C++ Time – Calculating Difference Between Two Points in Milliseconds

c++c++-chronoc++11time

How can I get (using the std::chrono library) the difference between two points in time in milliseconds?

I could do that using this:

std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();
    
std::chrono::time_point<std::chrono::system_clock> foo = now + std::chrono::milliseconds(100);
    
std::chrono::duration<float> difference = foo - now;
    
const int milliseconds = difference.count() * 1000;

How can I get this time in milliseconds, so I can use the duration as an unsigned int, and not a float and then multiply by 1000?

Best Answer

std::chrono::duration has two template parameters, the second being exactly the unit of measure. You can invoke std::chrono::duration_cast to cast from one duration type to another. Also, there is a predefined duration type for milliseconds: std::chrono::milliseconds. Composing this together:

auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(foo - now);

To get the actual number of milliseconds, use duration::count:

auto ms = milliseconds.count();

Its return type is duration::rep, which for standard duration types like std::chrono::milliseconds is a signed integer of unspecified size.

Related Solutions

C++ – Calculating Time Difference

I know this is an old question, but there's an updated answer for C++0x. There is a new header called <chrono> which contains modern time utilities. Example use:

#include <iostream>
#include <thread>
#include <chrono>

int main()
{
    typedef std::chrono::high_resolution_clock Clock;
    typedef std::chrono::milliseconds milliseconds;
    Clock::time_point t0 = Clock::now();
    std::this_thread::sleep_for(milliseconds(50));
    Clock::time_point t1 = Clock::now();
    milliseconds ms = std::chrono::duration_cast<milliseconds>(t1 - t0);
    std::cout << ms.count() << "ms\n";
}

50ms

More information can be found here:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2661.htm

There is also now a boost implementation of <chrono>.

C++ – Using chrono for System Time in Milliseconds and Time Operations

You can do now.time_since_epoch() to get a duration representing the time since the epoch, with the clock's resolution. To convert to milliseconds use duration_cast:

auto duration = now.time_since_epoch();
auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();

Best Answer

Related Solutions

C++ – Calculating Time Difference

C++ – Using chrono for System Time in Milliseconds and Time Operations

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