C++ 中文周刊第96期

constexpr auto get = [](auto value) {
  if consteval {
    return value;
  } else {
    auto result = 0;
    asm("movl $42, %%eax\n" : "=r" (result) );
    return result;
  }
};

static_assert(0 == get(0));
static_assert(4 == get(4));
static_assert(2 == get(2));

consteval auto fn() {
    return get(0);
}

int main(int argc, char**) {
  assert(42 == get(0));
  assert(42 == get(argc));
  return fn();
}

C++20/clang-15 static reflection via __builtin_dump_struct

希望大家身体健康，活得久一点，就能用到静态反射了

On leading underscores and names reserved by the C and C++ languages

编译器标准库用到很多变量是两个下划线下划线大写字母开头的。自己定义变量尽量别用

Transcoding Unicode with AVX-512: AMD Zen 4 vs. Intel Ice Lake

测了一下simdutdf在Zen4的表现，挺强的

2022 APFS Advent Challenge Day 17 - Blazingly Fast Checksums with SIMD

blake3是不是就是simd加速的？

Pictures of a Working Garbage Collector

给oilshell设计的GC。没仔细看

Care is needed to use C++ std::optional with non-trivial objects

optional做函数参数是十分不恰当的。这玩意只适合做返回值

On QVarLengthArray and Uninitialized Storage in C++

QVarLengthArray类似vector，区别在于对小数据做SBO优化，且resize不会做额外的初始化动作。初始化动作是c++默认有的。很多场景来看是多余的，比如string。c++20/23做了许多修正

比如

std::unique_ptr<int[]> p3 = std::make_unique_for_overwrite<int[]>(100'000);

再比如string

// C++23
std::string s = ~~~;
 
auto oldSize = s.size();
 
s.resize_and_overwrite(100'000, [oldSize](char *buf, std::size_t count) {
 
  // For starters, s will *reserve* enough space, without initializing it.
  //
  // - buf points to the string's storage (i.e. s.data()) *after* the reserve;
  // - count is the 1st argument to resize_and_overwrite (100k), so
  //   we can re-use this function with different `count`s.
 
 
  // Populate the range [buf, buf+count]. We can mutate the entirety of
  // the string's buffer. But let's say we're just interested in populating
  // the new contents -- from position oldSize up to count.
  for (size_it i = oldSize; i < count; ++i)
    buf[i] = generateData(i);
 
  // Notes:
  // - If we're growing, the newly created storage is *uninitialized*.
  //   Don't read from it!
  //
  // - The old contents are still there, and we can access them freely.
  //   If needed, carry `oldSize` manually, to identify where to start 
  //   writing (and leave the old contents alone).
  //
  // - It is legal to write into buf[count],
  //   but it will be overwritten with \0 when we're done.
     
  // We don't need to populate the *entire* buffer -- we may stop short!
  // The returned value will be the new size of the string.
 
  return actual_new_size;
});

QVarLengthArray不会做多余的初始化，请注意(不过QT这套东西会玩的越来越少了，大部分读者应该不玩QT)

What's an executable and how is it structured

基础知识，不会的可以去看《程序员的自我修养链接/库》这本书

What does it mean when I get a mismatch from MSVC for _COROUTINE_ABI?

MSVC有两套coroutine API

// in <experimental/coroutine>
#ifndef _ALLOW_COROUTINE_ABI_MISMATCH
#pragma detect_mismatch("_COROUTINE_ABI", "1")
#endif // _ALLOW_COROUTINE_ABI_MISMATCH

// in <coroutine>
#ifndef _ALLOW_COROUTINE_ABI_MISMATCH
#pragma detect_mismatch("_COROUTINE_ABI", "2")
#endif // _ALLOW_COROUTINE_ABI_MISMATCH

使用 /std:c++20和 /std:c++latest，才会用最新的api，experimental是c++17旧的

How should I interpret the various values of NLM_CONNECTIVITY?

又是Windows API，如何处理flags，我直接贴下面，不懂windows

Flag	Meaning	Recommendation
DISCONNECTED	No network interface detects any network	Treat as offline.
NOTRAFFIC	An interface is connected, but it cannot send or receive network traffic.	Treat as offline.
SUBNET/LOCALNETWORK	An interface has been configured to send traffic, but the system cannot confirm Internet connectivity.	Make one attempt to contact service.
INTERNET	The system has confirmed access to Microsoft Internet sites.	Treat as fully online.