C++

Understanding the inner workings of C++ smart pointers - The shared_ptr

After last months Understanding the inner workings of C++ smart pointers - The unique_ptr with custom deleter you're curious about how the shared_ptr is implemented? Great! Here we go.

A minimalistic shared_ptr implementation

Well, minimalistic is a simple word. A shared_ptr is a little more complex than a unique_ptr. Below is the implementation of a shared_ptr.

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template<typename T>
class shared_ptr {
  ctrl_blk_base* ctrl_blk_{};
  T*             t_{};

  shared_ptr(ctrl_blk_with_storage<T>* cb)
  : shared_ptr{cb, cb->get()}
  {}

  shared_ptr(ctrl_blk_base* cb, T* t)
  : ctrl_blk_{cb}
  , t_{t}
  {}

  template<typename U, typename... Args>
  friend shared_ptr<U> make_shared(Args&&... vals);

public:
  shared_ptr() = default;

  shared_ptr(T* t)
  : shared_ptr{new ctrl_blk<T>{t}, t}
  {}

  ~shared_ptr()
  {
    if(ctrl_blk_) { ctrl_blk_->release_shared(); }
  }

  shared_ptr(const shared_ptr& rhs)
  : ctrl_blk_{rhs.ctrl_blk_}
  , t_{rhs.t_}
  {
    if(ctrl_blk_) { ctrl_blk_->add_shared(); }
  }

  shared_ptr(shared_ptr&& rhs)
  : ctrl_blk_{rhs.ctrl_blk_}
  , t_{rhs.t_}
  {
    rhs.ctrl_blk_ = nullptr;
    rhs.t_        = nullptr;
  }

  shared_ptr& operator=(const shared_ptr& rhs)
  {
    shared_ptr{rhs}.swap(*this);  // forward to copy ctor
    return *this;
  }

  shared_ptr& operator=(shared_ptr&& rhs)
  {
    shared_ptr{std::move(rhs)}.swap(*this);  // forward to move-ctor
    return *this;
  }

  void swap(shared_ptr& rhs)
  {
    std::swap(t_, rhs.t_);
    std::swap(ctrl_blk_, rhs.ctrl_blk_);
  }
};

template<typename T, typename... Args>
shared_ptr<T> make_shared(Args&&... vals)
{
  return new ctrl_blk_with_storage<T>(std::forward<Args>(vals)...);
}

A shared_ptr needs to track the use count. This tracking is done via a control block. Since multiple shared_ptr that point to the same data must use the same control block, the shared_ptr implementation stores only a pointer to that control block, next to the data pointer.

When you create a new shared_ptr by passing an already allocated pointer to the constructor, the first thing the shared_ptr does is allocate a new control block where it also stores the data pointer. If you look closely, you can see that in my implementation, shared_ptr has a data member of type ctrl_blk_base. At the same time, a ctrl_blk is allocated in the constructor. I will show you that implementation later. For now, it is safe to assume that inheritance is used.

In the destructor, release_shared is called on the control block if the latter isn't a nullptr.

The next thing are the copy and move operations. They use one clever trick. They create a new temporary shared_ptr object and call swap. That's an easy and robust way to maintain the use count.

The shared_ptr is special due to its control block. You can see this in the implementation of make_shared as well. I'm returning a dynamically allocated object of type ctrl_blk_with_storage, which triggers another shared_ptr constructor. Hopefully, obviously, ctrl_blk_with_storage is also derived from ctrl_blk_base.

Peaking into shared_ptrs control block implementation

Below, you will find the control block base class ctrl_blk_base and the two derived classes ctrl_blk and ctrl_blk_with_storage you've seen earlier.

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struct ctrl_blk_base {
  std::atomic_uint64_t shared_ref_count_{1};

  void add_shared() { ++shared_ref_count_; }
  auto dec() { return --shared_ref_count_; }

  virtual void release_shared() = 0;
};

template<typename T>
struct ctrl_blk : ctrl_blk_base {
  T* data_;

  explicit ctrl_blk(T* data)
  : ctrl_blk_base{}
  , data_{data}
  {}

  void release_shared() override
  {
    if(0 == dec()) {
      delete data_;
      delete this;  // self delete
    }
  }
};

template<typename T>
struct ctrl_blk_with_storage : ctrl_blk_base {
  T in_place_;

  template<typename... Args>
  explicit ctrl_blk_with_storage(Args&&... vals)
  : ctrl_blk_base{}
  , in_place_{std::forward<Args>(vals)...}
  {}

  T* get() { return &in_place_; }

  void release_shared() override
  {
    if(0 == dec()) {
      delete this;  // self delete
    }
  }
};

The base class ctrl_blk_base contains an atomic unsigned long data member and the three member functions add_shared, dec, and release_shared. The first two are helpers to maintain the use count. More interesting is release_shared since it is virtual, pure virtual to be precise. The virtuality here is necessary because the two derived classes have different implementations.

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Let's start by looking at ctrl_blk. This class has a pointer data member that contains the original data. If the use count reaches zero in release_shared, the function first deletes the payload data before deleting itself. The last step is necessary because the shared_ptr destructor cannot do this.

If you now switch to ctrl_blk_with_storage, you can see that this class brings its own storage space for the T it is constructed with. This is what make_shared uses. In release_shared, it is enough to perform a self-delete. This also invokes the destructor of T.

You just saw a minimalistic example of a shared_ptr. I ignored the weak_ptr and its implications. However, I hope this helps you understand the two smart pointers better.

More about smart pointers

I will continue writing about smart pointers in my next post.

Andreas

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