FBString分析与使用FBString简介

struct MediumLarge {
    Char * data_;
    size_t size_;
    size_t capacity_;
    size_t capacity() const {
      return kIsLittleEndian
        ? capacity_ & capacityExtractMask
        : capacity_ >> 2;
    }
    void setCapacity(size_t cap, Category cat) {
        capacity_ = kIsLittleEndian
          ? cap | static_cast<category_type>(cat)
          : (cap << 2) | static_cast<category_type>(cat);
    }
  };

  union {
    Char small_[sizeof(MediumLarge) / sizeof(Char)];
    MediumLarge ml_;
  };

struct RefCounted {
    std::atomic<size_t> refCount_;
    Char data_[1];

    static RefCounted * fromData(Char * p) {
      return static_cast<RefCounted*>(
        static_cast<void*>(
          static_cast<unsigned char*>(static_cast<void*>(p))
          - sizeof(refCount_)));
    }

    static size_t refs(Char * p) {
      return fromData(p)->refCount_.load(std::memory_order_acquire);
    }

    static void incrementRefs(Char * p) {
      fromData(p)->refCount_.fetch_add(1, std::memory_order_acq_rel);
    }

    static void decrementRefs(Char * p) {
      auto const dis = fromData(p);
      size_t oldcnt = dis->refCount_.fetch_sub(1, std::memory_order_acq_rel);
      assert(oldcnt > 0);
      if (oldcnt == 1) {
        free(dis);
      }
    }

    static RefCounted * create(size_t * size) {
      // Don't forget to allocate one extra Char for the terminating
      // null. In this case, however, one Char is already part of the
      // struct.
      const size_t allocSize = goodMallocSize(
        sizeof(RefCounted) + *size * sizeof(Char));
      auto result = static_cast<RefCounted*>(checkedMalloc(allocSize));
      result->refCount_.store(1, std::memory_order_release);
      *size = (allocSize - sizeof(RefCounted)) / sizeof(Char);
      return result;
    }

    static RefCounted * create(const Char * data, size_t * size) {
      const size_t effectiveSize = *size;
      auto result = create(size);
      fbstring_detail::pod_copy(data, data + effectiveSize, result->data_);
      return result;
    }

    static RefCounted * reallocate(Char *const data,
                                   const size_t currentSize,
                                   const size_t currentCapacity,
                                   const size_t newCapacity) {
      assert(newCapacity > 0 && newCapacity > currentSize);
      auto const dis = fromData(data);
      assert(dis->refCount_.load(std::memory_order_acquire) == 1);
      // Don't forget to allocate one extra Char for the terminating
      // null. In this case, however, one Char is already part of the
      // struct.
      auto result = static_cast<RefCounted*>(
             smartRealloc(dis,
                          sizeof(RefCounted) + currentSize * sizeof(Char),
                          sizeof(RefCounted) + currentCapacity * sizeof(Char),
                          sizeof(RefCounted) + newCapacity * sizeof(Char)));
      assert(result->refCount_.load(std::memory_order_acquire) == 1);
      return result;
    }
  };

fbstring_core(fbstring_core&& goner) noexcept {
    // Take goner's guts
    ml_ = goner.ml_;
    // Clean goner's carcass
    goner.reset();
  }

实现简单，用处多多

### 访问函数
- const Char * data() const
- const Char * c_str()
- Char * mutable_data()
large状态的，要作拷贝啦～～～COW

### 回缩函数
- void shrink(const size_t delta)
size减少delta. "得儿塔" 让我想起了中学的数学课～～～还有我同桌

### Reserve函数
- void reserve(size_t minCapacity, bool disableSSO)
让这个string的capacity至少有minCapacity这么大，这个函数完美诠释了三种状态的转换。

## basic_fbstring
- 这个其实没什么说的，用fbstring_core的接口把std::string的所有接口都实现了一遍。
- 最后还附赠了一个 **typedef basic_fbstring<char> fbstring** 外加一个**FOLLY_FBSTRING_HASH**

# FBString作者：[Andrei Alexandrescu](https://zh.wikipedia.org/wiki/%E5%AE%89%E5%BE%B7%E7%83%88%C2%B7%E4%BA%9E%E6%AD%B7%E5%B1%B1%E5%BE%B7%E9%9B%B7%E6%96%AF%E5%BA%AB) C++和D語言專家