c++11 输出时间

C++11中输出当前时间最直接的方法：

std::time_t t2 = std::time(nullptr);
cout << std::put_time(std::localtime(&t), "%Y-%m-%d %H.%M.%S") << "." << msecs << endl;

这种方法可以输出年月日时分秒，不过却不能输出毫秒，如果要输出毫秒需要用下面的方法：

auto n = chrono::system_clock::now();
auto m = n.time_since_epoch();
auto diff = duration_cast<milliseconds>(ms).count();
auto const msecs = diff % 1000;

std::time_t t = system_clock::to_time_t(n1);
cout << std::put_time(std::localtime(&t), "%Y-%m-%d %H.%M.%S") << "." << msecs << endl;

将绝对时间转换为标准时间字符串的方法：

#include <string>
#include <chrono>
#include <cinttypes>
#include <ctime>
#include <sstream>
#include <iomanip>

std::string millisecond_to_str(std::int64_t milliseconds)
{
    std::chrono::milliseconds ms(milliseconds);
    std::chrono::time_point<std::chrono::high_resolution_clock, std::chrono::milliseconds> t1(ms);
    std::time_t t = std::chrono::system_clock::to_time_t(t1);

    std::stringstream ss;
    auto const msecs = ms.count() % 1000;
    ss << std::put_time(std::localtime(&t), "%Y-%m-%d %H.%M.%S") << "." << msecs;
    return ss.str();
}