C++ 中文周刊第40期

从reddit/hackernews/lobsters/meetingcpp摘抄一些c++动态

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最近有点忙cppcon视频没看完

资讯

Qt Creator 6 released

文章

c++ execution 与 coroutine (三)：最简单的receiver与最简单的sender

手把手教你写sender/receiver

I spent some time optimizing memset and memcpy in x86 assembly. Here are a few interesting things about memset and memcpy. 代码地址在这里https://github.com/nadavrot/memset_benchmark

核心想法就是这个

Uninitialized Stack Variables 栈变量不初始化一个bug

这个看个乐，代码

$ cat a.c
#include <arpa/inet.h>
#include <sys/socket.h>

#include <netinet/in.h>

#include <err.h>
#include <stdlib.h>

#define PORT 12345

int
main() {
	int sock;
  // 加这行就不报错了	socklen_t length, s_size;
	struct sockaddr_storage server;
	struct sockaddr_in6 *sin = (struct sockaddr_in6 *)&server;
  // 加这行也不报错 memset(sin, 0, sizeof(*sin));
	if ((sock = socket(PF_INET6, SOCK_STREAM, 0)) < 0) {
		err(EXIT_FAILURE, "socket");
		/* NOTREACHED */
	}

	if (inet_pton(PF_INET6, "::1", &(sin->sin6_addr)) != 1) {
		err(EXIT_FAILURE, "inet_pton");
		/* NOTREACHED */
	}

	sin->sin6_family = PF_INET6;
	sin->sin6_port = htons(PORT);

	if (bind(sock, (struct sockaddr *)sin, sizeof(*sin)) != 0) {
		err(EXIT_FAILURE, "bind");
		/* NOTREACHED */
	}

	return 0;
}
$ cc a.c
$ ./a.out
a.out: bind: Can't assign requested address
$ 

这个问题定位下来就是未初始化，解错了数据

一种永远不会忘记写break的switch case写法

switch (f)
{
    break; case Foo::Alpha:
        s += "is nothing";
    break; case Foo::Beta:
        s += "is important";
        f = Foo::Alpha;
    break; case Foo::Gamma:
        s += "is very important";
        f = Foo::Alpha;
    break; case Foo::Delta:
        s += "is not very important";
        f = Foo::Alpha;
}

哈哈，给我看笑了

c++ tip of week Did you know about static reflection proposal for C++2X?

简单反射，可以这里体验一下https://godbolt.org/z/x55GPdTxE。虽然在clang reflect分支里，但是没合并到主干。这个反射到提案进度也不知道鸽成什么样子了

#include <experimental/reflect>
#include <iostream>

namespace meta = std::experimental::reflect;

template<class... Ts>
struct print_enum {
  void operator()() {
    ((std::cout << meta::get_name_v<meta::get_type_t<Ts>> << ':' << meta::get_name_v<Ts> << std::endl), ...);
  }
};

enum class Trading { Quant, Lab };

int main() {
  std::cout << meta::get_name_v<reflexpr(Trading)> << std::endl; // prints Trading
  std::cout << meta::get_name_v<reflexpr(Trading::Quant)> << std::endl; // prints Quant
  std::cout << meta::get_name_v<reflexpr(Trading::Lab)> << std::endl; // prints Lab

  meta::unpack_sequence_t<print_enum, meta::get_enumerators_t<reflexpr(Trading)>>{}(); // prints Trading::Quant, Trading::Lab
}