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 */
// Local Variables:
// mode:C++
// End:
《STL源码剖析》---stl_pair.h阅读笔记,布布扣,bubuko.com
原文：http://blog.csdn.net/kangroger/article/details/38635681