C ++ 20へのもう1つの小さなステップ。 アルバカーキでの会議

演算子<=>

struct point3d {
    int x;
    int y;
    int z;
};

using p_ref = const point3d&;

bool operator==(p_ref a, p_ref b) {
    return a.x == b.x
        && a.y == b.y
        && a.z == b.z
    ;
}
bool operator< (p_ref a, p_ref b) {
    return a.x < b.x
        || (a.x == b.x && a.y < b.y)
        || (a.x == b.x && a.y == b.y
            && a.z < b.z)
    ;
}

bool operator!=(p_ref a, p_ref b) {
    return !(a == b);
}
bool operator<=(p_ref a, p_ref b) {
    return !(b < a);
}
bool operator> (p_ref a, p_ref b) {
    return b < a;
}
bool operator>=(p_ref a, p_ref b) {
    return !(a < b);
}

struct point3d {
    int x;
    int y;
    int z;

    auto operator<=>(const point3d&)
        const = default;
};

class weak_equality;
class strong_equality;
class partial_ordering;
class weak_ordering;
class strong_ordering;

#include <compare> // weak_ordering, is_neq

struct point3d {
    int x;
    int y;
    int z;

    std::weak_ordering operator<=>(const point3d& p) const {
        using std::abs;
        if (auto cmp = abs(z) <=> abs(p.z); std::is_neq(cmp)) return cmp;
        if (auto cmp = abs(x) <=> abs(p.x); std::is_neq(cmp)) return cmp;
        return abs(y) <=> abs(p.y);
    }
};

lexicographical_compare_3way(InputIterator1 b1, InputIterator1 e1, InputIterator2 b2, InputIterator2 e2);

osyncstream

#include <iostream>
#include <thread>
#include <string_view>

void say_hello(std::string_view username) {
    std::cerr << "Hello " << username;
}

void beware_of(std::string_view something) {
    std::cerr << something << " is dangerous";
}

int main() {
    std::thread t1([]{
        say_hello("Rachel");
    });

    std::thread t2([]{
        beware_of("darkness");
    });

    std::cerr << '\n';
 
    t2.join();
    t1.join();

    /* Possible output:
            Hello darkness
            Rachel is dangerous
    */
}

#include <iostream>
#include <thread>
#include <string_view>

void say_hello(std::string_view username) {
    std::osyncstream{std::cerr} << "Hello " << username;
}

void beware_of(std::string_view something) {
    std::osyncstream(std::cerr) << something << " is dangerous";
}

int main() {
    std::thread t1([]{
        say_hello("Rachel");
    });

    std::thread t2([]{
        beware_of("darkness");
    });

    std::cerr << '\n';
 
    t2.join();
    t1.join();
}

21

• P0539R2 — integers, ( ) . , , . proposal.
• P0415R0* — constexpr std::complex. C++20 constexpr.
• P0202R2* — constexpr <algorithm>. C++20 , , std::swap. , proposal.
• P0275R2 — shared_library , . Evolution, , . Library Evolution .
• P0858R0 ( ) — . std::basic_string_view std::array constexpr . C++20.

• P0457R1 — starts_with ends_with . C++20. !
• P0458R0 — contains(key) member [unordered_]map/set/multimap/multiset. LWG, , , C++20.

• range based for :
  
  

  for (T thing = f(); auto& x : thing.items())

• :
  
  

  using greater_t = decltype([](auto x, auto y) { return x > y; });
  std::map<std::string, int, greater_t> map;
  constexpr greater_t comparator{}; // OK

• std::is_pod std::is_pod_v deprecated.
• type trait std::remove_cvref.
• [[nodiscard]] async(), new, allocate(), empty() launder().
• std::memory_order* scoped enum.
• :
  
  

  template <class T> struct atomic<shared_ptr<T>>;
  template <class T> struct atomic<weak_ptr<T>>;

• std::atomic.
• .