boost库学习之 timer库

timer是一个很小的库,提供简易的时间度量和进度显示,包括timer、progress_timer、progress_display三个类。不适合高精度的时间测量任务，精度依赖于操作系统或编译器，可提供的最大时间跨度只有几百个小时，不适合大跨度的时间段测量。

timer类

先来看一下标准库如何计时:

#include <iostream>
#include <ctime>

using namespace std;

int main()
{
	clock_t begin, end;

	begin = clock();

	for (long i=0; i<1000000; ++i);

	end = clock();
	cout << "loop time is:" << (double)(end-begin) / CLOCKS_PER_SEC << endl;
	system("pause");
}

使用了标准库<ctime>头文件中的clock()，它返回自进程启动以来的clock数,每秒的clock数由宏CLOCKS_PER_SEC定义。在win32下是1000,在linux下是1000000。也就是说在win32下的精度为毫秒,在linux下的精度为微妙。

再看timer类源码

//  boost timer.hpp header file  ---------------------------------------------//

//  Copyright Beman Dawes 1994-99.  Distributed under the Boost
//  Software License, Version 1.0. (See accompanying file
//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

//  See http://www.boost.org/libs/timer for documentation.

//  Revision History
//  01 Apr 01  Modified to use new <boost/limits.hpp> header. (JMaddock)
//  12 Jan 01  Change to inline implementation to allow use without library
//             builds. See docs for more rationale. (Beman Dawes)
//  25 Sep 99  elapsed_max() and elapsed_min() added (John Maddock)
//  16 Jul 99  Second beta
//   6 Jul 99  Initial boost version

#ifndef BOOST_TIMER_HPP
#define BOOST_TIMER_HPP

#include <boost/config.hpp>
#include <ctime>
#include <boost/limits.hpp>

# ifdef BOOST_NO_STDC_NAMESPACE
    namespace std { using ::clock_t; using ::clock; }
# endif

namespace boost {

//  timer  -------------------------------------------------------------------//

//  A timer object measures elapsed time.

//  It is recommended that implementations measure wall clock rather than CPU
//  time since the intended use is performance measurement on systems where
//  total elapsed time is more important than just process or CPU time.

//  Warnings: The maximum measurable elapsed time may well be only 596.5+ hours
//  due to implementation limitations.  The accuracy of timings depends on the
//  accuracy of timing information provided by the underlying platform, and
//  this varies a great deal from platform to platform.

class timer
{
 public:
         timer() { _start_time = std::clock(); } // postcondition: elapsed()==0
//         timer( const timer& src );      // post: elapsed()==src.elapsed()
//        ~timer(){}
//  timer& operator=( const timer& src );  // post: elapsed()==src.elapsed()
  void   restart() { _start_time = std::clock(); } // post: elapsed()==0
  double elapsed() const                  // return elapsed time in seconds
    { return  double(std::clock() - _start_time) / CLOCKS_PER_SEC; }

  double elapsed_max() const   // return estimated maximum value for elapsed()
  // Portability warning: elapsed_max() may return too high a value on systems
  // where std::clock_t overflows or resets at surprising values.
  {
    return (double((std::numeric_limits<std::clock_t>::max)())
       - double(_start_time)) / double(CLOCKS_PER_SEC);
  }

  double elapsed_min() const            // return minimum value for elapsed()
   { return double(1)/double(CLOCKS_PER_SEC); }

 private:
  std::clock_t _start_time;
}; // timer

} // namespace boost

#endif  // BOOST_TIMER_HPP

timer也使用了标准库<ctime>头文件中的clock()，timer对象一旦被声明，构造函数就启动计时，之后可以用elapsed测量自创建到elapsed()被调用所流逝的时间。如

#include <boost/timer.hpp> //头文件
#include <iostream>

using namespace boost;     //命名空间
using namespace std;

int main()
{
	timer t;                 //timer对象一旦被创建，它的构造函数就启动了计时工作

	for(int i=0; i<1000; ++i)
		for (int j=0; j<1000; ++j);

	cout << t.elapsed() << "s" << endl;  //调用elapsed()测量自创建后流逝的时间
	system("pause");
}

progress_timer类

progress_timer继承自timer,可以像用timer的用法用progress_timer。progress_timer更简单，只需要声明progress_timer对象即可，在析构时会自动输出流逝的时间。

#include <boost/progress.hpp>
#include <iostream>

using namespace boost;
using namespace std;

void func()
{
	progress_timer pt;

	for(long i=0; i<1000000; ++i);
}

int main()
{
	func();
	system("pause");
}

progress_display类

progress_display可以在控制台上显示程序执行进度，它的构造函数接受一个long型的参数作为进度显示的基数，之后显示一个进度条界面，重载的操作符+=和++用来增加基数，成员count()可返回当前计数。

#include <boost/progress.hpp>
#include <iostream>

using namespace boost;
using namespace std;

void func()
{
	progress_display pd(10000000);

	for(int i=0; i<10000000; ++i) {
		++pd;
	}
}

int main()
{
	func();
	system("pause");
}