refcnt.hh revision 8221:8b5f900233ee 
1/*
2 * Copyright (c) 2002-2005 The Regents of The University of Michigan
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions are
7 * met: redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer;
9 * redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution;
12 * neither the name of the copyright holders nor the names of its
13 * contributors may be used to endorse or promote products derived from
14 * this software without specific prior written permission.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
17 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
18 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
19 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
20 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
26 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 *
28 * Authors: Nathan Binkert
29 */
30
31#ifndef __BASE_REFCNT_HH__
32#define __BASE_REFCNT_HH__
33
34/**
35 * @file base/refcnt.hh
36 *
37 * Classes for managing reference counted objects.
38 */
39
40/**
41 * Derive from RefCounted if you want to enable reference counting of
42 * this class.  If you want to use automatic reference counting, you
43 * should use RefCountingPtr<T> instead of regular pointers.
44 */
45class RefCounted
46{
47  private:
48    // The reference count is mutable because one may want to
49    // reference count a const pointer.  This really is OK because
50    // const is about logical constness of the object not really about
51    // strictly disallowing an object to change.
52    mutable int count;
53
54  private:
55    // Don't allow a default copy constructor or copy operator on
56    // these objects because the default operation will copy the
57    // reference count as well and we certainly don't want that.
58    RefCounted(const RefCounted &);
59    RefCounted &operator=(const RefCounted &);
60
61  public:
62    /**
63     * We initialize the reference count to zero and the first object
64     * to take ownership of it must increment it to one.
65     *
66     * @attention A memory leak will occur if you never assign a newly
67     * constructed object to a reference counting pointer.
68     */
69    RefCounted() : count(0) {}
70
71    /**
72     * We make the destructor virtual because we're likely to have
73     * virtual functions on reference counted objects.
74     *
75     * @todo Even if this were true, does it matter?  Shouldn't the
76     * derived class indicate this?  This only matters if we would
77     * ever choose to delete a "RefCounted *" which I doubt we'd ever
78     * do.  We don't ever delete a "void *".
79     */
80    virtual ~RefCounted() {}
81
82    /// Increment the reference count
83    void incref() { ++count; }
84
85    /// Decrement the reference count and destroy the object if all
86    /// references are gone.
87    void decref() { if (--count <= 0) delete this; }
88};
89
90/**
91 * If you want a reference counting pointer to a mutable object,
92 * create it like this:
93 * @code
94 * typedef RefCountingPtr<Foo> FooPtr;
95 * @endcode
96 *
97 * @attention Do not use "const FooPtr"
98 * To create a reference counting pointer to a const object, use this:
99 * @code
100 * typedef RefCountingPtr<const Foo> ConstFooPtr;
101 * @endcode
102 *
103 * These two usages are analogous to iterator and const_iterator in the stl.
104 */
105template <class T>
106class RefCountingPtr
107{
108  protected:
109    /// The stored pointer.
110    /// Arguably this should be private.
111    T *data;
112
113    /**
114     * Copy a new pointer value and increment the reference count if
115     * it is a valid pointer.  Note, this does not delete the
116     * reference any existing object.
117     * @param d Pointer to store.
118     */
119    void
120    copy(T *d)
121    {
122        data = d;
123        if (data)
124            data->incref();
125    }
126
127    /**
128     * Delete the reference to any existing object if it is non NULL.
129     * @attention this doesn't clear the pointer value, so a double
130     * decref could happen if not careful.
131     */
132    void
133    del()
134    {
135        if (data)
136            data->decref();
137    }
138
139    /**
140     * Drop the old reference and change it to something new.
141     */
142    void
143    set(T *d)
144    {
145        // Need to check if we're actually changing because otherwise
146        // we could delete the last reference before adding the new
147        // reference.
148        if (data != d) {
149            del();
150            copy(d);
151        }
152    }
153
154  public:
155    /// Create an empty reference counting pointer.
156    RefCountingPtr() : data(0) {}
157
158    /// Create a new reference counting pointer to some object
159    /// (probably something newly created).  Adds a reference.
160    RefCountingPtr(T *data) { copy(data); }
161
162    /// Create a new reference counting pointer by copying another
163    /// one.  Adds a reference.
164    RefCountingPtr(const RefCountingPtr &r) { copy(r.data); }
165
166    /// Destroy the pointer and any reference it may hold.
167    ~RefCountingPtr() { del(); }
168
169    // The following pointer access functions are const because they
170    // don't actually change the pointer, though the user could change
171    // what is pointed to.  This is analagous to a "Foo * const".
172
173    /// Access a member variable.
174    T *operator->() const { return data; }
175
176    /// Dereference the pointer.
177    T &operator*() const { return *data; }
178
179    /// Directly access the pointer itself without taking a reference.
180    T *get() const { return data; }
181
182    /// Assign a new value to the pointer
183    const RefCountingPtr &operator=(T *p) { set(p); return *this; }
184
185    /// Copy the pointer from another RefCountingPtr
186    const RefCountingPtr &operator=(const RefCountingPtr &r)
187    { return operator=(r.data); }
188
189    /// Check if the pointer is empty
190    bool operator!() const { return data == 0; }
191
192    /// Check if the pointer is non-empty
193    operator bool() const { return data != 0; }
194};
195
196/// Check for equality of two reference counting pointers.
197template<class T>
198inline bool operator==(const RefCountingPtr<T> &l, const RefCountingPtr<T> &r)
199{ return l.get() == r.get(); }
200
201/// Check for equality of of a reference counting pointers and a
202/// regular pointer
203template<class T>
204inline bool operator==(const RefCountingPtr<T> &l, const T *r)
205{ return l.get() == r; }
206
207/// Check for equality of of a reference counting pointers and a
208/// regular pointer
209template<class T>
210inline bool operator==(const T *l, const RefCountingPtr<T> &r)
211{ return l == r.get(); }
212
213/// Check for inequality of two reference counting pointers.
214template<class T>
215inline bool operator!=(const RefCountingPtr<T> &l, const RefCountingPtr<T> &r)
216{ return l.get() != r.get(); }
217
218/// Check for inequality of of a reference counting pointers and a
219/// regular pointer
220template<class T>
221inline bool operator!=(const RefCountingPtr<T> &l, const T *r)
222{ return l.get() != r; }
223
224/// Check for inequality of of a reference counting pointers and a
225/// regular pointer
226template<class T>
227inline bool operator!=(const T *l, const RefCountingPtr<T> &r)
228{ return l != r.get(); }
229
230#endif // __BASE_REFCNT_HH__
231