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 to 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 with C++20 new 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 for the calendar and time-zone proposal, also created a playground for it on Wandbox.

#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 stands for 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 be 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 initialise a std::span<T>, you have to provide the size for the constructor.

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) don'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 potentially 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 can not be used in a constexpr evaluation, because there were 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 have to call the destructor explicitly. Finally, the last line frees the memory.

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


cppreference.com/ has a concise description of 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 with a future post into 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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Tags: C++20


0 #1 Peter Lauro 2020-06-06 14:02
Concerning "constexpr" function definition, it would be worth to mention a new std trait


which helps the function to detect the context in which the function is invoked.
0 #2 Peter Lauro 2020-06-06 16:24
concerning the std::span example:
in the case of template "copy" function (with the current signature), the compilers are not capable to find the function for int[3] types. the implicit conversion from int[3] to spawn is not applied.

the call of copy method needs to employ the explicit conversion from array to span.

copy(std::span(arr1), std::span(arr2));
0 #3 Rainer Grimm 2020-06-11 08:18
I will write a post about std::is_contant_evaluated in the future. This was only an overview.

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