skia2/include/core/SkTLazy.h

/*
    Copyright 2011 Google Inc.

    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
 */


#ifndef SkTLazy_DEFINED
#define SkTLazy_DEFINED

#include "SkTypes.h"

/**
 *  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) {
        const T* ptr = src.get();
        if (ptr) {
            fPtr = new (fStorage) T(*ptr);
        }
    }

    ~SkTLazy() {
        if (fPtr) {
            fPtr->~T();
        }
    }

    /**
     *  Return a pointer to a default-initialized instance of the class. If a
     *  previous instance had been initialzied (either from init() or set()) it
     *  will first be destroyed, so that a freshly initialized instance is
     *  always returned.
     */
    T* init() {
        if (fPtr) {
            fPtr->~T();
        }
        fPtr = new (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 (fPtr) {
            *fPtr = src;
        } else {
            fPtr = new (fStorage) T(src);
        }
        return fPtr;
    }
    
    /**
     *  Returns either NULL, or a copy of the object that was passed to
     *  set() or the constructor.
     */
    T* get() const { return fPtr; }
    
private:
    T*   fPtr; // NULL or fStorage
    char fStorage[sizeof(T)];
};

#endif