Re: [Boost-users] [date-time] boost::time_duration wrapping

Subject: Re: [Boost-users] [date-time] boost::time_duration wrapping
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2014-08-07 00:17:32

On Aug 6, 2014, at 7:07 PM, Justin Muncaster <jmuncaster_at_[hidden]> wrote:

> Hello,
> 
> I’m having issues with wrapping of boost::time_duration derived types. I can’t find anywhere in the documentation for when the time_duration types are supposed to wrap around.

If the duration types are based on signed integers (which I believe they are), then "wrapping around” is undefined behavior.
Once you’ve invoked undefined behavior, then you can’t expect *anything*, or, as I say, “there are no wrong answers”

— Marshall

> 
> I have attached some sample source code below. The output is:
>  SecCount = 80000000 is OK.
>  Strange
>  Stranger
>  Problematic
>  SecCount = 80000001 is STRANGE.
> 
> I expect the output to be
>  SecCount = 80000000 is OK.
>  SecCount = 80000001 is OK.
> 
> The problem we ran into had to do with seconds and milliseconds having different maximum permissible values. Any idea why boost::posix_time::seconds goes negative at 0x80000001 while milliseconds is not until much later?
> 
> Thanks,
> 
> Justin
> 
> 
> //
> // test.cpp
> //
> 
> #include <boost/date_time.hpp>
> #include <iostream>
> 
> using namespace std;
> using namespace boost::posix_time;
> 
> //-------------------------------------------------------
> //-------------------------------------------------------
> void Strange(long SecCount)
> {
>  long MilliCount = 1000 * SecCount;
> 
>  bool IsStrange = false;
> 
>  if (SecCount >= 0 && seconds(SecCount).is_negative())
>  {
>    cout << "Strange" << endl;
>    IsStrange = true;
> 
>    if (!milliseconds(MilliCount).is_negative())
>    {
>      // If seconds wraps, why doesn't milliseconds?
>      cout << "Stranger" << endl;
>      IsStrange = true;
>    }
>  }
> 
>  if (seconds(SecCount) != milliseconds(MilliCount))
>  {
>    cout << "Problematic" << endl;
>    IsStrange = true;
>  }
> 
>  if (!IsStrange)
>  {
>    cout << "SecCount = " << SecCount << " is OK." << endl;
>  }
>  else
>  {
>    cout << "SecCount = " << SecCount << " is STRANGE." << endl;
>  }
> }
> 
> //-------------------------------------------------------
> //-------------------------------------------------------
> int main()
> {
>  cout << hex;
>  Strange(0x80000000);
>  Strange(0x80000001);
>  return 0;
> }
> 
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