auto it = vector.begin()结果类型不能转换为const_iterator

容器需要提供一种iterator可隐式转换为a 的类型const_iterator.鉴于此,我尝试使用auto初始化对象vector::begin(),并std::distance在RHS所在的位置使用该结果对象const_iterator.这不起作用.这是一个完整的例子:

#include 
#include 
#include 
#include 

typedef std::vector  Packet;
typedef std::vector  Packets;

template  
Iter next_upto (Iter begin, Iter end, size_t n)
{
    Iter ret = begin;
      for (; n > 0 && ret != end; ++ret, --n)
            ;   
        return ret;
}

Packets::const_iterator Process (Packets::const_iterator begin, Packets::const_iterator end)
{
  Packets::const_iterator ret = begin;
  while (ret != end)
    ++ret;  // do something
  return ret;
}

int main()
{
  Packets test (100); // vector of 100 default-initialized packets

  // process them 10 at a time
  for (auto it = test.begin();
    it != test.end();
    it = next_upto (it, test.end(), 10))
  {
     auto itr = Process (it, next_upto (it, test.end(), 10));
     Packets::const_iterator it2 = it; 
     const size_t n1 = std::distance (it2, itr);
     const size_t n = std::distance (it, itr);
     std::cout << "Processed " << n << " packets\n";
  }
}

在g ++ 4.8.1(和4.8.2)下编译,得到:

[1/2] Building CXX object CMakeFiles/hacks.dir/main.o
FAILED: /usr/bin/g++    -Wall -std=c++11 -g -MMD -MT CMakeFiles/hacks.dir/main.o -MF "CMakeFiles/hacks.dir/main.o.d" -o CMakeFiles/hacks.dir/main.o -c main.cpp
main.cpp: In function ‘int main()’:
main.cpp:39:45: error: no matching function for call to ‘distance(__gnu_cxx::__normal_iterator*, std::vector > >&, __gnu_cxx::__normal_iterator*, std::vector > >&)’
      const size_t n = std::distance (it, itr);
                                             ^
main.cpp:39:45: note: candidate is:
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/4.8/vector:60,
                 from main.cpp:2:
/usr/include/c++/4.8/bits/stl_iterator_base_funcs.h:114:5: note: template typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator)
     distance(_InputIterator __first, _InputIterator __last)
     ^
/usr/include/c++/4.8/bits/stl_iterator_base_funcs.h:114:5: note:   template argument deduction/substitution failed:
main.cpp:39:45: note:   deduced conflicting types for parameter ‘_InputIterator’ (‘__gnu_cxx::__normal_iterator*, std::vector > >’ and ‘__gnu_cxx::__normal_iterator*, std::vector > >’)
      const size_t n = std::distance (it, itr);
                                             ^

我知道,我可以通过调用解决这种特定情况下cbegin()和cend(),而不是begin()和end(),但由于begin()并end()返回一个类型,应该是转换为const_iterator,我不知道我理解为什么这是必要的.

为什么auto在这种情况下推导出一种不可转换的类型const_iterator？