C++20: The Library

My last post “C++20: The Core Language” presented the new features of the C++20 core language. Today, I continue my journey with an overview of the C++20 library.

TimelineCpp20LibrariesThe image shows you my plan for today.


Calendar and Time-Zone

The chrono library from C++11/14 was extended with a calendar and time-zone facility.  If you don’t know the Chrono library, read my posts to time.


Calendar: consists of types, which represent a year, a month, a day of a weekday, and an n-th weekday of a month. These elementary types can be combined into complex types such for example year_month, year_month_day, year_month_day_last, years_month_weekday, and year_month_weekday_last. The operator “/” is overloaded for the convenient specification of time points. Additionally, we will get new with C++20 literals: d for a day and y for a year.


Time points can be displayed in various specific time zones.

Due to the extended chrono library, the following use cases are easy to implement:

  • representing dates in various forms
auto d1 = 2019y/oct/28;
auto d2 = 28d/oct/2019;
auto d3 = oct/28/2019; 


  • get the last day of a month
  • get the number of days between two dates
  • printing the current time in various time-zones

If you want to play with these features, use Howard Hinnards implementation on GitHub. Howard Hinnard, the author of the calendar and time-zone proposal, also created a playground for it on Wandbox.


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    #include "date.h"
    #include <iostream>
        using namespace date;
        using namespace std::chrono;
        auto now = system_clock::now();
        std::cout << "The current time is " << now << " UTC\n";
        auto current_year = year_month_day{floor<days>(now)}.year();
        std::cout << "The current year is " << current_year << '\n';
        auto h = floor<hours>(now) - sys_days{jan/1/current_year};
        std::cout << "It has been " << h << " since New Years!\n";


    Of course, C++20 uses the std::chrono namespace instead of the date namespace. Here is the output of the program:




    A std::span is an object that can refer to a contiguous sequence of objects. A std::span, sometimes also called a view, is never an owner. This contiguous memory can be an array, a pointer with a size, or a std::vector. A typical implementation needs a pointer to its first element and a size. The main reason for having a std::span<T> is that a plain array will decay to a pointer if passed to a function; therefore, the size is lost. std::span<T> automatically deduces the size of the plain array or the std::vector. If you use a pointer to initialize a std::span<T>, you must provide the constructor’s size.

    template <typename T>
    void copy_n(const T* p, T* q, int n){}
    template <typename T>
    void copy(std::span<const T> src, std::span<T> des){}
    int main(){
      int arr1[] = {1, 2, 3};
      int arr2[] = {3, 4, 5};
      copy_n(arr1, arr2, 3);         // (1)
      copy(arr1, arr2);              // (2)


    In contrast to the function copy_n (1), copy (2) doesn’t need the number of elements. Hence, a common cause of errors is gone with std::span<T>.

    constexpr Containers

    C++ becomes more and more constexpr. For example, many algorithms of the Standard Template Library get with C++20 a constexpr overload.  constexpr for a function or function template means that it could be performed at compile time. The question is now, which containers can be used at compile time? With C++20, the answer is std::string and std::vector.

    Before C++20, both could not be used in a constexpr evaluation because of three limiting aspects.

    1. Destructors couldn’t be constexpr.
    2. Dynamic memory allocation/deallocation wasn’t available.
    3. In-place construction using placement-new wasn’t available.

    These limiting aspects are now solved.

    Point 3 talks about placement-new, which is quite unknown. Placement-new is often used to instantiate an object in a pre-reserved memory area. Besides, you can overload placement-new globally or for your data types.

    char* memory = new char[sizeof(Account)];        // allocate memory
    Account* account = new(memory) Account;          // construct in-place
    account->~Account();                             // destruct
    delete [] memory;                                // free memory


    Here are the steps to use placement-new. The first line allocates memory for an Account, which is used in the second line to construct an account in place. Admittedly, the expression account->~Account() looks strange. This expression is one of these rare cases in which you must explicitly call the destructor. Finally, the last line frees the memory.

    I will not go further into the details of constexpr Containers. If you are curious, read proposal 784R1.


    cppreference.com/concisely describes the new formatting library: “The text formatting library offers a safe and extensible alternative to the printf family of functions. It is intended to complement the existing C++ I/O streams library and reuse some of its infrastructure such as overloaded insertion operators for user-defined types.”. This concise description includes a straightforward example:

    std::string message = std::format("The answer is {}.", 42);


    Maybe, this reminds you of Pythons format string. You are right. There is already an implementation of the std::format on GitHub available: fmt. Here are a few examples from the mentioned implementation. Instead of std, it uses the namespace fmt.

    • Format and use positional arguments
    std::string s = fmt::format("I'd rather be {1} than {0}.", "right", "happy");
    // s == "I'd rather be happy than right."


    •  Convert an integer to a string in a safe way
    fmt::memory_buffer buf;
    format_to(buf, "{}", 42);    // replaces itoa(42, buffer, 10)
    format_to(buf, "{:x}", 42);  // replaces itoa(42, buffer, 16)
    // access the string with to_string(buf) or buf.data()


    • Format user-defined types
    struct date {
      int year, month, day;
    template <>
    struct fmt::formatter<date> {
      template <typename ParseContext>
      constexpr auto parse(ParseContext &ctx) { return ctx.begin(); }
      template <typename FormatContext>
      auto format(const date &d, FormatContext &ctx) {
        return format_to(ctx.out(), "{}-{}-{}", d.year, d.month, d.day);
    std::string s = fmt::format("The date is {}", date{2012, 12, 9});
    // s == "The date is 2012-12-9"


    What’s next?


    As promised, I will dive deeper into a future post in the library. But first, I have to finish my high-level overview of C++20. My next post is about the concurrency features.



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