In early March, the meeting of the WG21 international working group on C ++ standardization, in which Yandex employees participated, ended in the US city of Kona.

Sessions, as usual, took a whole day plus an additional subgroup on working with numbers.

The main time was devoted to polishing a draft of C ++ 17, but some small and interesting innovations still managed to slip into C ++ 17.

20 years waited ...

std::ofstream(L"      .txt") << u8"!";

std::optional

std::optional :

template <class T>
bool operator < (const optional<T>&, const T&);
// ...

:

optional<const int> x = 42;
int y = 666;
x == y; //  
...

optional<string> s = "Hello";
s == "Hello"; //  

, :

template <class T, class U>
bool operator < (const optional<T>&, const U&);
// ...

, 's == «Hello»' .

Deduction guides

deduction guides, :

// !     .
std::tuple      a = {42, "Hello", 3.14};
std::array      b = {1, 2, 3, 4, 5, 6, 7, 8};
std::pair       c = {42, "Hello"};
std::function   d = [](int i) { std::cout << i; };
std::set        e = {1, 2, 3, 4, 5, 6, 7, 8};

// P.S.:     '='     :)

std::byte

- , - « »? C ++17 std::byte

unsigned char raw_buffer[buffer_size];

std::byte raw_buffer[buffer_size];

.

,

. :

• std::filesystem::path , , Windows POSIX (, zOS).
• std::filesystem::directory_entry , 2.
• std::hash noexcept, ( , unordered_* ).
• std::atomic_* ,

  long example(atomic<long> * a) { return atomic_fetch_add(a, 42); }

  .
• - inline, ODR. , , .

, ++ . C++17, C++20. .

Numbers TS

, , , , . Numbers TS — . :

• wide float — , ;
• wide integers ( stdcpp.ru) — , ;
• unbounded floats — , ;
• unbounded integers — , ;
• rational — ;
• safe numbers — , UB , UB ;
• decimal;
• small float.

, .

operator<=>()

operator<=>(). , C++20 :

struct foo { /*     */ };

auto operator<=>(const foo&, const foo&) = default; //      foo c foo
auto operator<=>(const foo&, const char*) = default; //      foo c const char*      const char*  foo

, //«-», , constexpr constexpr , constexpr « », , . , 21 , ++ ++17 . — WG21 C++.

C++17 , !

C++20, C++17/C++14/C++11, C++, stdcpp.ru. !

! ++ , , .