c++中输入输出流迭代器及copy源码解析

c++中的流迭代器是迭代器中一种特殊的迭代器，其中包含两种迭代器：输入流迭代器(istream_iterator)，输出流迭代器(ostream_iterator)。使用这两种迭代器可以把输入输出流当作容器来操作。（头文件iterator)

下面是输入输出流常见的简单用法：

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>

using std::cout;
using std::endl;
using std::cin;
using std::ostream_iterator;
using std::istream_iterator;
using std::vector;
using std::back_insert_iterator;
using std::copy;


int main()
{
  vector<int> vec;//定义一个vector
  copy(istream_iterator<int>(cin),istream_iterator<int>(),back_insert_iterator<vector<int>>(vec));//从输入流中读取int数据插入到vector结尾
  copy(vec.begin(),vec.end(),ostream_iterator<int>(cout," "));//从vector中读取数据写入输出流
  return 0;
}


                                //代码1

copy是stl中操作迭代器的算法，三个参数都是迭代器，可能的一种实现方式如下:

template<class InputIt, class OutputIt>
OutputIt copy(InputIt first, InputIt last, 
              OutputIt d_first)
{
    while (first != last) {
        *d_first++ = *first++;
    }
    return d_first;
}
                            //代码2

看到了代码2中copy的实现方式，对代码1中做了啥有了大概了解。第一个copy将输入流迭代器（cin)中的东西赋值给vector的后插迭代器(back_insert_iterator)，然后的输入流迭代器和vector的后插迭代器分别+1直到设定的终止条件，第二个copy将vector中的数据赋给输出流迭代器。

难点：1。为什么终止条件是输入流迭代器等于istream_iterator<int>？

2.输入流迭代器为什么要赋值给*back_insert_iterator，普通的迭代器可以吗？

3.copy中迭代器都有++操作，输入流输出流迭代器也有这个++操作？

问题3最好解答，因为输入输出流迭代器重载了++操作符，返回的是自身，所以++操作符无所谓。

就像这样： ostream_iterator<_Tp>& operator++() { return *this; } 
  ostream_iterator<_Tp>& operator++(int) { return *this; }

问题2是因为要完成*d_first++ = *first++;的操作，此时d_first为back_insert_iterator，而back_insert_iterator重载了=运算符和*运算符，如下

 operator=(const typename _Container::value_type& __value) { 
    container->push_back(__value);
    return *this;
  }
  back_insert_iterator<_Container>& operator*() { return *this; }

所以=操作就相当于vector.push_back()，如果是front_insert_iterator则相当于vector.push_front()。

问题1则需要看istream_iterator源码了：

template <class _Tp, 
          class _CharT = char, class _Traits = char_traits<_CharT>,
          class _Dist = ptrdiff_t> 
class istream_iterator {
public:
  typedef _CharT                         char_type;
  typedef _Traits                        traits_type;
  typedef basic_istream<_CharT, _Traits> istream_type;

  typedef input_iterator_tag             iterator_category;
  typedef _Tp                            value_type;
  typedef _Dist                          difference_type;
  typedef const _Tp*                     pointer;
  typedef const _Tp&                     reference;

  istream_iterator() : _M_stream(0), _M_ok(false) {}
  istream_iterator(istream_type& __s) : _M_stream(&__s) { _M_read(); }//有参构造函数，以构造就会执行_M_read()函数

  reference operator*() const { return _M_value; }
  pointer operator->() const { return &(operator*()); }

  istream_iterator& operator++() { //每次前置++会执行一次_M_read函数一次
    _M_read(); 
    return *this;
  }
  istream_iterator operator++(int)  {//和前置++差不多
    istream_iterator __tmp = *this;
    _M_read();
    return __tmp;
  }

  bool _M_equal(const istream_iterator& __x) const//判断终止条件的关键
    { return (_M_ok == __x._M_ok) && (!_M_ok || _M_stream == __x._M_stream); }

private:
  istream_type* _M_stream;///数据成员
  _Tp _M_value;
  bool _M_ok;

  void _M_read() {
    _M_ok = (_M_stream && *_M_stream) ? true : false;//流不为空，且流状态ok时_M_ok为true
    if (_M_ok) {
      *_M_stream >> _M_value;
      _M_ok = *_M_stream ? true : false;
    }
  }
};

template <class _Tp, class _CharT, class _Traits, class _Dist>
inline bool 
operator==(const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __x,
           const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __y) {
  return __x._M_equal(__y);
}

#ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDER

template <class _Tp, class _CharT, class _Traits, class _Dist>
inline bool 
operator!=(const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __x,
           const istream_iterator<_Tp, _CharT, _Traits, _Dist>& __y) {
  return !__x._M_equal(__y);
}
                        //代码3

首先看数据成员

istream_type* _M_stream;//流，例子中时cin

_Tp _M_value;//暂存从cin中读取的一个数据

bool _M_ok;//存储流状态是否ok

然后看构造函数，有参构造函数构造一个流状态ok的对象，无参构造函数构造一个流状态goodbit不为1(不可读)的对象，而将有参构造函数构造的对象等于无参构造函数构造的对象作为终止条件，首先要看看==运算符

return (_M_ok == __x._M_ok) && (!_M_ok || _M_stream == __x._M_stream);

就相当于(_M_ok == __x._M_ok && !_M_ok ) || (_M_ok == __x._M_ok&&_M_stream == __x._M_stream)

翻译以下就是两个流迭代器对象_M_OK都为false或者两个都ok，且两者对应的流对象相等。

而无参构造的流迭代器对象_M_stream为0，两者不可能相等，所以只可能两者_M_OK都为false时才会相等

而要使_M_OK为false，则流对象的goodbit为0，即不可读，所以要么读到文件结束符，要么读取失败(fail，比如cin>>int.却读到了一个string）。

ostream_iterator输出流迭代器与输入流迭代器类似，实现如下：

template <class _Tp,
          class _CharT = char, class _Traits = char_traits<_CharT> >
class ostream_iterator {
public:
  typedef _CharT                         char_type;
  typedef _Traits                        traits_type;
  typedef basic_ostream<_CharT, _Traits> ostream_type;

  typedef output_iterator_tag            iterator_category;
  typedef void                           value_type;
  typedef void                           difference_type;
  typedef void                           pointer;
  typedef void                           reference;

  ostream_iterator(ostream_type& __s) : _M_stream(&__s), _M_string(0) {}
  ostream_iterator(ostream_type& __s, const _CharT* __c) 
    : _M_stream(&__s), _M_string(__c)  {}
  ostream_iterator<_Tp>& operator=(const _Tp& __value) { 
    *_M_stream << __value;
    if (_M_string) *_M_stream << _M_string;
    return *this;
  }
  ostream_iterator<_Tp>& operator*() { return *this; }
  ostream_iterator<_Tp>& operator++() { return *this; } 
  ostream_iterator<_Tp>& operator++(int) { return *this; } 
private:
  ostream_type* _M_stream;
  const _CharT* _M_string;
};

// The default template argument is declared in iosfwd

// We do not read any characters until operator* is called.  The first
// time operator* is called, it calls getc.  Subsequent calls to getc 
// return a cached character, and calls to operator++ use snextc.  Before
// operator* or operator++ has been called, _M_is_initialized is false.
template<class _CharT, class _Traits>
class istreambuf_iterator
  : public iterator<input_iterator_tag, _CharT,
                    typename _Traits::off_type, _CharT*, _CharT&>
{
public:
  typedef _CharT                           char_type;
  typedef _Traits                          traits_type;
  typedef typename _Traits::int_type       int_type;
  typedef basic_streambuf<_CharT, _Traits> streambuf_type;
  typedef basic_istream<_CharT, _Traits>   istream_type;

public:
  istreambuf_iterator(streambuf_type* __p = 0) { this->_M_init(__p); }
  istreambuf_iterator(istream_type& __is) { this->_M_init(__is.rdbuf()); }

  char_type operator*() const 
    { return _M_is_initialized ? _M_c : _M_dereference_aux(); }

