Question

当我尝试在gcc 4.7下使用std::distance自定义迭代器时，它会抱怨找不到difference_type。我遗憾地不知道它为什么会失败。

#include <iterator>

class nit {
public:
    typedef int difference_type;
};

int main() {
  const nit test1;
  std::distance( test1, test1 );
  return 0;
}

给出错误：

/usr/include/c++/4.7/bits/stl_iterator_base_funcs.h:114:5: error: no type named ‘difference_type’ in ‘struct std::iterator_traits<nit>’

Answer 1

您是否尝试过定义所有必需的类型/运算符？

#include <iterator>

struct nit
{
  typedef std::random_access_iterator_tag iterator_category;
  typedef int value_type;
  typedef int difference_type;
  typedef int* pointer;
  typedef int& reference;

  bool operator==(nit const&)
  {
    return true;
  }

  bool operator!=(nit const&)
  {
    return false;
  }

  int operator-(nit const&)
  {
    return 0;
  }

  nit()
  {
  }
};

int main()
{
  nit const test1;
  std::distance(test1, test1);

  return 0;
}

Answer 2

或者，您必须在类中提供std :: iterator_traits所期望的所有typedef（有或没有std :: iterator的帮助），或者您必须自己专门化std :: iterator_traits。

GCC的{p> This version会发出其他错误消息，但不会改变您的代码非法的事实。

prog.cpp: In function ‘int main()’:
prog.cpp:9: error: uninitialized const ‘test1’
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++v4/bits/stl_iterator_base_types.h: At global scope:
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_types.h: In instantiation of ‘std::iterator_traits<nit>’:
prog.cpp:10:   instantiated from here
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_types.h:133: error: no type named ‘iterator_category’ in ‘class nit’
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_types.h:134: error: no type named ‘value_type’ in ‘class nit’
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_types.h:136: error: no type named ‘pointer’ in ‘class nit’
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_types.h:137: error: no type named ‘reference’ in ‘class nit’
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_funcs.h: In function ‘typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator) [with _InputIterator = nit]’:
prog.cpp:10:   instantiated from here
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_iterator_base_funcs.h:119: error: no matching function for call to ‘__iterator_category(nit&)’

找不到difference_type

2 个答案: