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C++ Korea 5th Seminar
C++20 Key Features Summary
???
Nexon Korea, Microsoft MVP
utilForever@gmail.com

??
??? (Chris Ohk)
? ?? ??? ?? ?????
? Microsoft Developer Technologies MVP
? ???? ?? C++ Korea ??
? IT ??? ?? ? ?? ??
? ?????? ?? ? ? ?? ?? ??? (2013)
? ??? Shader? Effect ?? (2014)
? 2D ?? ????? (2014)
? ??? ?? ?? (2017)
? ?? C++ ?? (2017)
? C++ ??? (2019)

?? ?? ?? ?? C++ Korea 5th Seminar

???? ??...
? C++20? ??? ?? ???? ?????.
? C++20? ??? ?? ? C++11/14/17 ??? ??? ??,
??? ?? ?? ?? ??? ?? ?????.
? ?? ?????? ??? ? ?? ?? ??? ?????.
(?? ?? ??? ??? ????? ???? ??)
? ?? ?? ??? ?? ?? Github? ?? ?????.
(https://github.com/utilForever/ModernCpp)

?? ?? C++ ?? ?? C++ Korea 5th Seminar

?? ?? C++ ?? ?? C++ Korea 5th Seminar
https://twitter.com/GorNishanov

??? ?? C++ Korea 5th Seminar
https://twitter.com/walletfox

C++20 ?? ??
? Concepts
? Contracts
? Ranges
? Module
? Coroutines

Concepts
? ?? ??? ?? ????
#include <algorithm>
#include <list>
int main()
{
std::list<int> l = { 2, 1, 3 };
std::sort(l.begin(), l.end());
}

Concepts
? ?? ??? ?? ????

Concepts
? ? ??? ??????
? template <class RandomIt>
void sort(RandomIt first, RandomIt last);
? ??? RandomIt? ValueSwappable? RandomAccessIterator? ?? ??
? ???? ???. ??? int?? ???(Iterator)? ???? ????.

Concepts
? C#??? ?? ??? ?? ??? ???? ?????? ?????.
? Icloneable
? Icomparable
? Iconvertible
? Iformattable
? Nullable
? ...
? C++?? ???? ??? ??? ???? ?????.
Concept? ???? ???? ???.

Concepts
? Concept? ??? ??? ?? ??? ??? ? ? ????.
? ?? ?? ?? concept ???? ?????.
??? ?? ??? bool??? ??, ??? ?? ?? ??? bool??? ???.
? requires ???? ?? ?? ??? ?????.
?? ??? constexpr bool ?? concept bool??? ???.
template <class T>
concept bool EqualityComparable()
{
return requires(T a, T b) {
{a == b}->Boolean; // Boolean is the concept defining
{a != b}->Boolean; // a type usable in boolean context
};
}

Concepts
? ???? ??? ?? ???? ????.
void f(const EqualityComparable&);
// OK, std::string satisfies EqualityComparable
f("abc"s);
// Error: not EqualityComparable
f(std::use_facet<std::ctype<char>>(std::locale{}));

Concepts
? Concept? ???? ?? ?? ? ??? ?????? ? ? ????.
In instantiation of 'void std::__sort(_RandomAccessIterator,
_RandomAccessIterator, _Compare) [with _RandomAccessIterator
= std::_List_iterator<int>;
_Compare = __gnu_cxx::__ops::_Iter_less_iter]':
error: no match for 'operator-' (operand types are
'std::_List_iterator<int>' and 'std::_List_iterator<int>')
std::__lg(__last - __first) * 2,
error: cannot call function 'void std::sort(_RAIter, _RAIter)
[with _RAIter = std::_List_iterator<int>]'
note: concept 'RandomAccessIterator()' was not satisfied

Concepts
? Concept ???? ??? ????.
? Basic : DefaultConstructible, MoveConstructible,
CopyConstructible, MoveAssignable, CopyAssignable,
Destructible
? Library-wide : EqualityComparable, LessThanComparable,
Swappable, ValueSwappable, NullablePointer, Hash, Allocator,
FunctionObject, Callable, Predicate
? Iterator : InputIterator, OutputIterator, ForwardIterator,
BidirectionalIterator, RandomAccessIterator,
ContiguousIterator

Contract
? ??? ?? ??? ?????. (maybe bugs be with you)

Contract
? ??? ??? ? ?? ??? ?? ???? ??
assert? static_assert? ?????.
void print_number(int* myInt)
{
assert(myInt != nullptr);
std::cout << *myInt << "n";
}
int main()
{
int a = 10;
int *b = nullptr, *c = nullptr;
b = &a;
print_number(b); print_number(c);
}

template<typename T>
class ArrayView
{
T& operator[](size_t i)[[expects:i < size()]];
ArrayView(const vector<T>& v)[[ensures:data() == v.data()]];
};
Contract
? Contract? ?? ?? ?? ??? ?? ??? ???
assert()?? ?? ??? ????? ??? ? ?? ?????.
? expects : ?? ??(Pre-condition)? ?? ???
? ensures : ?? ??(Post-condition)? ?? ???

struct A {
bool f() const; bool g() const;
virtual std::string bhar()[[expects:f() && g()]];
virtual int hash()[[ensures:g()]];
virtual void gash()[[expects:g()]];
virtual double fash(int i) const[[expects:i > 0]];
};
struct B : A {
std::string bhar() override[[expects:f()]]; // ERROR: weakening.
int hash() override[[ensures:f() && g()]]; // ERROR: strengthening.
void gash() override[[expects:g()]]; // OK: repeat from base.
double fash(int) override; // OK: inherited from base.
};
Contract
? ??? ? ??? ?? ?? ???? ??? ?????? ?
?? ???? ?? ??? ????? ???? ????.

Ranges
? Ranges? ??? ??? ??? ????????.
? Ranges? ??
? ??? (Convenience) : ???? ??
? ??? ?? (Lazy Evaluation) : ?? ?? ?? ??, ???? ?? ??
? ??? (Composability) : ???? ?? ??, ?? ??? ??
? Ranges? ?? ? ? ?? ?? ??? ?? ???, ?? ?????
???? ��Ranges for C++ Standard Library��? ?????.
(/seao/c-korea-2nd-seminar-ranges-for-the-cpp-standard-library)

Ranges
? [0 ~ 10)? ?????.
int arr[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
std::vector<int> v(10);
for (int i = 0; i < 10; ++i)
v[i] = i;
std::vector<int> v = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

Ranges
? [0 ~ 10)? ?????.
#include <numeric>
std::iota(v.begin(), v.end(), 0);
ranges::iota(v, 0);

Ranges
? [0 ~ 15)?? 5?? ??? ??? ? ??? ?? ??? ??
vector<int> v =
view::iota(0, 15) |
view::group_by(quotient) |
view::transform(sum);
for (auto e : v)
cout << e << " "; cout << endl;
auto quotient = [](int a, int b) {
return a / 5 == b / 5;
};
auto sum = [](auto rng) {
return ranges::accumulaate(rng, 0);
};

Ranges
? ??2
+ ??2
= ??2
? ???? ? ??? ? ??, ??, ?? 100? ??? ??
auto triples = view::ints(1) >>= [] (int z) { return
view::ints(1, z + 1) >>= [=](int x) { return
view::ints(x, z + 1) >>= [=](int y) { return
yield_if(x*x + y*y == z*z,
std::make_tuple(x, y, z));
}; }; };
auto triangles = triples | view::take(100);

Module
? C++??? #include? ?? ?? ??? ?????.
? ??? ?? ?? ?? ?? ?? ??? ????? ????.
(?? ?? ???? ?? ?? ??? ???? ???.)
? ?? ??? ??? ??? ??? ? ?? ??? ???? ????.
? C# : using System.Linq;
? Java : import java.util.Scanner;
? Python : from path import pi
? ?? C++?? Module? ???? ?? ? ? ?? ?????.

Module
? ???? Module?? ???? ? ???? 3?????.
? module : ??? ??? ??? ?????.
?) module M1 : ??? ??? ??? M1??.
? import : ??? ??? ??? ?????.
?) import M1 : ??? ??? ??? M1??.
? export : ??? ??? ?????? ?????.
?) export int f(int, int)
??? ??? ??? f?? ?? ??? int, ?? ??? (int, int)?.

Module
? ? cpp ???? ??? ??? ?? cpp ???? ???? ??
// m1.cpp
module M1;
export int f(int, int);
int g(int x) { return x * x; }
int f(int x, int y) { return g(x) + g(y); }
// m2.cpp
module M2;
export int g(int, int);
import std.core;
int f(int x, int y) { return x + y; }
int g(int x, int y)
{
return f(abs(x), abs(y));
}
// main.cpp
import M1;
import M2;
int main()
{
printf(��%dn��,
f(3, 4) + g(3, 4));
}

Coroutine
? ��C++ Coroutine ????: ???, ????, ??? ???�� ????
Coroutine? ?? ??? ??? ?????.
(?? ??? ?? ?? ??? ??? ???? ?? ??????.)

C++20 ?? ??
? Designated initializers
? operator<=>
? Relation operator/comparison
? Nested inline namespaces
? consteval fuctions
? Feature test macros

C++20 ?? ??
? std::span
? std::endian
? std::bit_cast
? Calender and timezone library

Designated initializers
? ??? ?? ???? ??? ???.
? ??? ?? ??? ??? ???.
? ?? x, z?? ?? ???? ??? ??? ?? ????
??? ??? ?? ? ????.
struct A { int x; int y; int z; };
A a1{3, 4};
A a2{3, 4, 5};
A a1;
a1.x = 3;
a1.z = 5;

Designated initializers
? C++20??? ? ? ?? ???? ??? ? ????.
? ?, ??? ??? ??? ?? ??? ???? ???.
? ????? ??? ? ????.
A a1{.x = 3, .z = 5};
A a1{.x = 3, .y = 4}; // OK
A a2{.y = 4, .x = 3}; // Error
union u { int a; const char* b; };
u f = { .b = "asdf" }; // OK, active member of the union is b
u g = { .a = 1,.b = "asdf" }; // Error, only one initializer may be provided

operator<=>
? strcmp? ??? ??? ?? 3??? ?? ???? ??????.
(Spaceship operator, 3-way comparison???? ???.)
? a? b?? ?? 1? ?????.
? a? b?? ??? -1? ?????.
? a? b? ��???(equal)�� ????(equivalent)�� 0? ?????.
(?? : ��??��? ��????��? ?? ?????.)
(a <=> b) < 0 // true if a < b
(a <=> b) < 0 // true if a > b
(a <=> b) == 0 // true if a is equal/equivalent to b

Relation operator/comparison
? ?? ? ????? ????.
??? 4m, ??? 1m? ???? ??? 1m, ??? 4m? ????

? ? ????? ??? ????. ??? ??? ????.
??? ? ???? ????? ??? ?? ??????
??? 4m, ??? 1m? ???? ??? 1m, ??? 4m? ????

? ? ? ? ????? ��????��? ???.
?? ?? weak ordering??? ???.
??? 4m, ??? 1m? ???? ??? 1m, ??? 4m? ????

? ?? ? ????? ????.
??? 4m, ??? 4m? ???? ??? 4m, ??? 4m? ????

? ? ????? ????.
??/?? ??? ?? ??? ????.
??? 4m, ??? 4m? ???? ??? 4m, ??? 4m? ????

? ?? ? ????? ��??��?? ???.
?? ?? strong ordering??? ???.
??? 4m, ??? 4m? ???? ??? 4m, ??? 4m? ????

? operator<=>? ?? ??? ?? ?? ?? ? ??? ???.
https://medium.com/@barryrevzin/implementing-the-spaceship-operator-for-optional-4de89fc6d5ec

? ?? ??? ?? ???? ?? ??? ????.
https://medium.com/@barryrevzin/implementing-the-spaceship-operator-for-optional-4de89fc6d5ec

