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
17 template <typename T> class SkTLazy;
18 template <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 */
24 template <typename T> class SkTLazy {
25 public:
SkTLazy()26 SkTLazy() : fPtr(NULL) {}
27
SkTLazy(const T * src)28 explicit SkTLazy(const T* src) : fPtr(NULL) {
29 if (src) {
30 fPtr = new (fStorage) T(*src);
31 }
32 }
33
SkTLazy(const SkTLazy<T> & src)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
~SkTLazy()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 */
init()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 */
set(const T & src)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 * Destroy the lazy object (if it was created via init() or set())
79 */
reset()80 void reset() {
81 if (this->isValid()) {
82 fPtr->~T();
83 fPtr = NULL;
84 }
85 }
86
87 /**
88 * Returns true if a valid object has been initialized in the SkTLazy,
89 * false otherwise.
90 */
isValid()91 bool isValid() const { return SkToBool(fPtr); }
92
93 /**
94 * Returns the object. This version should only be called when the caller
95 * knows that the object has been initialized.
96 */
get()97 T* get() const { SkASSERT(this->isValid()); return fPtr; }
98
99 /**
100 * Like above but doesn't assert if object isn't initialized (in which case
101 * NULL is returned).
102 */
getMaybeNull()103 T* getMaybeNull() const { return fPtr; }
104
105 private:
106 friend void* operator new<T>(size_t, SkTLazy* lazy);
107
108 T* fPtr; // NULL or fStorage
109 char fStorage[sizeof(T)];
110 };
111
112 // Use the below macro (SkNEW_IN_TLAZY) rather than calling this directly
new(size_t,SkTLazy<T> * lazy)113 template <typename T> void* operator new(size_t, SkTLazy<T>* lazy) {
114 SkASSERT(!lazy->isValid());
115 lazy->fPtr = reinterpret_cast<T*>(lazy->fStorage);
116 return lazy->fPtr;
117 }
118
119 // Skia doesn't use C++ exceptions but it may be compiled with them enabled. Having an op delete
120 // to match the op new silences warnings about missing op delete when a constructor throws an
121 // exception.
delete(void *,SkTLazy<T> *)122 template <typename T> void operator delete(void*, SkTLazy<T>*) { SK_CRASH(); }
123
124 // Use this to construct a T inside an SkTLazy using a non-default constructor.
125 #define SkNEW_IN_TLAZY(tlazy_ptr, type_name, args) (new (tlazy_ptr) type_name args)
126
127 /**
128 * A helper built on top of SkTLazy to do copy-on-first-write. The object is initialized
129 * with a const pointer but provides a non-const pointer accessor. The first time the
130 * accessor is called (if ever) the object is cloned.
131 *
132 * In the following example at most one copy of constThing is made:
133 *
134 * SkTCopyOnFirstWrite<Thing> thing(&constThing);
135 * ...
136 * function_that_takes_a_const_thing_ptr(thing); // constThing is passed
137 * ...
138 * if (need_to_modify_thing()) {
139 * thing.writable()->modifyMe(); // makes a copy of constThing
140 * }
141 * ...
142 * x = thing->readSomething();
143 * ...
144 * if (need_to_modify_thing_now()) {
145 * thing.writable()->changeMe(); // makes a copy of constThing if we didn't call modifyMe()
146 * }
147 *
148 * consume_a_thing(thing); // could be constThing or a modified copy.
149 */
150 template <typename T>
151 class SkTCopyOnFirstWrite {
152 public:
SkTCopyOnFirstWrite(const T & initial)153 SkTCopyOnFirstWrite(const T& initial) : fObj(&initial) {}
154
155 // Constructor for delayed initialization.
SkTCopyOnFirstWrite()156 SkTCopyOnFirstWrite() : fObj(NULL) {}
157
158 // Should only be called once, and only if the default constructor was used.
init(const T & initial)159 void init(const T& initial) {
160 SkASSERT(NULL == fObj);
161 SkASSERT(!fLazy.isValid());
162 fObj = &initial;
163 }
164
165 /**
166 * Returns a writable T*. The first time this is called the initial object is cloned.
167 */
writable()168 T* writable() {
169 SkASSERT(fObj);
170 if (!fLazy.isValid()) {
171 fLazy.set(*fObj);
172 fObj = fLazy.get();
173 }
174 return const_cast<T*>(fObj);
175 }
176
177 /**
178 * Operators for treating this as though it were a const pointer.
179 */
180
181 const T *operator->() const { return fObj; }
182
183 operator const T*() const { return fObj; }
184
185 const T& operator *() const { return *fObj; }
186
187 private:
188 const T* fObj;
189 SkTLazy<T> fLazy;
190 };
191
192 #endif
193