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Expand Up @@ -32,7 +32,7 @@ RSS使用仓库的release RSS [链接](https://github.com/wanghenshui/cppweeklyn
| --------------------- | --------------------- | --------------------- | --------------------- | --------------------- | --------------------- | --------------------- | --------------------- | --------------------- | --------------------- |
| [五十一](./posts/051.md) | [五十二](./posts/052.md) | [五十三](./posts/053.md) | [五十四](./posts/054.md) | [五十五](./posts/055.md) | [五十六](./posts/056.md) | [五十七](./posts/057.md) | [五十八](./posts/058.md) | [五十九](./posts/059.md) | [六十期](./posts/060.md) |
| [六十一](./posts/061.md) | [六十二](./posts/062.md) | [六十三](./posts/063.md) | [六十四](./posts/064.md) | [六十五](./posts/065.md) | [六十六](./posts/066.md) | [六十七](./posts/067.md) | [六十八](./posts/068.md) | [六十九](./posts/069.md) | [七十期](./posts/070.md) |
| [七十一](./posts/071.md) ||||||||||
| [七十一](./posts/071.md) |[七十二](./posts/072.md)|||||||||


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layout: post
title: 第72期
---
# C++ 中文周刊 第72期

从[reddit](https://www.reddit.com/r/cpp/)/[hackernews](https://news.ycombinator.com/)/[lobsters](https://lobste.rs/)/摘抄一些c++动态



[周刊项目地址](https://github.com/wanghenshui/cppweeklynews)｜[在线地址](https://wanghenshui.github.io/cppweeklynews/) ｜[知乎专栏](https://www.zhihu.com/column/jieyaren) |[腾讯云+社区](https://cloud.tencent.com/developer/column/92884)

弄了个qq频道，[手机qq点击进入](https://qun.qq.com/qqweb/qunpro/share?_wv=3&_wwv=128&inviteCode=xzjHQ&from=246610&biz=ka)

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)

2022 0726 最近有点忙

---

## 资讯

标准委员会动态/ide/编译器信息放在这里
[七月邮件汇总](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/#mailing2022-07)

[编译器信息最新动态推荐关注hellogcc公众号 本周更新 2022-07-20 第159期](https://github.com/hellogcc/osdt-weekly/blob/master/weekly-2022/2022-07-20.md)



## 文章

- [趣谈哈希表优化：从规避 Hash 冲突到利⽤ Hash 冲突](https://mp.weixin.qq.com/s/oeuExiW3DYQnBG8HvDgBAg)

讲完美哈希表的。这个知识好像没怎么普及开，这里推荐一下

- [Making sure that people use make_unique and make_shared to make your object](https://devblogs.microsoft.com/oldnewthing/20220721-00/?p=106879)

一个思路，禁止各种make，只能从静态函数构造

```cpp
template<typename T>
struct require_make_shared :
public std::enable_shared_from_this<T>
{
protected:
struct use_the_create_method {
explicit use_the_create_method() = default;
};

public:
template<typename...Args>
static auto create(Args&&... args)
{
return std::make_shared<T>
(use_the_create_method{},
std::forward<Args>(args)...);
}

// Deny copy construction
require_make_shared(require_make_shared const&) = delete;
};

class Widget : public require_make_shared<Widget>
{
public:
Widget(use_the_create_method);
Widget(use_the_create_method, int);
Widget(use_the_create_method, int, int);
};

void test()
{
auto v1 = Widget::create(); // okay
auto v2 = std::make_shared<Widget>(); // nope
auto v3 = std::make_unique<Widget>(); // nope
Widget v4; // nope
}
```
- [6 C++23 features improving string and string_view](https://www.sandordargo.com/blog/2022/07/20/6-features-improving-string-string_view-in-cpp23)
```cpp
std::string s{"there is a needle in the haystack"};
std::string_view sv{"acdef"};
if (s.contains("needle")) { // 总算有这个api了
std::cout << "we found a needle in: " << std::quoted(s) << '\n';
}
if (!sv.contains('b')) {
std::cout << "we did not find a 'b' in: " << std::quoted(sv) << '\n';
}
std::string s(nullptr); //不会挂了
```

basic_string::resize_and_overwrite() 可以省一个写，用不上的可以先用gcc内置函数

```cpp
std::string s { "Food: " };
s.resize_and_overwrite(10, [](char* buf, int n) {
return std::find(buf, buf + n, ':') - buf;
});
std::cout << "2. " << std::quoted(s) << '\n';
```

- [finally. #embed](https://thephd.dev/finally-embed-in-c23)

c23支持embed了，c++23还不支持std::embed。服了

介绍一下embed，看代码你就懂了

```c
#include <assert.h>

int main (int, char*[]) {
static const char sound_signature[] = {
#embed <sdk/jump.wav>
};
static_assert((sizeof(sound_signature) / sizeof(*sound_signature)) >= 4,
"There should be at least 4 elements in this array.");

// verify PCM WAV resource signature (at run-time)
assert(sound_signature[0] == 'R');
assert(sound_signature[1] == 'I');
assert(sound_signature[2] == 'F');
assert(sound_signature[3] == 'F');

return 0;
}

```
- [Custom types and std::format from C++20 ](https://www.cppstories.com/2022/custom-stdformat-cpp20/)
定制formatter，代码不贴了
- [Did you know that C++23 added auto(x): decay-copy in the language?](https://github.com/QuantlabFinancial/cpp_tip_of_the_week/blob/master/287.md)
```cpp
auto foo(auto&&);
int main() {
int i{};
const int c{};
foo(42); // int
foo(i); // int&
foo(int{i}); // int
foo(auto{i}); // int
foo(auto(42)); // int
foo(c); // const int&
foo(int{c}); // int
foo(auto(c)); // int
}
```

