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();
}

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

// 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;
});

// 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