refcnt.hh

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#ifndef __BASE_REFCNT_HH__
#define __BASE_REFCNT_HH__

#include <type_traits>

class RefCounted
{
  private:
    // The reference count is mutable because one may want to
    // reference count a const pointer.  This really is OK because
    // const is about logical constness of the object not really about
    // strictly disallowing an object to change.
    mutable int count;

  private:
    // Don't allow a default copy constructor or copy operator on
    // these objects because the default operation will copy the
    // reference count as well and we certainly don't want that.
    RefCounted(const RefCounted &);
    RefCounted &operator=(const RefCounted &);

  public:
    RefCounted() : count(0) {}

    virtual ~RefCounted() {}

    void incref() const { ++count; }

    void decref() const { if (--count <= 0) delete this; }
};

template <class T>
class RefCountingPtr
{
  public:
    using PtrType = T*;

  protected:
    static constexpr auto TisConst = std::is_const<T>::value;
    using ConstT = typename std::conditional<TisConst,
            RefCountingPtr<T>,
            RefCountingPtr<typename std::add_const<T>::type>>::type;
    friend ConstT;
    using NonConstT = typename std::conditional<TisConst,
            RefCountingPtr<typename std::remove_const<T>::type>,
            RefCountingPtr<T>>::type;
    friend NonConstT;
    T *data;

    void
    copy(T *d)
    {
        data = d;
        if (data)
            data->incref();
    }

    void
    del()
    {
        if (data)
            data->decref();
    }

    void
    set(T *d)
    {
        // Need to check if we're actually changing because otherwise
        // we could delete the last reference before adding the new
        // reference.
        if (data != d) {
            del();
            copy(d);
        }
    }

  public:
    RefCountingPtr() : data(0) {}

    RefCountingPtr(T *data) { copy(data); }

    RefCountingPtr(const RefCountingPtr &r) { copy(r.data); }

    RefCountingPtr(RefCountingPtr&& r)
    {
        data = r.data;
        r.data = nullptr;
    }

    template <bool B = TisConst>
    RefCountingPtr(const NonConstT &r) { copy(r.data); }

    ~RefCountingPtr() { del(); }

    // The following pointer access functions are const because they
    // don't actually change the pointer, though the user could change
    // what is pointed to.  This is analagous to a "Foo * const".

    T *operator->() const { return data; }

    T &operator*() const { return *data; }

    T *get() const { return data; }

    template <bool B = TisConst>
    operator RefCountingPtr<typename std::enable_if<!B, ConstT>::type>()
    {
        return RefCountingPtr<const T>(*this);
    }

    const RefCountingPtr &operator=(T *p) { set(p); return *this; }

    const RefCountingPtr &operator=(const RefCountingPtr &r)
    { return operator=(r.data); }

    const RefCountingPtr &operator=(RefCountingPtr&& r)
    {
        /* This happens regardless of whether the pointer is the same or not,
         * because of the move semantics, the rvalue needs to be 'destroyed'.
         */
        del();
        data = r.data;
        r.data = nullptr;
        return *this;
    }

    bool operator!() const { return data == 0; }

    operator bool() const { return data != 0; }
};

template<class T>
inline bool operator==(const RefCountingPtr<T> &l, const RefCountingPtr<T> &r)
{ return l.get() == r.get(); }

template<class T>
inline bool operator==(const RefCountingPtr<T> &l, const T *r)
{ return l.get() == r; }

template<class T>
inline bool operator==(const T *l, const RefCountingPtr<T> &r)
{ return l == r.get(); }

template<class T>
inline bool operator!=(const RefCountingPtr<T> &l, const RefCountingPtr<T> &r)
{ return l.get() != r.get(); }

template<class T>
inline bool operator!=(const RefCountingPtr<T> &l, const T *r)
{ return l.get() != r; }

template<class T>
inline bool operator!=(const T *l, const RefCountingPtr<T> &r)
{ return l != r.get(); }

#endif // __BASE_REFCNT_HH__