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Dave Amiana
Dave Amiana

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Implementing Building Blocks of Reference Semantics: Unique Reference

Our previous discussion explores the anatomy of pointer types in C and C++ on par with hinting nuances of reference semantics. We discussed the use cases of raw pointers and smart pointers in modern C++ and found that there are degrees of aptness in choosing between automatic resource management over manual resource management. More specifically, we introduced std::unique_ptr<T> and demonstrated some of its functionalities.

This article discusses the design goals and implementation of std::unique_ptr<T>. However, instead of discussing the intricacies of the STL implementation in different C++ compiler distributions, we simplify this to express the design goals of a unique reference.

At the end of this article, we develop another layer of understanding with the functionalities of unique reference and implement our version of it.

Design goals of unique reference

Let us list down the requirements we must satisfy for implementing our version of the unique reference. Our implementation must satisfy the following.

  • Must accept any type
  • Automatic resource management
  • Maintain the property of uniqueness
  • Pointer-like interface

Our simple version of the std::unique_ptr<T> requires to be explicitly initialized with the object it refers to.

Implementation

Let us build up our implementation by fulfilling our design requirements one by one.

Requirement: Must Accept any Type

We begin with the simplest requirement It must take any type. We simply have to add a template parameter to our type (class) for this requirement to suffice this.

template<typename T> class unique_reference;
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That's it! We satisfied our first requirement.

Requirement: Automatic Resource Management

This is the elegant solution of the C++ abstraction mechanism that comes in: the answer is Resource Acquisition Is Initialization or RAII.

RAII provides guarantees regarding the state of our class. Resource management is tied with class invariance the lifetime of an object. This is achieved by object constructors and destructors which are responsible for initialization and clean-up of resources.

template<typename T> class unique_reference{
T* m_ptr{nullptr};
public:
  unique_reference() = default;

  ~unique_reference() {
    delete m_ptr; // seems right, but dangerous!
    m_ptr = nullptr;
  }
};
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The constructor is triggered when an instance of unique_reference is initialized -- that is why in other languages like Python, and Swift, these are known as initializers. Meanwhile, destructors are triggered when an instance of unique_reference goes out of scope. Together, they serve our need for automatic resource management that gives us deterministic behavior which guarantees resource management. The deterministic behavior of our reference management gives superior property over implementing garbage collectors which may or may not clean up a piece of resource at the time an object went out of scope.

It seems that we managed our resources well. We initialized our pointer to nullptr upon construction and de-initialized it by deleting the contents of m_ptr. However, there is a subtle element of trouble that we still might leak our resources, even after setting m_ptr = nullptr. The compiler will not warn you with this, but you are shooting yourself in the foot.

To resolve this, we need to re-think our mental model of unique reference. At the time our object goes out of scope we want to destroy the object, not necessarily delete them abruptly. So we call the object's destructor ~T().

Note that types that do not have user-provided destructors such as primitives are marked as * trivial destructors *, which is why ~T() is valid for primitives.

template<typename T> class unique_reference{
T* m_ptr{nullptr};
public:
  unique_reference() = default;

    ~unique_reference(){
      if(m_ptr != nullptr){
        m_ptr->~T();
        m_ptr = nullptr;
      }
    }
};
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We're doing great. Let's keep going!

Requirement: Maintain Property of Uniqueness

For this, we want our unique reference to be able to transfer its ownership inside a function or a class when need be. By implementing the mechanism of transference, we need to maintain the property of uniqueness. We do so by incorporating move semantics. There are two parts for enabling moveable ownership, our unique reference has to be: move constructible, and move assignable.

Let us work with our moveable constructors first.

  explicit unique_reference(unique_reference<T> &&other) noexcept {
    std::swap(other.m_ptr, m_ptr);
  }
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Let us walk through what we are actually saying with the above snippet of code. First, we need to mark our constructors as explicit to avoid unexpected implicit conversions. Then we make use of * rvalue reference * for our parameter which marks our class as move constructible. But before we handle move construction, we have to guarantee that upon construction we will not throw exceptions so we mark our noexcept specifier.

