Edd Dawson wrote:
Hi again, Kirit!
Kirit Sælensminde wrote:
That should force a string constructor only at the end of the chain, the rest of the time passing a pointer.
Based on your suggestion I've come up with a nice and simple solution. Rather than doing a single copy at the end I can do once at the beginning, using this crazy looking contraption:
template<typename T> class ref_once_copied { public: ref_once_copied(const T &obj) : obj_(new T(obj)), referenced_(false) { } ~ref_once_copied() { if (referenced_) delete obj_; } operator T &() { referenced_ = true; return *obj_; }
private: T *obj_; bool referenced_; };
That looks like a pretty smart move. The first question I had was about using types so as to eliminate the bool. I.e. arrange for it to return a slightly different type, maybe via a function used to wrap this class. Now that I'm looking at it again though I'm not sure that I completely understand your implementation. It looks to me like you're always forcing a copy (in the constructor) so what job does referenced_ do? I can see that it has value if you move the copy from the constructor into the type cast (the operator T &()), but as it is in the constructor if the reference isn't taken for some reason then it will leak a T. It looks like you've changed your mind about the semantics half way through implementing it.
I started out using a boost::shared_ptr<> for the obj_ member, but I realised that approach outlined above would work just as well and also avoid the extra overhead that the reference counting entails.
I agree with this. In a multi-threaded environment those interlocked increments and decrements aren't cheap. A policy based implementation might be nice where we can provide the right threading semantics to the shared_ptr would be cool.
So I just wanted to say thanks! I wouldn't have gone down this road if it wasn't for your suggestion. Perhaps you'll find a use for this in your implementation?
I certainly will! :-) You're the same Edd that has a C++ JPEG library as well aren't you? I've bookmarked it and will have a play with that once I get a chance. K