C++11 为 std::chrono::time_point 添加流输出运算符

Question

我希望能够做到以下几点:

std::cerr << std::chrono::system_clock::now() << std::endl;

并得到以下内容:

Wed May  1 11:11:12 2013

所以我写了以下内容:

template<typename Clock, typename Duration>
std::ostream &operator<<(std::ostream &stream,
  const std::chrono::time_point<Clock, Duration> &time_point) {
  const time_t time = Clock::to_time_t(time_point);
#if __GNUC__ > 4 || \
    ((__GNUC__ == 4) && __GNUC_MINOR__ > 8 && __GNUC_REVISION__ > 1)
  // Maybe the put_time will be implemented later?
  struct tm tm;
  localtime_r(&time, &tm);
  return stream << std::put_time(tm, "%c");
#else
  char buffer[26];
  ctime_r(&time, buffer);
  buffer[24] = '\0';  // Removes the newline that is added
  return stream << buffer;
#endif
}

这行得通，但是从不同的命名空间调用它时我总是遇到问题。这应该只在全局命名空间中是否正确？

Answer 1

当您想确保调用正确的函数时，您应该在将调用它的代码范围内放置一个 using 声明。

例如:

namespace pretty_time {
  /* your operator<< lives here */
}


void do_stuff() {
  using namespace pretty_time;   // One way to go is this line
  using pretty_time::operator<<; // alternative that is more specific (just use one of these two lines, but not both)
  std::cout << std::chrono::system_clock::now();
}