? operator<=> + relation operator? ??? ??
class Rectangle {
public:
Rectangle(int x, int y) : m_x(x), m_y(y) { }
int GetArea() const { return m_x * m_y; }
std::weak_ordering operator<=>(const Rectangle& r) const {
//if (auto cmp = m_x <=> r.m_x; cmp != 0) return cmp;
//if (auto cmp = m_y <=> r.m_y; cmp != 0) return cmp;
return GetArea() <=> r.GetArea();
}
private:
int m_x, m_y;
};

consteval functions
? constexpr ??
? ??? ??? ?? ???? ??? ??? ?????.
? ??? ??? ??? ????? ???? ?????.
? consteval ??
? ??? ??? ?? ???? ??? ??? ?????.
? ??? ??? ??? ????? ??? ??? ?????.

consteval functions
? ??? ??? ??? ??? ??? ?? ???? ?????.
? consteval? ??? ??
consteval int sqr(int n) { return n * n; }
constexpr int r = sqr(100); // OK
consteval int sqrsqr(int n)
{
return sqr(sqr(n)); // OK
}
constexpr int dblsqr(int n)
{
return 2 * sqr(n); // Error
}

Nested inline namespaces
? ?? ?????? + ???
namespace A::B::inline C
{
...
}
namespace A::B
{
inline namespace C
{
...
}
}
=

Nested inline namespaces
? ?? ?????? + ???
namespace A::inline B::C
{
...
}
namespace A
{
inline namespace B
{
namespace C
{
...
}
}
}
=

Feature test macros
? ???? ??? C++ ????? ????.
? GCC
? Clang
? MSVC
? EDG eccp
? Intel C++
? Portland Group (PGI)
? ...

Feature test macros
? ?????? ??? ?????.

Feature test macros
? ???? ?? ? ??? ?? ?? ?? ??? ????.
(apple-clang ?????... ??? ?? ????. ��brew install clang��)
#if defined(ROSETTASTONE_WINDOWS)
#include <filesystem>
#elif defined(ROSETTASTONE_LINUX)
#include <experimental/filesystem>
#elif defined(ROSETTASTONE_MACOSX)
#include <sys/stat.h>
#endif
#if defined(ROSETTASTONE_WINDOWS)
namespace filesystem = std::filesystem;
#elif defined(ROSETTASTONE_LINUX)
namespace filesystem = std::experimental::filesystem;
#endif

Feature test macros
? ???? ?? ??? ???? ?? ??? ??? ??? ???.
#ifdef __has_include
# if __has_include(<optional>)
# include <optional>
# elif __has_include(<experimental/optional>)
# include <experimental/optional>
# elif __has_include(<boost/optional.hpp>)
# include <boost/optional.hpp>
# else
# error "Missing <optional>"
# endif
#endif

Feature test macros
? C++20?? ?? ??? ????? ??? ??????,
???? ??? C++? ?? ??? ???? ???? ? ????.
#if __cpp_constexpr >= 201304
# define CONSTEXPR constexpr
#else
# define CONSTEXPR inline
#endif
CONSTEXPR int bar(unsigned i)
{
...
}

Feature test macros
? C++20?? ?? ??? ????? ??? ??????,
???? ??? C++? ?? ??? ???? ???? ? ????.
CONSTEXPR int bar(unsigned i)
{
#if __cpp_binary_literals
unsigned mask1 = 0b11000000;
unsigned mask2 = 0b00000111;
#else
unsigned mask1 = 0xC0;
unsigned mask2 = 0x07;
#endif
if ( i & mask1 ) return 1;
if ( i & mask2 ) return 2;
return 0;
}

Feature test macros
? ??? C++20? ?? ??? __has_cpp_attribute? ????
??(attribute) ?? ??? ??? ? ????.
#ifdef __has_cpp_attribute
# if __has_cpp_attribute(deprecated)
# define DEPRECATED(msg) [[deprecated(msg)]]
# endif
#endif
#ifndef DEPRECATED
# define DEPRECATED(msg)
#endif
DEPRECATED("foo() has been deprecated") void foo();

std::span
? std::array_view? ??? ??????.
? std::string <-> std::string_view? ?? ?????.
? ?? ??? ?? ?? ????? ?? ?????.
? std::vector
? std::string
? T[]
? T* + size

std::span
? std::span? ??? ??
https://cor3ntin.github.io/posts/span/
std::vector<std::string> greeting = { "hello", "world" };
std::span<std::string> span(greeting);
for (auto&& s : span)
{
s[0] = std::toupper(s[0]);
}
for (const auto& word : greeting)
{
std::cout << word << ' ';
}

