尝试编写std :: iterator：编译错误

Question

I am trying to write an std::iterator for the CArray<Type,ArgType> MFC class. This is what I have done till now:

template <class Type, class ArgType>
class CArrayIterator : public std::iterator<std::random_access_iterator_tag, ArgType>
{
public:
    CArrayIterator(CArray<Type,ArgType>& array_in, int index_in = 0)
        : m_pArray(&array_in), m_index(index_in)
    {
    }

    void operator++() { ++m_index; }
    void operator++(int) { ++m_index; }
    void operator--() { --m_index; }
    void operator--(int) { --m_index; }
    void operator+=(int n) { m_index += n; }
    void operator-=(int n) { m_index -= n; }
    typename ArgType operator*() const{ return m_pArray->GetAt(m_index); }
    typename ArgType operator->() const { return m_pArray->GetAt(m_index); }
    bool operator==(const CArrayIterator& other) const
    {
        return m_pArray == other.m_pArray && m_index == other.m_index;
    }
    bool operator!=(const CArrayIterator& other) const
    {
        return ! (operator==(other));
    }

private:
    CArray<Type,ArgType>* m_pArray;
    int m_index;
};

I also provided two helper functions to create the iterators like this:

template<class Type, class ArgType>
CArrayIterator<Type,ArgType> make_begin(CArray<Type,ArgType>& array_in)
{
    return CArrayIterator<Type,ArgType>(array_in, 0);
}

template<class Type, class ArgType>
CArrayIterator<Type,ArgType> make_end(CArray<Type,ArgType>& array_in)
{
    return CArrayIterator<Type,ArgType>(array_in, array_in.GetSize());
}

To test the code, I wrote a simple class A and tried to use it like this:

class A
{
public:
    A(int n): m_i(n)
    {
    }

    int get() const
    {
        return m_i;
    }

private:
    int m_i;
};
struct Test
{
    void operator()(A* p)
    {
        std::cout<<p->get()<<"\n";
    }
};

int main(int argc, char **argv) 
{
    CArray<A*, A*> b;

    b.Add(new A(10));
    b.Add(new A(20));

    std::for_each(make_begin(b), make_end(b), Test());
        return 0;
}

But when I compile this code, I get the following error:

Error 4 error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'CArrayIterator' C:\\Program Files\\Microsoft Visual Studio 9.0\\VC\\include\\xutility 1564 Vs8Console

Can anybody throw some light on what I am doing wrong and how it can be corrected? I am using VC9 compiler if it matters.

Answer 1

You've said that your iterator is a "random access iterator". Part of the requirements for random access iterators is that you provide < , <= , > and >= comparision operators, with < giving a strict weak ordering and the usual relationships between them.

You need to provide the appropriate comparison operators, or you could consider 'downgrading' to a bi-directional iterator.

Answer 2

Depending on what you're trying to do, you may not need to write an iterator class at all. CArray s are similar to vector s in that the underlying data store is just a C-style array and the class manages allocation & deallocation for you. You can use GetData() to get a pointer to the data itself, and use simple pointer math to find the end; much the same way you would use raw C-style arrays with the STL <algorithm> s. To wit:

#define _AFXDLL
#include <afx.h>
#include <afxwin.h>         // MFC core and standard components
#include <afxtempl.h>

#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

int main()
{
    CArray<int, int> ints;
    srand((unsigned)time(0));
    for( int i = 0; i < 10; ++i )
        ints.Add(rand()%10);

    vector<int> ints2;
    copy(ints.GetData(), ints.GetData()+ints.GetCount(), back_inserter(ints2));

    cout << "Original : ";
    copy(ints.GetData(), ints.GetData()+ints.GetCount(), ostream_iterator<int>(cout, " "));
    cout << endl
        << endl
        << "Copy : ";

    copy(ints2.begin(), ints2.end(), ostream_iterator<int>(cout, " "));
    cout << endl;

    return 0;

}