#ifndef __BASE_REFCNT_HH__
#define __BASE_REFCNT_HH__
+/**
+ * @file base/refcnt.hh
+ *
+ * Classes for managing reference counted objects.
+ */
+
+/**
+ * Derive from RefCounted if you want to enable reference counting of
+ * this class. If you want to use automatic reference counting, you
+ * should use RefCountingPtr<T> instead of regular pointers.
+ */
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:
RefCounted &operator=(const RefCounted &);
public:
+ /**
+ * We initialize the reference count to zero and the first object
+ * to take ownership of it must increment it to one.
+ *
+ * @attention A memory leak will occur if you never assign a newly
+ * constructed object to a reference counting pointer.
+ */
RefCounted() : count(0) {}
+
+ /**
+ * We make the destructor virtual because we're likely to have
+ * virtual functions on reference counted objects.
+ *
+ * @todo Even if this were true, does it matter? Shouldn't the
+ * derived class indicate this? This only matters if we would
+ * ever choose to delete a "RefCounted *" which I doubt we'd ever
+ * do. We don't ever delete a "void *".
+ */
virtual ~RefCounted() {}
+ /// Increment the reference count
void incref() { ++count; }
+
+ /// Decrement the reference count and destroy the object if all
+ /// references are gone.
void decref() { if (--count <= 0) delete this; }
};
+/**
+ * If you want a reference counting pointer to a mutable object,
+ * create it like this:
+ * @code
+ * typedef RefCountingPtr<Foo> FooPtr;
+ * @endcode
+ *
+ * @attention Do not use "const FooPtr"
+ * To create a reference counting pointer to a const object, use this:
+ * @code
+ * typedef RefCountingPtr<const Foo> ConstFooPtr;
+ * @endcode
+ *
+ * These two usages are analogous to iterator and const_iterator in the stl.
+ */
template <class T>
class RefCountingPtr
{
protected:
+ /// The stored pointer.
+ /// Arguably this should be private.
T *data;
- void copy(T *d)
+ /**
+ * Copy a new pointer value and increment the reference count if
+ * it is a valid pointer. Note, this does not delete the
+ * reference any existing object.
+ * @param d Pointer to store.
+ */
+ void
+ copy(T *d)
{
data = d;
if (data)
data->incref();
}
- void del()
+
+ /**
+ * Delete the reference to any existing object if it is non NULL.
+ * @attention this doesn't clear the pointer value, so a double
+ * decref could happen if not careful.
+ */
+ void
+ del()
{
if (data)
data->decref();
}
- void set(T *d)
- {
- if (data == d)
- return;
- del();
- copy(d);
+ /**
+ * Drop the old reference and change it to something new.
+ */
+ 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:
+ /// Create an empty reference counting pointer.
RefCountingPtr() : data(0) {}
+
+ /// Create a new reference counting pointer to some object
+ /// (probably something newly created). Adds a reference.
RefCountingPtr(T *data) { copy(data); }
+
+ /// Create a new reference counting pointer by copying another
+ /// one. Adds a reference.
RefCountingPtr(const RefCountingPtr &r) { copy(r.data); }
+
+ /// Destroy the pointer and any reference it may hold.
~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".
+
+ /// Access a member variable.
T *operator->() const { return data; }
+
+ /// Dereference the pointer.
T &operator*() const { return *data; }
+
+ /// Directly access the pointer itself without taking a reference.
T *get() const { return data; }
+ /// Assign a new value to the pointer
const RefCountingPtr &operator=(T *p) { set(p); return *this; }
+
+ /// Copy the pointer from another RefCountingPtr
const RefCountingPtr &operator=(const RefCountingPtr &r)
{ return operator=(r.data); }
+ /// Check if the pointer is empty
bool operator!() const { return data == 0; }
+
+ /// Check if the pointer is non-empty
operator bool() const { return data != 0; }
};
+/// Check for equality of two reference counting pointers.
template<class T>
inline bool operator==(const RefCountingPtr<T> &l, const RefCountingPtr<T> &r)
{ return l.get() == r.get(); }
+/// Check for equality of of a reference counting pointers and a
+/// regular pointer
template<class T>
inline bool operator==(const RefCountingPtr<T> &l, const T *r)
{ return l.get() == r; }
+/// Check for equality of of a reference counting pointers and a
+/// regular pointer
template<class T>
inline bool operator==(const T *l, const RefCountingPtr<T> &r)
{ return l == r.get(); }
+/// Check for inequality of two reference counting pointers.
template<class T>
inline bool operator!=(const RefCountingPtr<T> &l, const RefCountingPtr<T> &r)
{ return l.get() != r.get(); }
+/// Check for inequality of of a reference counting pointers and a
+/// regular pointer
template<class T>
inline bool operator!=(const RefCountingPtr<T> &l, const T *r)
{ return l.get() != r; }
+/// Check for inequality of of a reference counting pointers and a
+/// regular pointer
template<class T>
inline bool operator!=(const T *l, const RefCountingPtr<T> &r)
{ return l != r.get(); }
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