std::endian
? ????? ?? ???? ???? ???? ??? ?? ? ????.
?? ?? ?? ��0x12345678��? ???? ??? ?,
? ? ??? (Big Endian) : 12 34 56 78
? ?? ??? (Little Endian) : 78 56 34 12

std::endian
? C++?? ??? ??? ??? ??? ?? ???? ???.
//! Byte swap unsigned int
uint32_t swap_uint32(uint32_t val)
{
val = ((val << 8) & 0xFF00FF00) | ((val >> 8) & 0xFF00FF);
return (val << 16) | (val >> 16);
}
//! Byte swap int
int32_t swap_int32(int32_t val)
{
val = ((val << 8) & 0xFF00FF00) | ((val >> 8) & 0xFF00FF);
return (val << 16) | ((val >> 16) & 0xFFFF);
}
https://stackoverflow.com/questions/2182002/convert-big-endian-to-little-endian-in-c-without-using-provided-func

std::endian
? ??? ?????? ?? ???? ????? ??? ???.
https://stackoverflow.com/questions/2182002/convert-big-endian-to-little-endian-in-c-without-using-provided-func
#if defined(__BYTE_ORDER) && __BYTE_ORDER == __BIG_ENDIAN ||
defined(__BIG_ENDIAN__) || defined(__ARMEB__) || defined(__THUMBEB__) ||
defined(__AARCH64EB__) || defined(_MIBSEB) || defined(__MIBSEB) ||
defined(__MIBSEB__)
// It's a big-endian target architecture
#elif defined(__BYTE_ORDER) && __BYTE_ORDER == __LITTLE_ENDIAN ||
defined(__LITTLE_ENDIAN__) || defined(__ARMEL__) ||
defined(__THUMBEL__) || defined(__AARCH64EL__) || defined(_MIPSEL) ||
defined(__MIPSEL) || defined(__MIPSEL__)
// It's a little-endian target architecture
#else
#error "I don't know what architecture this is!"
#endif

std::endian
? std::endian? ???? ??????.
template <typename T, std::endian endianess = std::endian::native>
std::string HEX__1(const T & value, size_t value_size = sizeof(T)) {
using namespace std;
uint8_t* buffer = (uint8_t*)(&value);
char converted[value_size * 2 + 1];
if (endianess == std::endian::big)
for (size_t i = 0; i < value_size; ++i) {
sprintf(&converted[i * 2], "%02X", buffer[i]);
}
else
for (size_t i = 0; i < value_size; ++i) {
sprintf(&converted[i * 2], "%02X", buffer[value_size - 1 - i]);
}
return converted;
}
https://gist.github.com/ivanstepanovftw/9159efe123140c45d9df02e3d0454152

std::bit_cast
? ?? ?? ??? ??????.
https://en.cppreference.com/w/cpp/numeric/bit_cast
constexpr double f64v = 19880124.0;
constexpr auto u64v = std::bit_cast<std::uint64_t>(f64v);
constexpr std::uint64_t u64v2 = 0x3fe9000000000000ull;
constexpr auto f64v2 = std::bit_cast<double>(u64v2);
int main() {
std::cout
<< std::fixed << f64v << "f64.to_bits() == 0x"
<< std::hex << u64v << "u64n";
std::cout
<< "f64::from_bits(0x" << std::hex << u64v2 << "u64) == "
<< std::fixed << f64v2 << "f64n";
}

Calender and timezone library
? C++11 ??? <chrono> ?????? ???????.
??? ??? ??? ? ???? ?? ???? ?????.
? C++20? <chrono> ??????? ?? ??? ?????.
? ?? ?? ??? ??
? ??? ?? ??? ??
? ?? ??? ?? ??? ??

