如何使用 fmt 库以小数秒打印当前时间

Question

This code

#include <chrono>
#include <fmt/format.h>
#include <fmt/chrono.h>
...
auto now = std::chrono::system_clock::now();
fmt::print("The time is: {:%Y-%m-%d %H:%M:%S}\n", now);

Prints this:

The time is: 2023-01-02 15:51:23

How do I get it to print sub-second precision, for example, milliseconds? Something like:

The time is: 2023-01-02 15:51:23.753

Answer 1

I think you have to do this in two steps -- YMD and HM followed by the high-res seconds. Here is a worked example, using fmt version 9.0 on Ubuntu:

#include <chrono>
#include <fmt/format.h>
#include <fmt/chrono.h>

int main() {
    auto now = std::chrono::system_clock::now();
    auto sse = now.time_since_epoch();
    fmt::print("{:%FT%H:%M:}{:%S}\n", now, sse);
    exit(0);
}

for which I get

$ g++ -o answer answer.cpp -lfmt; ./answer 
2023-01-02T16:26:17.009359309
$ 

---

(Initial, earlier, slightly off attempt to reason based on spdlog follows. But spdlog wraps fmt and overlays its format.)

It is on the format page , you want something like %Y-%m-%d %H:%M:%S.%e . Note that %e , %f and %F give you milli, micro and nanoseconds. Edit: That was the spdlog format though.

Here is a quick example, for simplicity from my R package RcppSpdlog accessing fmt via spdlog :

> library(RcppSpdlog)
> RcppSpdlog::log_set_pattern("[%Y-%m-%d %H:%M:%S.%f] [%L] %v")
> RcppSpdlog::log_warn("Hello")
[2023-01-02 15:00:49.746031] [W] Hello
> RcppSpdlog::log_set_pattern("[%Y-%m-%d %H:%M:%S.%e] [%L] %v")
> RcppSpdlog::log_warn("World")
[2023-01-02 15:01:02.137] [W] World
> 

Note the display of micro- and milliseconds as stated.

Edit: There is something else going on with your example. Here is a simplified answer, printing only seconds and it comes by default fractionally down to nanoseconds (for me on Linux):

$ cat answer.cpp
#include <chrono>
#include <fmt/format.h>
#include <fmt/chrono.h>

int main() {
    fmt::print("{:%S}\n", std::chrono::system_clock::now().time_since_epoch());
    exit(0);
}
$ g++ -o answer answer.cpp -lfmt
$ ./answer 
02.461241690
$