Chrono: Input – MC++ BLOG

You can also apply the format specifier for formatted input.

This post is the eleventh in my detailed journey through the chrono extension in C++20.

The chrono library supports formatted input in two ways. You can use the function std::chrono::from_stream or std::chrono::parse. Both functions require an input stream and parse the input into a time point according to the format specification. All format specifier except %q for unit suffixed according to the literals for time durations can be used.

`std::chrono::from_stream`

std::chrono::from_stream has overloads for the various clocks.

Clocks

std::chrono::system_time
std::chrono::utc_time
std::chrono::tai_time
std::chrono::gps_time
std::chrono::file_time
std::chrono::local_time

Calendar dates

std::chrono::year_month_day
std::chrono::year_month
std::chrono::month_day
std::chrono::weekday
std::chrono::year
std::chrono::month
std::chrono::day

The various overloads require in the elementary form an input stream is, a format string fmt, and a time point or a calendar object chro: std::chrono::from_stream(is, fmt, chro). The chrono object from the input stream is then parsed according to the format string.

You can also provide an abbreviation abb for a time zone and an offset off to the UTC time: std::chrono::from_stream(is, fmt, chro, abb, off). The offset has the type std::chrono::minutes.

The program inputChrono.cpp uses formatted input to read a time point and a calendar date from an input stream.

 // inputChrono.cpp

#include <chrono>
#include <iostream>
#include <string>
#include <sstream>

int main() {

    std::cout << '\n';

    std::chrono::sys_seconds timePoint;
    std::istringstream iStream1{"2021-08-11 21:49:35"};              //(1)
    std::chrono::from_stream(iStream1, "%F %T", timePoint);          //(2)
    if (iStream1) std::cout << "timePoint: "  << timePoint << '\n';
    else std::cerr << "timepoint: Reading failed\n";

    std::chrono::year_month_day date1;
    std::istringstream iStream2{"11/08/21"};                         //(3)
    std::chrono::from_stream(iStream2, "%x", date1);                 //(4)
    if (iStream2) std::cout << "date1: "  << date1 << '\n';
    else std::cerr << "date1: Reading failed\n";

    std::chrono::year_month_day date2;
    std::istringstream iStream3{"11/15/21"};
    std::chrono::from_stream(iStream3, "%x", date2);                  //(5)
    if (iStream3) std::cout << "date2: "  << date2 << '\n';
    else std::cerr << "date2: Reading failed\n";

    std::cout << '\n';

}

On lines 1 and 2, the data on the input stream (iStream1) matches the format string (“%F %T"). The same holds for input stream iStream2 (line 3) and the corresponding format string “%x" (line 4). On the contrary, there is no 15th month, and the parse step in line 5 fails. Consequentially, the failbit of the iStream3 is set. Using the iStream3 in a boolean expression evaluates to false.

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`std::chrono::parse`

Accordingly to std::chrono::from_stream, you can use the function std::chrono::parse for parsing input. The following code snippet shows their equivalence.

std::chrono::from_stream(is, fmt, chro)
is >> std::chrono::parse(fmt, chro)

Instead of std::chrono::from_stream, std::chrono::parse is directly invoked on the input stream is. std::chrono::parse also needs a format string fmt and a chrono object chro.

Consequently, I can directly rewrite the previous program inputChrono.cpp using std::chrono::from_stream into the program inputChronoParse.cpp using std::chrono::parse.

// inputChronoParse.cpp

#include <chrono>
#include <iostream>
#include <string>
#include <sstream>

int main() {

    std::cout << '\n';

    std::chrono::sys_seconds timePoint;
    std::istringstream iStream1{"2021-08-11 21:49:35"};
    iStream1 >> std::chrono::parse("%F %T", timePoint);
    if (iStream1) std::cout << "timePoint: "  << timePoint << '\n';
    else std::cerr << "timepoint: Reading failed\n";

    std::chrono::year_month_day date1;
    std::istringstream iStream2{"11/08/21"};
    iStream2 >> std::chrono::parse("%x", date1);
    if (iStream2) std::cout << "date1: "  << date1 << '\n';
    else std::cerr << "date1: Reading failed\n";

    std::chrono::year_month_day date2;
    std::istringstream iStream3{"11/15/21"};
    iStream3 >> std::chrono::parse("%x", date2);
    if (iStream3) std::cout << "date2: "  << date2 << '\n';
    else std::cerr << "date2: Reading failed\n";

    std::cout << '\n';

}

What’s Next?

I’m done with my deep dive into the chrono library. In my next post, I will discuss concurrency.

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std::chrono::from_stream