C++ 中文周刊第110期

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资讯

cppiso有个开发者调查问卷有空闲的没事儿干的可以投一下 https://www.surveymonkey.com/r/isocpp-2023

标准委员会动态/ide/编译器信息放在这里

周六有深圳meetup

ACCU 2023要开始了。去年的没啥意思

视频信息密度低，看起来真的太费时间了

本周内容也不多

文章

operator<=> for C++20入门篇

老文了，这里列下代码

struct Name {
  string first_name;
  string mid_name;
  string last_name;
};

bool operator<(const Name& other) const {
  //before c++11
  return first_name<other.first_name
      || first_name == other.first_name && mid_name < other.mid_name
      || first_name == other.first_name && mid_name == other.mid_name && last_name < other.last_name;

  //after c++11
  return std::tie(first_name, mid_name, last_name) < 
      std::tie(other.first_name, other.mid_name, other.last_name);
}
// 干净
std::strong_ordering operator<=>(const Name&) const = default;

再比如，控制细节

struct ID {
    int id_number;
    auto operator<=>(const ID&) const = default;
};

struct Person {
    ID id;
    string name;
    string email;
    std::weak_ordering operator<=>(const Person& other) const
    {
        return id<=>other.id;
    }
};

还有就是要注意容器比较，要单独实现operator ==,operator<=>再相等情况下的性能不太行

struct SomeType {
    int int_property;
    std::vector<int> some_ints; // vector是容器
    std::strong_ordering operator<=>(const SomeType&) const = default;
    bool operator==(const SomeType&) const = default; // 加上这一行
};

Functional exception-less error handling with C++23’s optional and expected

直接列代码吧，以前的optional就是废物

std::optional<image_view> get_cute_cat (image_view img) {
    auto cropped = find_cat(img);
    if (!cropped) {
      return std::nullopt;
    }

    auto with_tie = add_bow_tie(*cropped);
    if (!with_tie) {
      return std::nullopt;
    }

    auto with_sparkles = make_eyes_sparkle(*with_tie);
    if (!with_sparkles) {
      return std::nullopt;
    }

    return add_rainbow(make_smaller(*with_sparkles));
}

愣是写出的go的味道

std::optional<image_view> get_cute_cat (image_view img) {
    return find_cat(img)
           .and_then(add_bow_tie)
           .and_then(make_eyes_sparkle)
           .transform(make_smaller)
           .transform(add_rainbow);
}

这多干净

Did you know that C++23 deprecated std::aligned_storage and std::aligned_union?

template<class T>
struct container {
    //std::aligned_storage_t<sizeof(T), alignof(T)> t_buff; // deprecated
    alignas(T) std::byte t_buff[sizeof(T)]; // okay
};

多用alignas(T)

Trivial functions can still be non-nothrow (modulo compiler bugs)

看不懂

老生常谈，concept比虚接口性能好

template <typename T>
concept is_iterable = requires(T v) {
                        { v.has_next() } -> std::convertible_to<bool>;
                        { v.next() } -> std::same_as<uint32_t>;
                        { v.reset() };
                      };
template <is_iterable T> size_t count(T &t) {
  t.reset();
  size_t count = 0;
  while (t.has_next()) {
    t.next();
    count++;
  }
  return count;
}


class iter_base {
public:
  virtual bool has_next() = 0;
  virtual uint32_t next() = 0;
  virtual void reset() = 0;
  virtual ~iter_base() = default;
};

size_t count_inheritance(iter_base &t) {
  t.reset();
  size_t count = 0;
  while (t.has_next()) {
    t.next();
    count++;
  }
  return count;
}


struct iterable_array : iter_base {
  std::vector<uint32_t> array{};
  size_t index = 0;
  void reset() { index = 0; }
  bool has_next() { return index < array.size(); }
  uint32_t next() {
    index++;
    return array[index - 1];
  }
};