue_inserter<int, std::deque<int, std::allocator<int> > >]’
/usr/include/c++/4.0.0/bits/stl_algobase.h:387:   instantiated from ‘_OutputIterator std::copy(_InputIterator, _InputIterator, _OutputIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, _OutputIterator = queue_inserter<int, std::deque<int, std::allocator<int> > >]’
i.cpp:53:   instantiated from here
/usr/include/c++/4.0.0/bits/stl_algobase.h:317: error: ‘__simple’ is not a valid template argument for type ‘bool’ because it is a non-constant expression
/usr/include/c++/4.0.0/bits/stl_algobase.h:317: error: ‘copy’ is not a member of ‘<declaration error>’

这是驱动程序:

int main() {
    vector<int> v;
    v.push_back( 1 );
    v.push_back( 2 );
    queue<int> q;
    copy( v.begin(), v.end(), make_queue_inserter(q) );
    while (q.size() > 0) {
        cout << q.front() << endl;
        q.pop();
    }
}

它到底为什么要专门化 iterator_traits。我的迭代器有什么问题？我不能只编写自己的简单迭代器吗？

Answer 1

您的 queue_inserter 需要派生自 std::iterator 以便正确定义所有 typedef，例如 value_type，因为它们已被使用在 STL 算法中这个定义有效:

template< typename T, typename U >
class queue_inserter : public std::iterator<std::output_iterator_tag, T>{
    queue<T, U> &qu;  
public:
    queue_inserter(queue<T,U> &q) : qu(q) { }
    queue_inserter<T,U> operator ++ (int) { return *this; }
    queue_inserter<T,U> operator ++ () { return *this; }
    queue_inserter<T,U> operator * () { return *this; }
    void operator = (const T &val) { qu.push(val); }
};