? ?? ?? ???
? UTC(?? ???) ?? : utc_clock
? system_time <-> utc_time? ?? from_sys/to_sys ?? ??
? TAI(?? ???) ?? : tai_clock
? utc_time <-> tai_time? ?? from_utc/to_utc ?? ??
? GPS ?? : gps_clock
? utc_time <-> utc_time? ?? from_utc/to_utc ?? ??
? ?? ?? ?? : clock_cast

? ?? ?? ??? ??
using sys_days = std::chrono::time_point<
std::chrono::system_clock, std::chrono::days>;
sys_days st(1997_y / dec / 12);
auto ut = utc_clock::from_sys(st);
auto tt = tai_clock::from_utc(ut);
auto gt = gps_clock::from_utc(ut);
assert(clock_cast<sys_clock>(st) == st);
assert(clock_cast<utc_clock>(ut) == ut);
assert(clock_cast<tai_clock>(tt) == tt);
assert(clock_cast<gps_clock>(gt) == gt);

? ??? ?? ???
? ?/?/? ?? : year_month_day
? ?? ? ??? ??? ? ???? (?/?/?, ?/?/?, ?/?/?)
? ??? ?? ?? ??
? year_month_day_last : ?? ?/?? ??? ?? ???? ???
? month_day_last : ?? ?? ??? ?? ???? ???
? weekday_last : ?? ?? ??? ??? ???? ???
? weekday_indexed : ?? ?? n?? ??? ???? ???

? ??? ?? ??? ??
auto d1 = 2018_y / mar / 27;
auto d2 = 27_d / mar / 2018;
auto d3 = mar / 27 / 2018;
year_month_day today = floor<days>(system_clock::now());
assert(d1 == d2);
assert(d2 == d3);
assert(d3 == today);
https://mariusbancila.ro/blog/2018/03/27/cpp20-calendars-and-time-zones/

? ??? ?? ??? ??
auto today = year_month_day{
floor<days>(system_clock::now()) };
auto ymdl = year_month_day_last(today.year(),
month_day_last{ month{ 2 } });
auto last_day_feb = year_month_day{ ymdl };
assert(last_day_feb == 2018_y / feb / 28); // for 2018

inline int number_of_days(sys_days const& first,
sys_days const& last)
{
return (last - first).count();
}
auto days = number_of_days(2018_y / apr / 1, 2018_y / dec / 25);
assert(days == 268);
? ??? ?? ??? ??

? ?? ??? ?? ???
? ?? ??
? locate_zone : ??? ??? ?? ???? ?? ??
? current_zone : ?? ?? ???? ?? ???? ?? ??
? ?? ??? ? ??
? sys_info, local_info
? zoned_traits, zoned_time
? choose, leap, link
? nonexistent_local_time
? ambiguous_local_time

auto time = floor<std::chrono::milliseconds>(system_clock::now());
std::cout << std::left << std::setw(25) << std::setfill(' ')
<< "Time"
<< time << std::endl;
auto localtime =
zoned_time<std::chrono::milliseconds>(date::current_zone(), time);
<< "Local"
<< localtime << std::endl;
? ?? ??? ?? ??? ??

auto zone_names = {
"Asia/Tokyo",
"Asia/Hong_Kong",
"Europe/Bucharest",
"Europe/Berlin",
"Europe/London",
"America/New_York",
"Pacific/Honolulu",
};
for(auto const& name : zone_names)
<< name
<< zoned_time<std::chrono::milliseconds>(name, localtime)
<< std::endl;
? ?? ??? ?? ??? ??

??? ?? C++20? ? ??
? Synchronized output
? char8_t
? [[likely]], [[unlikely]] attribute
? [[no_unique_address]] attribute
? constexpr
? virtual functions
? dynamic_cast
? typeid
? try-catch

??? ?? C++20? ? ??
? std::remove_cvref
? std::atomic_ref
? std::atomic_shared_ptr
? std::atomic_weak_ptr
? std::destroying_delete

C++20? ??? ?? ?? ??
? Expansion statements
? constexpr std::vector
? std::format
? std::source_location
? std::flatmap, std::flatset
? std::byteswap
? using enum

??
? C++20 ??? ??? ??? ??? ?? ?? ????.
? C++20? ???? C++11 ?? ? ??? ??? ?????.
? ??? ?? ??? ??? ?? / ?? ???? ????.
? ? ??? ?????? ??? ? ??? ? ??? ????.
? ?? C++? ??? ??? ???????,
?? ???? ???? ??? ?? ??? ?????.

?????
http://github.com/utilForever
?? ?????!

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