switch fall-through

最新推荐文章于 2025-07-10 09:44:47 发布

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最新推荐文章于 2025-07-10 09:44:47 发布

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在阅读LevelDB的util/hash.cc源码时，注意到一个宏用于表明在case语句中执行完毕后不使用break，而是继续执行下一个case的情况。这种编程特性被称为fall-through。

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最近看leveldb代码

util/hash.cc实现有这么一段代码

uint32_t Hash(const char* data, size_t n, uint32_t seed) {
  // Similar to murmur hash
  const uint32_t m = 0xc6a4a793;
  const uint32_t r = 24;
  const char* limit = data + n;
  uint32_t h = seed ^ (n * m);

  // Pick up four bytes at a time
  while (data + 4 <= limit) {
    uint32_t w = DecodeFixed32(data);
    data += 4;
    h += w;
    h *= m;
    h ^= (h >> 16);
  }

  // Pick up remaining bytes
  switch (limit - data) {
    case 3:
      h += static_cast<unsigned char>(data[2]) << 16;
      FALLTHROUGH_INTENDED;
    case 2:
      h += static_cast<unsigned char>(data[1]) << 8;
      FALLTHROUGH_INTENDED;
    case 1:
      h += static_cast<unsigned char>(data[0]);
      h *= m;
      h ^= (h >> r);
      break;
  }
  return h;
}

// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through
// between switch labels. The real definition should be provided externally.
// This one is a fallback version for unsupported compilers.
#ifndef FALLTHROUGH_INTENDED
#define FALLTHROUGH_INTENDED do { } while (0)
#endif

其实这个宏就是啥都不干的意思，之所以写这个宏，只是为了说明当前case执行完后，不break，而是fall throuth到下一个case里