SkTLazy.h revision bb5c46591c50d05418467cd1c4e927ceb85c2ba9 
1
2/*
3 * Copyright 2011 Google Inc.
4 *
5 * Use of this source code is governed by a BSD-style license that can be
6 * found in the LICENSE file.
7 */
8
9
10
11#ifndef SkTLazy_DEFINED
12#define SkTLazy_DEFINED
13
14#include "SkTypes.h"
15#include <new>
16
17template <typename T> class SkTLazy;
18template <typename T> void* operator new(size_t, SkTLazy<T>* lazy);
19
20/**
21 *  Efficient way to defer allocating/initializing a class until it is needed
22 *  (if ever).
23 */
24template <typename T> class SkTLazy {
25public:
26    SkTLazy() : fPtr(NULL) {}
27
28    explicit SkTLazy(const T* src) : fPtr(NULL) {
29        if (src) {
30            fPtr = new (fStorage) T(*src);
31        }
32    }
33
34    SkTLazy(const SkTLazy<T>& src) : fPtr(NULL) {
35        if (src.isValid()) {
36            fPtr = new (fStorage) T(*src->get());
37        } else {
38            fPtr = NULL;
39        }
40    }
41
42    ~SkTLazy() {
43        if (this->isValid()) {
44            fPtr->~T();
45        }
46    }
47
48    /**
49     *  Return a pointer to a default-initialized instance of the class. If a
50     *  previous instance had been initialized (either from init() or set()) it
51     *  will first be destroyed, so that a freshly initialized instance is
52     *  always returned.
53     */
54    T* init() {
55        if (this->isValid()) {
56            fPtr->~T();
57        }
58        fPtr = new (SkTCast<T*>(fStorage)) T;
59        return fPtr;
60    }
61
62    /**
63     *  Copy src into this, and return a pointer to a copy of it. Note this
64     *  will always return the same pointer, so if it is called on a lazy that
65     *  has already been initialized, then this will copy over the previous
66     *  contents.
67     */
68    T* set(const T& src) {
69        if (this->isValid()) {
70            *fPtr = src;
71        } else {
72            fPtr = new (SkTCast<T*>(fStorage)) T(src);
73        }
74        return fPtr;
75    }
76
77    /**
78     *  Returns true if a valid object has been initialized in the SkTLazy,
79     *  false otherwise.
80     */
81    bool isValid() const { return NULL != fPtr; }
82
83    /**
84     * Returns the object. This version should only be called when the caller
85     * knows that the object has been initialized.
86     */
87    T* get() const { SkASSERT(this->isValid()); return fPtr; }
88
89    /**
90     * Like above but doesn't assert if object isn't initialized (in which case
91     * NULL is returned).
92     */
93    T* getMaybeNull() const { return fPtr; }
94
95private:
96    friend void* operator new<T>(size_t, SkTLazy* lazy);
97
98    T*   fPtr; // NULL or fStorage
99    char fStorage[sizeof(T)];
100};
101
102// Use the below macro (SkNEW_IN_TLAZY) rather than calling this directly
103template <typename T> void* operator new(size_t, SkTLazy<T>* lazy) {
104    SkASSERT(!lazy->isValid());
105    lazy->fPtr = reinterpret_cast<T*>(lazy->fStorage);
106    return lazy->fPtr;
107}
108
109// Skia doesn't use C++ exceptions but it may be compiled with them enabled. Having an op delete
110// to match the op new silences warnings about missing op delete when a constructor throws an
111// exception.
112template <typename T> void operator delete(void*, SkTLazy<T>*) { SK_CRASH(); }
113
114// Use this to construct a T inside an SkTLazy using a non-default constructor.
115#define SkNEW_IN_TLAZY(tlazy_ptr, type_name, args) (new (tlazy_ptr) type_name args)
116
117/**
118 * A helper built on top of SkTLazy to do copy-on-first-write. The object is initialized
119 * with a const pointer but provides a non-const pointer accessor. The first time the
120 * accessor is called (if ever) the object is cloned.
121 *
122 * In the following example at most one copy of constThing is made:
123 *
124 * SkTCopyOnFirstWrite<Thing> thing(&constThing);
125 * ...
126 * function_that_takes_a_const_thing_ptr(thing); // constThing is passed
127 * ...
128 * if (need_to_modify_thing()) {
129 *    thing.writable()->modifyMe(); // makes a copy of constThing
130 * }
131 * ...
132 * x = thing->readSomething();
133 * ...
134 * if (need_to_modify_thing_now()) {
135 *    thing.writable()->changeMe(); // makes a copy of constThing if we didn't call modifyMe()
136 * }
137 *
138 * consume_a_thing(thing); // could be constThing or a modified copy.
139 */
140template <typename T>
141class SkTCopyOnFirstWrite {
142public:
143    SkTCopyOnFirstWrite(const T& initial) : fObj(&initial) {}
144
145    // Constructor for delayed initialization.
146    SkTCopyOnFirstWrite() : fObj(NULL) {}
147
148    // Should only be called once, and only if the default constructor was used.
149    void init(const T& initial) {
150        SkASSERT(NULL == fObj);
151        SkASSERT(!fLazy.isValid());
152        fObj = &initial;
153    }
154
155    /**
156     * Returns a writable T*. The first time this is called the initial object is cloned.
157     */
158    T* writable() {
159        SkASSERT(NULL != fObj);
160        if (!fLazy.isValid()) {
161            fLazy.set(*fObj);
162            fObj = fLazy.get();
163        }
164        return const_cast<T*>(fObj);
165    }
166
167    /**
168     * Operators for treating this as though it were a const pointer.
169     */
170
171    const T *operator->() const { return fObj; }
172
173    operator const T*() const { return fObj; }
174
175    const T& operator *() const { return *fObj; }
176
177private:
178    const T*    fObj;
179    SkTLazy<T>  fLazy;
180};
181
182#endif
183