  istreambuf_iterator& operator++() { this->_M_nextc(); return *this; }
  istreambuf_iterator  operator++(int) {
    if (!_M_is_initialized)
      _M_postincr_aux();
    istreambuf_iterator __tmp = *this;
    this->_M_nextc();
    return __tmp;
  }

  bool equal(const istreambuf_iterator& __i) const {
    return this->_M_is_initialized && __i._M_is_initialized
      ? this->_M_eof == __i._M_eof
      : this->_M_equal_aux(__i);
  }

private:
  void _M_init(streambuf_type* __p) {
    _M_buf = __p;
    _M_eof = !__p;
    _M_is_initialized = _M_eof;
  }

  char_type _M_dereference_aux() const;
  bool _M_equal_aux(const istreambuf_iterator&) const;
  void _M_postincr_aux();

  void _M_nextc() {
    int_type __c = _M_buf->snextc();
    _M_c = traits_type::to_char_type(__c);    
    _M_eof = traits_type::eq_int_type(__c, traits_type::eof());
    _M_is_initialized = true;
  }

  void _M_getc() const {
    int_type __c = _M_buf->sgetc();
    _M_c = traits_type::to_char_type(__c);
    _M_eof = traits_type::eq_int_type(__c, traits_type::eof());
    _M_is_initialized = true;
  }

private:
  streambuf_type* _M_buf;
  mutable _CharT _M_c;
  mutable bool _M_eof : 1;
  mutable bool _M_is_initialized : 1;
};

template<class _CharT, class _Traits>
_CharT istreambuf_iterator<_CharT, _Traits>::_M_dereference_aux() const
{
  this->_M_getc();
  return _M_c;
}

template<class _CharT, class _Traits>
bool istreambuf_iterator<_CharT, _Traits>
  ::_M_equal_aux(const istreambuf_iterator& __i) const
{
  if (!this->_M_is_initialized)
    this->_M_getc();
  if (!__i._M_is_initialized)
    __i._M_getc();

  return this->_M_eof == __i._M_eof;
}

template<class _CharT, class _Traits>
void istreambuf_iterator<_CharT, _Traits>::_M_postincr_aux()
{
  this->_M_getc();
}

template<class _CharT, class _Traits>
inline bool operator==(const istreambuf_iterator<_CharT, _Traits>& __x,
                       const istreambuf_iterator<_CharT, _Traits>& __y) {
  return __x.equal(__y);
}

#ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDER

template<class _CharT, class _Traits>
inline bool operator!=(const istreambuf_iterator<_CharT, _Traits>& __x,
                       const istreambuf_iterator<_CharT, _Traits>& __y) {
  return !__x.equal(__y);
}

#endif /* __STL_FUNCTION_TMPL_PARTIAL_ORDER */

// The default template argument is declared in iosfwd
template<class _CharT, class _Traits>
class ostreambuf_iterator
  : public iterator<output_iterator_tag, void, void, void, void>
{
public:
  typedef _CharT                           char_type;
  typedef _Traits                          traits_type;
  typedef typename _Traits::int_type       int_type;
  typedef basic_streambuf<_CharT, _Traits> streambuf_type;
  typedef basic_ostream<_CharT, _Traits>   ostream_type;

public:
  ostreambuf_iterator(streambuf_type* __buf) : _M_buf(__buf), _M_ok(__buf) {}
  ostreambuf_iterator(ostream_type& __o)
    : _M_buf(__o.rdbuf()), _M_ok(__o.rdbuf() != 0) {}

  ostreambuf_iterator& operator=(char_type __c) {
    _M_ok = _M_ok && !traits_type::eq_int_type(_M_buf->sputc(__c),
                                               traits_type::eof());
    return *this;
  }    
  
  ostreambuf_iterator& operator*()     { return *this; }
  ostreambuf_iterator& operator++()    { return *this; }
  ostreambuf_iterator& operator++(int) { return *this; }

  bool failed() const { return !_M_ok; }

private:
  streambuf_type* _M_buf;
  bool _M_ok;
};

原文链接：https://blog.csdn.net/wuqindeyunque/article/details/81488113

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