学步园

pair是STL中的模板类型，它可以存储两个元素，它也被称作“对组”。在map中已经用到了它，pair其实就是一个struct结构，存有两个public的元素，重载了几个运算符，没有什么成员函数，源代码很简单。

G++ 2.91.57，cygnus\cygwin-b20\include\g++\stl_pair.h 完整列表
/*
 *
 * Copyright (c) 1994
 * Hewlett-Packard Company
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Hewlett-Packard Company makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 *
 * Copyright (c) 1996,1997
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */

/* NOTE: This is an internal header file, included by other STL headers.
 *   You should not attempt to use it directly.
 */

#ifndef __SGI_STL_INTERNAL_PAIR_H
#define __SGI_STL_INTERNAL_PAIR_H

__STL_BEGIN_NAMESPACE

template <class T1, class T2>
struct pair {
  typedef T1 first_type;
  typedef T2 second_type;
//没有用访问限定符，但是struct默认是public
  T1 first;		// 注意，它是 public
  T2 second;		// 注意，它是 public
  
  //默认构造函数是用了模板参数的默认构造函数
  //因此pair中元素要有默认构造函数
  pair() : first(T1()), second(T2()) {}
  pair(const T1& a, const T2& b) : first(a), second(b) {}

#ifdef __STL_MEMBER_TEMPLATES
  //用一个pair初始化另一个pair，类型可以不同，但是能转换就好。
  template <class U1, class U2>
  pair(const pair<U1, U2>& p) : first(p.first), second(p.second) {}
#endif
};
//两个pair相等，意味着它们中的两个元素都相等。
template <class T1, class T2>
inline bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y) { 
  return x.first == y.first && x.second == y.second; 
}

template <class T1, class T2>
inline bool operator<(const pair<T1, T2>& x, const pair<T1, T2>& y) { 
  return x.first < y.first || (!(y.first < x.first) && x.second < y.second); 
  // x的第一元素小于y的第一元素，或，x的第一元素不大于y的第一元素而
  // x的第二元素小于y的第二元素，才視為x小於y。
}

//根据两个数值，构造一个pair
template <class T1, class T2>
inline pair<T1, T2> make_pair(const T1& x, const T2& y) {
  return pair<T1, T2>(x, y);
}

__STL_END_NAMESPACE

#endif /* __SGI_STL_INTERNAL_PAIR_H */

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