On 02/08/2013 09:28 PM, Mathias Gaunard wrote:
On 08/02/13 18:35, Jeffrey Yasskin wrote:
WebKit has a class vaguely like this for your case #2: https://code.google.com/p/chromium/codesearch/#chrome/src/third_party/WebKit...
used at https://code.google.com/p/chromium/codesearch/#chrome/src/third_party/WebKit/Source/WebCore/rendering/style/RenderStyle.h&rcl=1360310731&l=137. Semantically every copy is a real copy, and, contrary to Mathias' assertion, couldn't be replaced by a move in C++11, but they want to share identical values when doing so is cheap.
If you want identical values to use the same resource, you should use a flyweight factory.
A flyweight factory has different use cases: - Normally there is no reference count in a flyweight factory or their objects. Hence, when you want to modify an object, then a copy is made in every case, even if there is only one copy. Therefore, we can see, that a flyweight factory does usually not implement copy-on-write and can be less efficient in the case that one single object is modified regularly. - If there's a single factory in a multi threaded environment, then synchronization can be a real bottleneck. This is not the case for cow_ptr<T> which works lock free. Hence flyweight factories have different use cases and performance characteristics. COPY-ON-WRITE HAS ITS OWN VALID USE CASES. A cow_ptr<T> template class can be utilized to implement copy-on-write for different situations. With cow_ptr<T> come some other nice use-cases such as - using it as a pimpl pointer without the need to implement copy and move constructors and assignments or the destructor, - adding cloning to a type hierarchy non-intrusively and - building arrays of polymorphic types with value semantics. (See the file cow_ptr.h in the github repository <https://github.com/ralphtandetzky/cow_ptr.git> for details.) Sometimes these use-cases mix. I've seen it in my own production code. Believe me, there are legit use-cases of cow_ptr. My questions are: 1. Would this fit into the boost libraries? 2. What improvements can I make to the current design and implementation? I would really appreciate constructive feedback.