C ++ 20 dalam perjalanan! Bertemu di Rapperswil Yona

Pada awal Juni, pertemuan kelompok kerja internasional WG21 tentang standardisasi C ++ berakhir di kota Rapperswil-Yona.

Inilah yang menanti Anda di bawah pemotongan:

• Kontrak dan teman
• Konsep (tidak ada teman)
• __has_cpp_attribute (tidak mungkin)
• bit_cast <my_stuff> (some_array)
• berisi , shift_left , shift_right , ispow2 , ceil2 ... dan algoritma lama dengan saus baru
• atomic_ref
• Apa yang baru dapat ditulis dalam templat dan bagaimana berguna
• constexpr virtual foo ()
• Paralelisme 2, Refleksi, dan Pelaksana TS

Juga akan ada bonus: minisection untuk para ahli:

• destructor virtual yang dideklarasikan oleh pengguna tidak memengaruhi tipe remeh
• Di mana saya bisa memberi tanda seru dan bagaimana itu bisa bermanfaat
• constexpr std :: regex mail_regex (R "((?: (?: [^ <> () \ [\].,;: \ s @ \"] + (?: \. [^ <> () \ [ \].,;: \ s @ \ "] +) *) | \". + \ ") @ (?: (?: [^ <> () \ [\].,;: \ s @ \" ] + \.) + [^ <> () \ [\].,;: \ s @ \ "] {2,}))")

Kontrak

Di C ++ 20, mereka menerima kontrak. Jadi, akan segera mungkin untuk melupakan tentang menggunakan makro untuk pernyataan, dapatkan dokumentasi terbaik di luar kotak, dan bahkan melihat peningkatan kinerja. Dalam praktiknya, kontrak terlihat seperti ini:

std::string get_name_by_login(std::string_view login)
    [[expects: !login.empty() ]]
    [[ensures ret_value: !ret_value.empty() ]]
;

, , . , :

• ( ) .
• ( ) . , , .
• (, doxygen) / .
• , (, ).

.

21 Fails, . , :

void(const std::contract_violation & e) noexcept {
    std::cerr << "Contract violated in function " << e.function_name() << '\n'
        << std::stacktrace();
}

:

Contract violated in function std::array<T, N>::operator[](size_type) [with T = int; long unsigned int N = 5ul; ]': 
 0# std::array<int, 5ul>::operator[](unsigned long) at /usr/include/c++/array:124
 1# bar(int) at ../example/assert_handler.cpp:17
 2# foo(int) at ../example/assert_handler.cpp:25
 3# main at ../example/assert_handler.cpp:54
 4# 0x00007F991FD69F45 in /lib/x86_64-linux-gnu/libc.so.6
 5# 0x0000000000401139
}

std::stacktrace C++20, design review LEWG, LWG. .

, Ranges TS. compile-time :

template <class F>
    requires Invocable<F>
void my_executor::execute(F f) noexcept {
    lock_guard l{data_mutex_};
    push(std::move(f));
}

proposal.

Feature-test macros

. . unlikely, __has_cpp_attribute(unlikely).

, .

bit_cast

reinterpret_cast – . , . C++ .

,

 my_type my = reinterpret_cast<my_type&>(some_array); 

my_type my = std::bit_cast<my_type>(some_array); 

, some_array my_type , , . , type aliasing.

, C++20:

• shift_left(it_begin, it_end, unsigned n) – n, *it_begin = std::move(*(it_begin + n)), *(it_begin + 1) = std::move(*(it_begin + n + 1))...
• shift_right(it_begin, it_end, unsigned n) – , , *(it_begin + n) = std::move(*it_begin), *(it_begin + n + 1) = std::move(*(it_begin + 1))...
• ispow2(x) – true ,
• ceil2(x) – ,
• contains – bool contains(const key& v), true

, , std::swap std::swap constexpr. , std::nth_element . 21.

atomic_ref

, - ? atomic_ref<T> ( ).

, , atomic_ref<T>. atomic_ref, atomic_ref, .

, (string_view, atomic_ref), . , .

X, operator<=> :

struct X {
    // ...
    std::strong_equality operator<=>(const X&, const X&) = default;
    // ...
};

:

template <X x>
struct x_as_template_param {
    // ...
};

operator<=>.

constexpr virtual

, , constexpr .

constexpr virtual int foo();, int foo() constexpr, . foo() , constexpr .

, , std::type_info, Boost.TypeIndex, compile-time :

template <class T, class U>
constexpr bool is_same() {
    constexpr bool res = (typeid(T) == typeid(U));
    return res;
}

.

Parallelism 2, Reflection Executors TS

Parallelism 2 , . type traits, simd .

Reflection (TS). , <type_traits>. constexpr constexpr! (. ).

Executors C++20, , , TS. , .

user-declared virtual destructor

, C++20 ( ), , :

struct i_am_trivial {
    int foo;
    char bar;

    virtual ~i_am_trivial() = default;
};

, , . , , , , . , , std::vector<Base>, Base .

constexpr!

, C++20 – constexpr! .

, runtime . C++.

constexpr! . .. constexpr! , ( ) . , . , , Boost.Hana [Boost.]PFR.

: constexpr std::regex

/ . , , .

C++ :

bool is_valid_mail(std::string_view mail) {
    static const std::regex mail_regex(R"((?:(?:[^<>()\[\].,;:\s@\"]+(?:\.[^<>()\[\].,;:\s@\"]+)*)|\".+\")@(?:(?:[^<>()\[\].,;:\s@\"]+\.)+[^<>()\[\].,;:\s@\"]{2,}))");

    return std::regex_match(
        std::cbegin(mail),
        std::cend(mail),
        mail_regex
    );
}

is_valid_mail(). , .

constexpr (constexpr new, is_constexpr_evaluated() .) C++ , constexpr std::regex.

constexpr std::regex is_valid_mail() . , GCC static const, .. GCC-6 constexpr – , GCC .

, constexpr std::regex?

P.S.: C++ Yandex.Taxi Coding Fest. C++17.