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一直觉得boost的时间库不是很好用，当然，也有可能是我没有深入理解，所以，把代码弄出来看看或许要好些，时间处理中，取当前时间真的是太常见，而boost中各种clock又区分不清楚，然而，代码能说明一切，从下面代码可以看出，steadyclock和systemclock根本就代表不同的计数. QueryPerformanceCounter这个函数取的是硬件定时器的值，这样的话，这个主要是用着计数使用，跟平常时间的now其实是不同的，这里的now只是代表的是当前的计数值，而systemclock取的则是系统时间，虽然转换成了文件系统的时间，这只是更改了下计时起点罢了，所以，如果要取当前时间的话还是使用systemclock::now, 当然，可能还有其它办法，这个只有学完再总结了。
// win/chrono.cpp --------------------------------------------------------------//
// Copyright Beman Dawes 2008
// Copyright 2009-2010 Vicente J. Botet Escriba
// Distributed under the Boost Software License, Version 1.0.
// See http://www.boost.org/LICENSE_1_0.txt
//----------------------------------------------------------------------------//
// Windows //
//----------------------------------------------------------------------------//
#ifndef BOOST_CHRONO_DETAIL_INLINED_WIN_CHRONO_HPP
#define BOOST_CHRONO_DETAIL_INLINED_WIN_CHRONO_HPP
#include <boost/detail/winapi/time.hpp>
#include <boost/detail/winapi/timers.hpp>
#include <boost/detail/winapi/GetLastError.hpp>
namespace boost
{
namespace chrono
{
namespace chrono_detail
{
BOOST_CHRONO_INLINE double get_nanosecs_per_tic() BOOST_NOEXCEPT
{
boost::detail::winapi::LARGE_INTEGER_ freq;
if ( !boost::detail::winapi::QueryPerformanceFrequency( &freq ) )
return 0.0L;
return double(1000000000.0L / freq.QuadPart);
}
}
steady_clock::time_point steady_clock::now() BOOST_NOEXCEPT
{
double nanosecs_per_tic = chrono_detail::get_nanosecs_per_tic();
boost::detail::winapi::LARGE_INTEGER_ pcount;
if ( nanosecs_per_tic <= 0.0L )
{
BOOST_ASSERT(0 && "Boost::Chrono - get_nanosecs_per_tic Internal Error");
return steady_clock::time_point();
}
unsigned times=0;
while ( ! boost::detail::winapi::QueryPerformanceCounter( &pcount ) )
{
if ( ++times > 3 )
{
BOOST_ASSERT(0 && "Boost::Chrono - QueryPerformanceCounter Internal Error");
return steady_clock::time_point();
}
}
return steady_clock::time_point(steady_clock::duration(
static_cast<steady_clock::rep>((nanosecs_per_tic) * pcount.QuadPart)));
}
#if !defined BOOST_CHRONO_DONT_PROVIDE_HYBRID_ERROR_HANDLING
steady_clock::time_point steady_clock::now( system::error_code & ec )
{
double nanosecs_per_tic = chrono_detail::get_nanosecs_per_tic();
boost::detail::winapi::LARGE_INTEGER_ pcount;
if ( (nanosecs_per_tic <= 0.0L)
|| (!boost::detail::winapi::QueryPerformanceCounter( &pcount )) )
{
boost::detail::winapi::DWORD_ cause =
((nanosecs_per_tic <= 0.0L)
? ERROR_NOT_SUPPORTED
: boost::detail::winapi::GetLastError());
if (BOOST_CHRONO_IS_THROWS(ec)) {
boost::throw_exception(
system::system_error(
cause,
BOOST_CHRONO_SYSTEM_CATEGORY,
"chrono::steady_clock" ));
}
else
{
ec.assign( cause, BOOST_CHRONO_SYSTEM_CATEGORY );
return steady_clock::time_point(duration(0));
}
}
if (!BOOST_CHRONO_IS_THROWS(ec))
{
ec.clear();
}
return time_point(duration(
static_cast<steady_clock::rep>(nanosecs_per_tic * pcount.QuadPart)));
}
#endif
BOOST_CHRONO_INLINE
system_clock::time_point system_clock::now() BOOST_NOEXCEPT
{
boost::detail::winapi::FILETIME_ ft;
boost::detail::winapi::GetSystemTimeAsFileTime( &ft ); // never fails
return system_clock::time_point(
system_clock::duration(
((static_cast<__int64>( ft.dwHighDateTime ) << 32) | ft.dwLowDateTime)
- 116444736000000000LL
//- (134775LL*864000000000LL)
)
);
}
#if !defined BOOST_CHRONO_DONT_PROVIDE_HYBRID_ERROR_HANDLING
BOOST_CHRONO_INLINE
system_clock::time_point system_clock::now( system::error_code & ec )
{
boost::detail::winapi::FILETIME_ ft;
boost::detail::winapi::GetSystemTimeAsFileTime( &ft ); // never fails
if (!BOOST_CHRONO_IS_THROWS(ec))
{
ec.clear();
}
return system_clock::time_point(
system_clock::duration(
((static_cast<__int64>( ft.dwHighDateTime ) << 32) | ft.dwLowDateTime)
- 116444736000000000LL
//- (134775LL*864000000000LL)
));
}
#endif
BOOST_CHRONO_INLINE
std::time_t system_clock::to_time_t(const system_clock::time_point& t) BOOST_NOEXCEPT
{
__int64 temp = t.time_since_epoch().count();
temp /= 10000000;
return static_cast<std::time_t>( temp );
}
BOOST_CHRONO_INLINE
system_clock::time_point system_clock::from_time_t(std::time_t t) BOOST_NOEXCEPT
{
__int64 temp = t;
temp *= 10000000;
return time_point(duration(temp));
}
} // namespace chrono
} // namespace boost
#endif
BOOST CHRONO steadycolock::now分析
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原文地址：http://www.cnblogs.com/csxy/p/5453922.html
