127 (#67)

wanghenshui · Aug 18, 2023 · 00167d6 · 00167d6
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+---
+layout: post
+title:  第127期
+---
+# C++ 中文周刊 第127期
+
+
+[周刊项目地址](https://github.com/wanghenshui/cppweeklynews)
+
+公众号
+
+<img src="https://wanghenshui.github.io/cppweeklynews/assets/code.png" alt=""  width="30%">
+
+
+
+RSS https://github.com/wanghenshui/cppweeklynews/releases.atom
+
+欢迎投稿，推荐或自荐文章/软件/资源等
+
+请[提交 issue](https://github.com/wanghenshui/cppweeklynews/issues)
+
+内容不多
+
+---
+
+## 资讯
+
+标准委员会动态/ide/编译器信息放在这里
+
+[编译器信息最新动态推荐关注hellogcc公众号 本周更新 2023-08-16 第215期 ](https://github.com/hellogcc/osdt-weekly/blob/master/weekly-2023/2023-01-04.md)
+
+
+
+boost 1.8.3发布，最后一个支持c++03的版本
+
+https://www.boost.org/users/history/version_1_83_0.html
+
+重点发布就是`boost::concurrent_flat_map`
+
+```txt
+Unordered:
+Major update.
+Added boost::concurrent_flat_map, a fast, thread-safe hashmap based on open addressing.
+Sped up iteration of open-addressing containers.
+In open-addressing containers, erase(iterator), which previously returned nothing, now returns a proxy object convertible to an iterator to the next element. This enables the typical it = c.erase(it) idiom without incurring any performance penalty when the returned proxy is not used.
+
+```
+
+
+## 文章
+
+#### [Did you know that C++26 std.format added formatting pointers ability ](https://github.com/tip-of-the-week/cpp/blob/master/tips/343.md)
+
+```cpp
+int main() {
+    auto i = 42;
+    std::cout << std::format("{:#018X}", reinterpret_cast<uintptr_t>(&i)); // prints 0X00007FFD9D71776C
+}
+```
+
+有点用，但也不是很多
+
+#### [模板元编程的精妙之--2/16进制字面量转换为编译期字符串 ](http://www.purecpp.cn/detail?id=2375)
+
+
+```cpp
+  asio::const_buffer b5 = 0xaB_buf;
+  ASIO_CHECK(b5.size() == 1);
+  ASIO_CHECK(memcmp(b5.data(), "\xab", 1) == 0);
+
+  asio::const_buffer b6 = 0xABcd_buf;
+  ASIO_CHECK(b6.size() == 2);
+  ASIO_CHECK(memcmp(b6.data(), "\xab\xcd", 2) == 0);
+
+  asio::const_buffer b7 = 0x01ab01cd01ef01ba01dc01fe_buf;
+  ASIO_CHECK(b7.size() == 12);
+  ASIO_CHECK(memcmp(b7.data(),
+        "\x01\xab\x01\xcd\x01\xef\x01\xba\x01\xdc\x01\xfe", 12) == 0);
+```
+
+编译期hex转字符串。很妙
+
+代码在这里 https://github.com/chriskohlhoff/asio/blob/master/asio/include/asio/buffer.hpp#L2743
+#### [Inside STL: Smart pointers](https://devblogs.microsoft.com/oldnewthing/20230814-00/?p=108597)
+
+shared_ptr有额外的计数信息, 大概这样
+
+```cpp
+struct control_block {
+    virtual void Dispose() = 0;
+    virtual void Delete() = 0;
+    std::atomic<unsigned long> shareds;
+    std::atomic<unsigned long> weaks;
+};
+template<typename T>
+struct shared_ptr {
+    T* object;
+    control_block* control;
+};
+
+template<typename T>
+struct weak_ptr {
+    T* object;
+    control_block* control;
+};
+```
+
+unique_ptr比较简单，就是指针+deleter，然后空基类优化（compressed_pair）
+
+#### [Inside STL: The shared_ptr constructor vs make_shared](https://devblogs.microsoft.com/oldnewthing/20230815-00/?p=108602)
+
+讲智能指针的内存布局
+
+shared_ptr是有额外的信息的，这部分信息需要一个分配
+
+```cpp
+auto p = std::shared_ptr<S>(new S());
+auto p = std::make_shared<S>();
+```
+
+这两种构造，第一种，由于是接管，S的内存和shared_ptr内部信息不是连续的，这对局部性缓存是不友好的
+
+#### [Inside STL: The shared_ptr constructor and enable_shared_from_this](https://devblogs.microsoft.com/oldnewthing/20230816-00/?p=108608)
+
+enable_shared_from_this怎么实现的？大概思路就是weak_ptr
+
+```c++
+template<typename T>
+struct enable_shared_from_this {
+    using esft_detector = enable_shared_from_this;
+    std::weak_ptr<T> weak_this;
+
+    std::weak_ptr<T> weak_from_this()
+    { return weak_this; }
+
+    std::shared_ptr<T> shared_from_this()
+    { return weak_this.lock(); }
+
+};
+```
+
+weak_this由谁来赋值？肯定是shared_ptr拉
+
+```cpp
+template<typename T, typename D>
+struct shared_ptr {
+    shared_ptr(T* ptr)
+    {
+        ... do the usual stuff ...