The body of our function is very trivial to implement. We want to take all the resources of our moved-from reference (other), and swap it with our moved-in object (this). That said, the second requirement (move assignable) is equally trivial to implement. We simply need to overload the assignment operator.

  unique_reference &operator=(unique_reference<T> &&other) noexcept {
    std::swap(other.m_ptr, m_ptr);
    return *this;  
  }
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We are essentially repeating the body of our assignment and constructor, so for consistency, let us implement a little helper function we declare in private which shall be responsible for swapping the objects.

private:
  void swap(unique_reference<T> &other) noexcept {
    std::swap(m_ptr, other.m_ptr);
  }
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Effectively, this results to:

  explicit unique_reference(unique_reference<T> &&other) noexcept {
    other.swap(*this);
  }
  / ... /
  unique_reference& operator=(unique_reference<T> &&other) noexcept {
    other.swap(*this);
    return *this;
  }
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Since we are discussing the constructors and assignment operators it is a good point to note about how to suppress them and why. The reason we want to suppress constructors are two folds: we want to explicitly mention that we do not intend this to happen, and we want the compiler to check if users try to access such modalities.

To do this, we simply write:

  explicit unique_reference(const unique_reference<T> &Type) = delete;
  unique_reference& operator=(const unique_reference<T> &other) = delete;
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Here, we suppressed copy-assignment and copy-construction since copying will defy the meaning of being unique.

Requirement: Pointer-like Interface

We need an interface to communicate with the state of our unique reference. For consistency, it has to resemble the interface of a pointer.

Recall that a pointer can be dereferenced with * and -> operators. And we need & operator to inspect the location of our pointer in memory. These are the basic operators we need to overload for our unique reference. To do this, we write:

  T &operator*(void) { return *(this->m_ptr); }
  T *operator->(void) { return this->m_ptr; }
  T &operator&(unique_reference<T> &other) { return other.m_ptr; }
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Let us walk through the three lines.

The first line returns a reference of *(this->m_ptr) which means that the content of m_ptr is accessed that which we can modify and read. The same idea goes with the arrow operator, we return a pointer to m_ptr's location in memory. The last operator is slightly different in that it returns the address of the pointer and not the referent. Recall that a pointer has its own location in memory separate from the entities it points to.

Putting it all together:

template <typename T> class unique_reference {

public:
  T *m_ptr{nullptr};
  explicit unique_reference() = default;
  explicit unique_reference(const unique_reference<T> &Type) = delete;

  explicit unique_reference(T *Type) : m_ptr(Type) {}
  explicit unique_reference(unique_reference<T> &&other) noexcept {
    other.swap(*this);
  }

  unique_reference &operator=(const unique_reference<T> &other) = delete;
  unique_reference &operator=(unique_reference<T> &&other) noexcept {
    other.swap(*this);
    return *this;
  }

  ~unique_reference() {
    if (m_ptr != nullptr) {
      m_ptr->~T();
      m_ptr = nullptr;
    }
  }

  T &operator*(void) { return *(this->m_ptr); }
  T *operator->(void) { return this->m_ptr; }
  T &operator&(unique_reference<T> &other) { return other.m_ptr; }

private:
  void swap(unique_reference<T> &other) noexcept {
    std::swap(m_ptr, other.m_ptr);
  }
};
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Test Cases

Time to see if we satisfied our design requirements:


Summary

We fleshed out our design requirements and implemented our version of the unique reference to satisfy what we intend to do with unique references. Upon testing, we found that it sufficiently did what we want it to do: we now have the first piece of our automatic resource management!

Since that we implemented a simple unique reference class, try to think about how we can extend this for an array-like interface as your homework. Doing this part on your own gives you room to re-think what we did with our unique_reference.

Have fun hacking!


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