<开篇>
本篇紧接着boost上篇叙述Boost::DateTime的时间处理。在C++中，常见的时间有time_t, FILETIME和tm，而boost中用ptime。
构造ptime
1.ptime的构造函数有四种：
1: using namespace boost::posix_time;
2: using namespace boost::gregorian;
3: ptime pt(date(2013,Jan,24),time_duration(1,2,3)); //由date和time_duration构造
4: ptime pt1(date(2013,Jan,24),hours()+nanosec(5));//改变形式的time_duration也能使用
5: ptime pt2(p1);//拷贝构造函数
6: ptime pt3(neg_infin);//特殊值构造
7: ptime p;//默认构造函数，这里p等于not_a_date_time
2.用string构造ptime:
1: std::string ts1("2013-01-30 23:32:22.000");//固定格式，小数点后支持6位
2: ptime pt1(time_from_string(ts1));
3: std::string ts2("20130130T233222");//没有分隔符的date和time
4: ptime pt2(from_iso_string(ts2));
5:
3.通过时钟构造ptime:
1: ptime ct1(second_clock::local_time());
2: ptime ct2(second_clock::universal_time());
3: ptime ct3(microsec_clock::local_time());
4: ptime ct4(microsec_clock::universal_time());
5:
4.time_t和FILETIME构造ptime:
1: ptime t = from_time_t(tt); // 其中tt为time_t
2: ptime t1 = from_ftime<ptime>(ft); //其中ft为FILETIME
ptime访问日期时间
1: using namespace boost::posix_time;
2: using namespace boost::gregorian;
3: ptime now(second_clock::local_time());
4: std::cout << "today is: " << now.date() << std::endl;
5: std::cout << "time is: " << now.time_of_day() << std::endl;
6:
ptime转换为string
1: std::string now_str(to_simple_string(now));
2: std::string now_iso_str(to_iso_string(now));
3: std::string now_iso_ext_str(to_iso_extended_string(now));
4: std::cout << now_str << std::endl;
5: std::cout << now_iso_str << std::endl;
6: std::cout << now_iso_ext_str << std::endl;
ptime与tm，time_t，FILETIME互转
1.tm
1: using namespace boost::posix_time;
2: using namespace boost::gregorian;
3: tm pt_tm;
4: pt_tm.tm_year = 113;
5: pt_tm.tm_mon = 11;
6: pt_tm.tm_mday = 25;
7: pt_tm.tm_hour = 2;
8: pt_tm.tm_min = 23;
9: pt_tm.tm_sec = 40;
10:
11: ptime pt = data_from_tm(pt_tm);
12: std::cout << pt << std::endl;
13:
14: pt = pt + hours(2);
15: tm pt_tm1 = to_tm(pt);
2. time_t
1: using namespace boost::posix_time;
2: using namespace boost::gregorian;
3:
4: time_t now = time(NULL);
5: std::cout << "time_t : " << now << std::endl;
6: ptime now_pt = from_time_t(now);
7: std::cout << "ptime from time_t : " << now_pt.time_of_day() << std::endl;
8: tm* now_tm = gmtime(&now);
9: std::cout << "tm struct: hour : " << now_tm->tm_hour << std::endl;
10:
3.FILETIME
1: FILETIME ft;
2: ft.dwHighDateTime = 29715317;
3: ft.dwLowDateTime = 3865122988UL
4: ptime pt = from_ftime<ptime>(ft);
5: // pt ===> 2005-Jun-07 15:30:57.03958200
6:
time_duration和time_period
1: using namespace boost::posix_time;
2: using namespace boost::gregorian;
3:
4: time_duration td(100,200,3,9);
5: std::cout << td << std::endl;
6: date d(2013,Feb,5);
7: ptime pt(d,minutes(10));
8: ptime pt1(d,hours(10));
9: time_period tp(pt,pt1);
10: std::cout << tp << std::endl;
11:
对于这两者的区别，一个是时间间隔，一个是时间起止的一个窗口。time_duration用于ptime的时间偏移计算为主。而time_period可以计算一个ptime时间点是否在这个时间区间内(参考contains函数)。time_period在创建之后可以扩展，可以平移，函数分别为expand和shift。请大家自己细究。
下一篇将介绍关于boost.datetime的格式化输入输出。
<完结>
