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/*
* Copyright 2011 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkTLazy_DEFINED
#define SkTLazy_DEFINED
#include "SkTypes.h"
#include <new>
template <typename T> class SkTLazy;
template <typename T> void* operator new(size_t, SkTLazy<T>* lazy);
/**
* Efficient way to defer allocating/initializing a class until it is needed
* (if ever).
*/
template <typename T> class SkTLazy {
public:
SkTLazy() : fPtr(NULL) {}
explicit SkTLazy(const T* src) : fPtr(NULL) {
if (src) {
fPtr = new (fStorage) T(*src);
}
}
SkTLazy(const SkTLazy<T>& src) : fPtr(NULL) {
if (src.isValid()) {
fPtr = new (fStorage) T(*src->get());
} else {
fPtr = NULL;
}
}
~SkTLazy() {
if (this->isValid()) {
fPtr->~T();
}
}
/**
* Return a pointer to a default-initialized instance of the class. If a
* previous instance had been initialized (either from init() or set()) it
* will first be destroyed, so that a freshly initialized instance is
* always returned.
*/
T* init() {
if (this->isValid()) {
fPtr->~T();
}
fPtr = new (SkTCast<T*>(fStorage)) T;
return fPtr;
}
/**
* Copy src into this, and return a pointer to a copy of it. Note this
* will always return the same pointer, so if it is called on a lazy that
* has already been initialized, then this will copy over the previous
* contents.
*/
T* set(const T& src) {
if (this->isValid()) {
*fPtr = src;
} else {
fPtr = new (SkTCast<T*>(fStorage)) T(src);
}
return fPtr;
}
/**
* Returns true if a valid object has been initialized in the SkTLazy,
* false otherwise.
*/
bool isValid() const { return NULL != fPtr; }
/**
* Returns either NULL, or a copy of the object that was passed to
* set() or the constructor.
*/
T* get() const { SkASSERT(this->isValid()); return fPtr; }
private:
friend void* operator new<T>(size_t, SkTLazy* lazy);
T* fPtr; // NULL or fStorage
char fStorage[sizeof(T)];
};
// Use the below macro (SkNEW_IN_TLAZY) rather than calling this directly
template <typename T> void* operator new(size_t, SkTLazy<T>* lazy) {
SkASSERT(!lazy->isValid());
lazy->fPtr = reinterpret_cast<T*>(lazy->fStorage);
return lazy->fPtr;
}
// Skia doesn't use C++ exceptions but it may be compiled with them enabled. Having an op delete
// to match the op new silences warnings about missing op delete when a constructor throws an
// exception.
template <typename T> void operator delete(void*, SkTLazy<T>*) { SK_CRASH(); }
// Use this to construct a T inside an SkTLazy using a non-default constructor.
#define SkNEW_IN_TLAZY(tlazy_ptr, type_name, args) (new (tlazy_ptr) type_name args)
/**
* A helper built on top of SkTLazy to do copy-on-first-write. The object is initialized
* with a const pointer but provides a non-const pointer accessor. The first time the
* accessor is called (if ever) the object is cloned.
*
* In the following example at most one copy of constThing is made:
*
* SkTCopyOnFirstWrite<Thing> thing(&constThing);
* ...
* function_that_takes_a_const_thing_ptr(thing); // constThing is passed
* ...
* if (need_to_modify_thing()) {
* thing.writable()->modifyMe(); // makes a copy of constThing
* }
* ...
* x = thing->readSomething();
* ...
* if (need_to_modify_thing_now()) {
* thing.writable()->changeMe(); // makes a copy of constThing if we didn't call modifyMe()
* }
*
* consume_a_thing(thing); // could be constThing or a modified copy.
*/
template <typename T>
class SkTCopyOnFirstWrite {
public:
SkTCopyOnFirstWrite(const T& initial) : fObj(&initial) {}
// Constructor for delayed initialization.
SkTCopyOnFirstWrite() : fObj(NULL) {}
// Should only be called once, and only if the default constructor was used.
void init(const T& initial) {
SkASSERT(NULL == fObj);
SkASSERT(!fLazy.isValid());
fObj = &initial;
}
/**
* Returns a writable T*. The first time this is called the initial object is cloned.
*/
T* writable() {
SkASSERT(NULL != fObj);
if (!fLazy.isValid()) {
fLazy.set(*fObj);
fObj = fLazy.get();
}
return const_cast<T*>(fObj);
}
/**
* Operators for treating this as though it were a const pointer.
*/
const T *operator->() const { return fObj; }
operator const T*() const { return fObj; }
const T& operator *() const { return *fObj; }
private:
const T* fObj;
SkTLazy<T> fLazy;
};
#endif
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