+
+        /* Here comes enable_shared_from_this magic */
+        if constexpr (supports_esft<T>::value) {
+            using detector = T::esft_detector;
+            ptr->detector::weak_this = *this;
+        }
+    }
+
+    ... other constructors and stuff ...
+};
+
+```
+
+只要enable_shared_from_this实现 esft_detector就行了，类似这样
+
+```cpp
+template<typename T, typename = void>
+struct supports_esft : std::false_type {};
+
+template<typename T>
+struct inline bool supports_esft<T,
+    std::void_t<typename T::esft_detector>>
+    : std::true_type {};
+```
+
+这样继承的类都有特化的shared_ptr构造
+
+#### [C++23: mdspan](https://www.sandordargo.com/blog/2023/08/15/cpp23-mdspan-mdsarray)
+
+编译器还没加上这个能力，可以用这个体验 https://github.com/kokkos/mdspan，在线 https://godbolt.org/z/Mxa7cej1a
+
+之前也讲过很多次了，直接贴代码吧
+
+```cpp
+std::array numbers {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+stdex::mdspan<int, stdex::extents<int, 2, 5>, stdex::layout_right> mdspanOfNumbers {numbers.data()};
+for (size_t rowIndex=0; rowIndex < mdspanOfNumbers.extent(0); ++rowIndex) {
+    for (size_t columnIndex=0; columnIndex < mdspanOfNumbers.extent(1); ++columnIndex) {
+        std::cout << mdspanOfNumbers[rowIndex, columnIndex] << ' ';
+    }
+    std::cout << '\n';
+}
+
+/*
+1 2 3 4 5 
+6 7 8 9 10 
+*/
+
+std::array numbers {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+stdex::mdspan<int, stdex::extents<int, 2, 5>, stdex::layout_left> mdspanOfNumbers {numbers.data()};
+for (size_t columnIndex=0; columnIndex < mdspanOfNumbers.extent(0); ++columnIndex) {
+    for (size_t rowIndex=0; rowIndex < mdspanOfNumbers.extent(1); ++rowIndex) {
+        std::cout << mdspanOfNumbers[columnIndex, rowIndex] << ' ';
+    }
+    std::cout << '\n';
+}
+/*
+1 3 5 7 9 
+2 4 6 8 10 
+*/
+```
+#### [Transcoding UTF-8 strings to Latin 1 strings at 18 GB/s using AVX-512](https://lemire.me/blog/2023/08/12/transcoding-utf-8-strings-to-latin-1-strings-at-12-gb-s-using-avx-512/)
+
+SIMD时间
+
+常规
+
+```cpp
+uint8_t leading_byte = data[pos]; // leading byte
+if (leading_byte < 0b10000000) {
+  *latin_output++ = leading_byte;
+  pos++;
+} else if ((leading_byte & 0b11100000) == 0b11000000) {
+  *latin_output++ = (leading_byte & 1) << 6 | (data[pos + 1]);
+  pos += 2;
+}
+```
+
+simd
+
+```cpp
+__m512i input = _mm512_loadu_si512((__m512i *)(buf + pos));
+__mmask64 leading = _mm512_cmpge_epu8_mask(input, _mm512_set1_epi8(-64));
+__mmask64 bit6 = _mm512_mask_test_epi8_mask(leading, input, _mm512_set1_epi8(1));
+input = _mm512_mask_sub_epi8(input, (bit6<<1) | next_bit6, input, _mm512_set1_epi8(-64));
+next_bit6 = bit6 >> 63;
+__mmask64 retain = ~leading;
+__m512i output = _mm512_maskz_compress_epi8(retain, input);
+int64_t written_out = _popcnt64(retain);
+__mmask64 store_mask = (1ULL << written_out) - 1;