这个也讲过很多次了。有些场景是需要copy语义的，否则莫名其妙出现了改动，解决办法就是让他退化，拿到copy

这个例子也说过很多次了

```cpp
#include <iostream>

template<class T>
constexpr std::decay_t<T>
decay_copy(T&& v)
noexcept(std::is_nothrow_convertible_v<T, std::decay_t<T>>) // exposition only
{
return std::forward<T>(v);
}

int main() {
std::vector<int> v1{1, 2, 1, 1, 1};
auto v2 = v1;
v1.erase(
std::remove(begin(v1), end(v1), *std::max_element(begin(v1), end(v1))),
v1.end());
v2.erase(std::remove(
begin(v2), end(v2),
decay_copy(*std::max_element(begin(v2), end(v2)))),
v2.end());
std::cout << "v1.size()=" << v1.size() << std::endl
<< "v2.size()=" << v2.size()
<< std::endl;
return 0;
}
```
- [Upside Down Polymorphic Inheritance Leveraging P2162 for Fun & Profit](https://blog.nickelp.ro/posts/p2162/)
p2162继承variant各个编译器已经支持了。作者展示了一种优雅的用法
```cpp
#include <cstdint>
#include <iostream>
using namespace std;
struct Request {
static constexpr uint8_t id_num {1};
uint8_t req_val;
void encode(ostream& os) const {
os.put(req_val);
}
void decode(istream& is) {
req_val = is.get();
}
};
struct Affirmative {
static constexpr uint8_t id_num {2};
uint8_t resp_val;
void encode(ostream& os) const {
os.put(resp_val);
}
void decode(istream& is) {
resp_val = is.get();
}
};
struct Failure {
static constexpr uint8_t id_num {3};
uint8_t resp_val;
void encode(ostream& os) const {
os.put(resp_val);
}
void decode(istream& is) {
resp_val = is.get();
}
};
#include <variant>
#include <exception>
struct Packet : variant<Request, Affirmative, Failure> {
using variant::variant;
Packet(uint8_t id) : variant {from_id(id)} {}
Packet(istream& is) {
decode(is);
}
void encode(ostream& os) const {
visit([&](const auto& inner){
os.put(inner.id_num);
inner.encode(os);
}, *this);
}
void decode(istream& is) {
*this = from_id(is.get());
visit([&](auto& inner){ inner.decode(is); }, *this);
}
private:
static Packet from_id(uint8_t id) {
switch(id) {
case Request::id_num:
return Request {};
case Affirmative::id_num:
return Affirmative {};
case Failure::id_num:
return Failure {};
}
throw std::runtime_error {"Invalid Packet Id!"};
}
};
#include <concepts>
#include <sstream>
#include <fmt/format.h>
using fmt::print;
int main(void) {
Packet p {Request {.req_val {5}}};
stringstream buf;
p.encode(buf);
Packet decoded {buf};
visit([](const auto& inner){
using T = decltype(auto(inner));
if constexpr(same_as<T, Request>)
print("I am a Request with a val of: {}", inner.req_val);
else if constexpr(same_as<T, Affirmative>)
print("I am an Affirmative with a val of: {}", inner.resp_val);
else if constexpr(same_as<T, Failure>)
print("I am a Failure with a val of: {}", inner.resp_val);
}, decoded);
}
```

这种写法确实挺爽的，有点猥琐的模式匹配那个味了，https://godbolt.org/z/rqYeY5r1f

作者也展示了用concept达到类似的效果
```cpp
#include <concepts>

struct Stranger {};

using Person = variant<Alice, Bob, Stranger>;

template <typename T>
concept IsNamed = requires(T p) {
{ p.name() } -> convertible_to<string_view>;
};

void print_name(Person& p) {
visit([](auto& inner) {
if constexpr(IsNamed<decltype(inner)>)
print("Name: {}\n", inner.name());
else
print("This is a stranger!\n");
}, p);
}
```
也不是不行，就是concept一般人比较不好接受
- [Size Optimization Tricks](https://justine.lol/sizetricks/)
讲裁剪二进制的。
## 视频
- [C++ Weekly - Ep 334 - How to Put a Lambda in a Container ](https://www.youtube.com/watch?v=qmd_yxSOsAE)
通过函数make_lambda返回一个lambda，然后推导这个lambda的类型，就可以放到vector里了。直接用lambda不行，类型信息是匿名的，无法使用。
或者用std::function不就行了？
## 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
- [pika](https://github.com/OpenAtomFoundation/pika) 一个nosql 存储， redis over rocksdb，非常需要人贡献代码胖友们， 感兴趣的欢迎加群294254078前来对线
## 新项目介绍/版本更新
- [在线配置属于你自己的clang-format](https://clang-format-configurator.site/)
- [carbon-lang号称做下一个c++](https://github.com/carbon-language/carbon-lang)
说实话格局低了。自己开新坑不如隔壁meta推出新IR修缮
另外下一个c++不是rust么，都这么多年了，为啥会选你carbon，除非像go一样硬推
- [indirect_value](https://github.com/jbcoe/indirect_value) pimpl惯用法的另一种实现, 可以看这个设计https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1950r1.html 挺有意思的
- [BTree](https://github.com/frozenca/BTree/blob/main/fc_btree.h) 一个可罗盘的btree实现。看个乐
## 工作招聘
有没有需要招扫地僧式保洁的。我会拖地
---
看到这里或许你有建议或者疑问或者指出错误，请留言评论! 多谢! 你的评论非常重要！也可以帮忙点赞收藏转发！多谢支持！
[本文永久链接](https://wanghenshui.github.io/cppweeklynews/posts/072.html)

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