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c++11c++-chronomilliseconds

std::chrono::milliseconds .count() returns in microseconds?

I am trying to log milliseconds of time that has elapsed over a period of time.

I have a class like this

// class member declarations
class MyClass {

    std::chrono::high_resolution_clock::time_point   m_start;
    std::chrono::system_clock::duration              m_elapsed;
};

I have 2 methods in the class. One is called from main that is func1CalledFromMainThread. 

// Class methods
using namespace std::chrono;
void MyClass::func1CalledFromMainThread() {

    m_start = std::chrono::high_resolution_clock::now();
}

And another one that is func2CalledFromADifferentThread is called from a different thread

void MyClass::func2CalledFromADifferentThread() {

    // after some time following line of code runs from a different thread
    auto end = high_resolution_clock::now();

    m_elapsed = duration_cast<milliseconds>(end - m_start);
    std::cout << "Elapsed time in milliseconds is " << m_elapsed.count()/1000 << std::endl;
}

The issue is in the cout logging. I see that I have to divide by 1000 to get milliseconds out of m_elapsed. Doesn't count return the count of std::chrono::milliseconds here? Why should I have to divide by 1000 here? Does count() return always in microseconds or am I doing a mistake?

Solution

  • count returns the number of ticks of the type on which you invoke it. If you wrote this:

    duration_cast<milliseconds>(end - m_start).count()

    it would correctly give you the number of milliseconds. However, you're not storing the result in std::chrono::milliseconds, you're storing it in std::chrono::system_clock::duration (the type of m_elapsed). Therefore, m_elapsed.count() returns the number of ticks in std::chrono::system_clock::duration's frequency, which is probably microseconds on your platform.

    In other words, you're immediately undoing the cast to milliseconds by storing the result in something other than milliseconds.