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c++ clock?

Apr 22nd, 2008

I need help with manipulating my computers clock through a c++ program and i have no idea on how to make the program. Does anyone have any idea on how to do this?

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Re: c++ clock?

Apr 22nd, 2008

What kind of manipulation are you looking for? Standard C++ pretty much only gives you query capabilities for the clock, though you have comprehensive time and date control.

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Re: c++ clock?

Apr 22nd, 2008

well i want it so that i can take it and subtract a chosen amount of time from the clock.

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Re: c++ clock?

Apr 22nd, 2008

To what end? You can easily call std::clock() to get the current value and perform math on it. But the result may not be meaningful.

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Re: c++ clock?

Apr 22nd, 2008

like take the current time and subtract say 6 hours. This is all for a timezone project im doing and i have everything but the clock

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Re: c++ clock?

Apr 22nd, 2008

That's slightly harder, but you have more control and it's actually portable:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <ctime>
#include <iostream>
 
int main()
{
  std::time_t now = std::time ( 0 );
  std::tm *local = std::localtime ( &now );
 
  local->tm_hour -= 6;
 
  std::time_t before = std::mktime ( local );
 
  std::cout<<"Now:         "<< ctime ( &now ) <<'\n';
  std::cout<<"6 hours ago: "<< ctime ( &before ) <<'\n';
}
#include <ctime>
#include <iostream>

int main()
{
  std::time_t now = std::time ( 0 );
  std::tm *local = std::localtime ( &now );

  local->tm_hour -= 6;

  std::time_t before = std::mktime ( local );

  std::cout<<"Now:         "<< ctime ( &now ) <<'\n';
  std::cout<<"6 hours ago: "<< ctime ( &before ) <<'\n';
}

C++ supports something called a clock, and means system clock ticks, not wallclock time. Keep that in mind when asking questions about the "clock", because what you actually want to manipulate is date/time.

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Re: c++ clock?

Apr 22nd, 2008

thanks ill try it out

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Re: c++ clock?

Apr 23rd, 2008

While I'm no expert at C++ I think I can help you with your dilema.

I use the library time.h so you'd want to add

#include <time.h>

to your header files to use this. I saw an earlier post uses ctime but anyway

if you use time.h you'll be able to use things like this:
this is all assuming that you have using namespace std; in there as well, although I'm not sure that time.h requires it.

Anyway the main two things you want to use are clock() and clock_t

clock_t is a type. it is a variable type like int to store clock variables.
clock() returns the given system clock at a time. Now this number is some strange number that won't really make any sense to you, so you just have to work with it in the way that I explain.

Let's say you want to have something happen 5 seconds in the future. Here's what you do.

clock_t FiveSecondsLater; // This declares your time variable
FiveSecondsLater = clock() + 5 * CLOCKS_PER_SEC;

Now at this point, you'll have a variable "FiveSecondsLater" that equals the value that the system clock will be in five seconds. How does this work?

clock() will return a value something like 39492. This number goes up at a certain rate.
CLOCKS_PER_SEC is a constant. It is exactly how much clock() increases in a second.
So if you want to set a time for 5 seconds in the future, you multiply it by however many seconds you want to do.

So if you later want to do something after five seconds you'd use an if statement combined with the previous code.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
if (clock() > FiveSecondsLater)
{
      // Do whatever it is you want to do.
}
if (clock() > FiveSecondsLater)
{
      // Do whatever it is you want to do.
}

Another function you could do here would be a generic "wait" function.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
void wait (float seconds)
{
      clock_t EndTime = seconds * CLOCKS_PER_SEC;
      while (EndTime > clock())
      {
            // this will loop until clock() equals EndTime
      }
 
}
void wait (float seconds)
{
      clock_t EndTime = seconds * CLOCKS_PER_SEC;
      while (EndTime > clock())
      {
            // this will loop until clock() equals EndTime
      }

}

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Re: c++ clock?

Apr 23rd, 2008

Well all C-based compilers have to add in the headers with '.h' following them. Later after the formation of c++ the programmers have ruled out the usage of '.h' and added c in the begininning

So
#include <ctime>
or
#include <time.h>
Refers to a header file with same content in it.

time.h is available so that even old c programs can be compiled using a c++ compiler.

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Re: c++ clock?

#10

Apr 23rd, 2008

>While I'm no expert at C++ I think I can help you with your dilema.
I don't think you can, sorry.

>I use the library time.h so you'd want to add
time.h is deprecated in standard C++. New code should be written using the ctime header instead.

>this is all assuming that you have using namespace std; in
>there as well, although I'm not sure that time.h requires it.
time.h does not place any names in the std namespace, but ctime does.

>Anyway the main two things you want to use are clock() and clock_t
Did you even bother reading the thread before replying? We've already established that the OP wants to work with date/time, not clock ticks.

>clock_t FiveSecondsLater; // This declares your time variable
>FiveSecondsLater = clock() + 5 * CLOCKS_PER_SEC;
That's nice. Now explain how that can be used to given the current wall clock time for different timezones.

>Another function you could do here would be a generic "wait" function.
Which is an extremely bad idea. A busy wait is both inefficient and anti-social.

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