More Details to Formatting User-Defined Types in C++20

Implementing a formatter for a user-defined type having more than one value in C++20 is challenging.

This post is the 5th post in my miniseries about formatting in C++20. Read the previous ones here:

A Formatter for More Values

Point is a class with three members.

// formatPoint.cpp

#include <format>
#include <iostream>
#include <string>

struct Point {
    int x{2017};
    int y{2020};
    int z{2023};
};

template <>
struct std::formatter<Point> : std::formatter<std::string> {
    auto format(Point point, format_context& context) const {
        return formatter<string>::format(
               std::format("({}, {}, {})", point.x, point.y, point.y), context);
  }
};

int main() {

    std::cout << '\n';

    Point point;

    std::cout << std::format("{:*<25}", point) << '\n';        // (1)
    std::cout << std::format("{:*^25}", point) << '\n';        // (2)
    std::cout << std::format("{:*>25}", point) << '\n';        // (3)

    std::cout << '\n';

    std::cout << std::format("{} {} {}", point.x, point.y, point.z) << '\n';  // (4)
    std::cout << std::format("{0:*<10} {0:*^10} {0:*>10}", point.x) << '\n';  // (5)

    std::cout << '\n';

}

In this case, I derive from the standard formatter std::formatter<std::string>. A std::string_view is also possible. std::formatter<Point> creates the formatted output by calling format on std::formatter. This function call already gets a formatted string as a value. Consequentially, all format specifiers of std::string are applicable (lines 1 – 3). On the contrary, you can also format each value of Point. This is precisely happening in lines (4) and (5).

Internationalization

The formatting functions std::format*, and std::vformat* have overloads accepting a locale. These overloads allow you to localize your format string.

The following code snippet shows the corresponding overload of std::format:

template< class... Args >
std::string format( const std::locale& loc,
                    std::format_string<Args...> fmt, Args&&... args );

To use a given locale, specify L before the type specifier in the format string. Now, you apply the locale in each call of std::format or set it globally with std::locale::global.

In the following example, I explicitly apply the German locale to each std::format call.

// internationalization.cpp

#include <chrono>
#include <exception>
#include <iostream>
#include <thread>

std::locale createLocale(const std::string& localString) {                         // (1)
  try {
    return std::locale{localString};       
  }
  catch (const std::exception& e) {
    return std::locale{""};
  }
}

int main() {

    std::cout << '\n';

    using namespace std::literals;

    std::locale loc = createLocale("de_DE");

    std::cout << "Default locale: " << std::format("{:}", 2023) << '\n';
    std::cout << "German locale:  " << std::format(loc, "{:L}", 2023) << '\n';    // (2)

    std::cout << '\n';

    std::cout << "Default locale: " << std::format("{:}", 2023.05) << '\n';
    std::cout << "German locale:  " << std::format(loc, "{:L}", 2023.05) << '\n'; // (3)

    std::cout << '\n';

    auto start = std::chrono::steady_clock::now();
    std::this_thread::sleep_for(33ms);
    auto end = std::chrono::steady_clock::now();

    const auto duration = end - start;

    std::cout << "Default locale: " << std::format("{:}", duration) << '\n';
    std::cout << "German locale:  " << std::format(loc, "{:L}", duration) << '\n'; // (4)

    std::cout << '\n';

    const auto now = std::chrono::system_clock::now();
    std::cout << "Default locale: " << std::format("{}\n", now);
    std::cout << "German locale:  " << std::format(loc, "{:L}\n", now);            // (5)

    std::cout << '\n';

}

The function createLocale (line 1) creates the German locale. If this fails, it returns the default locale that uses American formatting. I use the German locale in lines (2), (3), (4), and (5). To see the difference, I also applied the std::format calls immediately afterward. Consequentially, the local-dependent thousand separator is used for the integral value (line 2), and the locale-dependent decimal point and thousand separator for the floating-point value (line 3). Accordingly, the time duration (line 4) and the time point (line 5) use the given German locale.

The following screenshot shows the program’s output.

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What’s Next?

std::formatter and its specializations also define the format specification for the chrono types. Before I write about them, I will dive deeper into the chrono extension of C++20.

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