+_mm512_mask_storeu_epi8((__m512i *)latin_output, store_mask, output);
+```
+
+完整代码 https://github.com/lemire/Code-used-on-Daniel-Lemire-s-blog/tree/master/2023/08/11
+
+
+#### [Some C++20 ranges aren’t const-iterable](https://quuxplusone.github.io/blog/2023/08/13/non-const-iterable-ranges/)
+
+range的坑，没有完全零开销，需要拷贝
+
+#### [Writing custom C++20 coroutine systems](https://www.chiark.greenend.org.uk/~sgtatham/quasiblog/coroutines-c++20/)
+
+手把手教你写协程
+
+
+#### [Parameter Passing in Flux versus Ranges](https://tristanbrindle.com/posts/parameter-passing-in-flux-vs-ranges)
+
+他也写了个range库 flux，对比了一下和range的差异，优缺点
+
+
+#### [How to convert an enum to string in C++](https://mariusbancila.ro/blog/2023/08/17/how-to-convert-an-enum-to-string-in-cpp/)
+
+推荐使用magic_enum
+
+## 视频
+
+
+CppNow基本上一周出几个视频，列个有意思的
+
+#### [The New C++ Library: Strong Library Foundation for Future Projects - Jonathan Müller & Arno Schödl ](https://www.youtube.com/channel/UCxHAlbZQNFU2LgEtiqd2Maw)
+
+ppt在这里 https://www.jonathanmueller.dev/talk/think-cell-library/
+
+代码在这里 https://github.com/think-cell/think-cell-library
+
+非常通用的组件介绍
+
+比如std::exchange
+
+```cpp
+template <typename T>
+class my_smart_ptr {
+    T* _ptr;
+  public:
+    my_smart_ptr(my_smart_ptr&& other) noexcept
+    : _ptr(other._ptr) {
+        other._ptr = nullptr;
+    }
+};
+```
+
+用上exchange
+```cpp
+template <typename T>
+class my_smart_ptr {
+    T* _ptr;
+  public:
+    my_smart_ptr(my_smart_ptr&& other) noexcept
+    : _ptr(std::exchange(other._ptr, nullptr))
+    {}
+};
+```
+
+这种语义是upsert，不只指针，其他value也可以这样优化，代码干净
+
+所以实现了tc::change
+
+```cpp
+void tc::optional<T>::reset() {
+    if (_has_value) {
+        _has_value = false;
+        value().~T();
+    }
+}
+```
+
+使用tc::change
+
+
+```cpp
+void tc::optional<T>::reset() {
+    if (tc::change(_has_value, false)) {
+        value().~T();
+    }
+}
+```
+
+为啥为了省一个if这么麻烦？，举个例子，异常+重入
+
+```cpp
+void foo1() {
+    …
+    if (dirty) {
+        clean();
+        dirty = false;
+    }
+    …
+}
+
+void foo2() {
+    …
+    if (tc::change(dirty, false)) {
+        try {
+            clean();
+        } catch (...) {
+            dirty = true;
+            throw;
+        }
+    }
+    …
+}
+```
+
+foo2比foo1更健壮点，避免了同时clean的场景
+
+假如多个线程都在clean，而clean时间较长，dirty更新不及时，就更新了多次
+
+foo2就避免了这种情况
+
+还有一些range string的就不列了，感兴趣的可以看看
+
+## 开源项目需要人手
+
+- [asteria](https://github.com/lhmouse/asteria) 一个脚本语言，可嵌入，长期找人，希望胖友们帮帮忙，也可以加群384042845和作者对线
+- [Unilang](https://github.com/linuxdeepin/unilang) deepin的一个通用编程语言，点子有点意思，也缺人，感兴趣的可以github讨论区或者deepin论坛看一看。这里也挂着长期推荐了
+- [gcc-mcf](https://gcc-mcf.lhmouse.com/) 懂的都懂
+
+
+
+## 工作招聘
+
+有没有数据库相关的工作推荐我一下，我要失业了快(能远程更好)
+---
+
+
+
+[本文永久链接](https://wanghenshui.github.io/cppweeklynews/posts/127.html)
+
+如果有疑问评论最好在上面链接到评论区里评论，这样方便搜索，微信公众号有点封闭/知乎吞评论