Stefan B Stefan B - 3 years ago 199
C++ Question

c++ std chrono overflow

I am trying to implement a stopwatch for debugging as follows:

#include <chrono>

namespace sbstd
{
class timer
{
private:
bool m_running = false;
std::chrono::time_point<std::chrono::system_clock> m_temp;
std::chrono::nanoseconds m_time;
public:
void start()
{
if (m_running) { throw std::exception("timer already running"); }
m_temp = std::chrono::system_clock::now();
m_running = true;
}

void stop()
{
if (!m_running) { throw std::exception("timer not started"); }
m_time += std::chrono::system_clock::now() - m_temp;
m_running = false;
}

long long get_ms() const
{
return std::chrono::duration_cast<std::chrono::milliseconds>(m_time).count();
}
};
}


In the main method I have the following:

sbstd::timer t1;
t1.start();
std::this_thread::sleep_for(std::chrono::seconds(5));
t1.stop();
cout << t1.get_ms();


Instead of 5000 I get a negative int.

What is wrong?

Answer Source

m_time is not initialised so its value is indeterminate - its just some garbage value. You probably want to initialise it to 0 - use std::chrono::nanoseconds::zero(). Here it is in action.

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