On 09/02/13 09:49, Ralph Tandetzky wrote:
Under your assertions the STL is just as broken. For example std::vector<T>::push[_]back() might change the address of the contained data and therefore invalidate all pointers and iterators to it.
That is intrinsic to this data structure. Modifying the size of a sequence is a whole other thing than modifying its elements.
For client code of a class using cow_ptrs for data members internally it is even easier not to worry about making copies, but the class automatically does it for you. cow_ptr helps to implement that behaviour.
So you admit the only rationale for your cow_ptr is to support sloppy programming practices. Making everything slow and unnatural just to make it "easier for the user".
It can be an implementation-specific detail and can make code faster without the client code knowing about it. But it can also be a thing the client code relies upon as with the Matrix class above. Sometimes client code might want to know, if copies can be made cheaply.
A client should never rely on copying not being a 0(n) operation, because it is. Don't want to copy? Don't copy. Now if you want to add an option to turn your copies into copy-on-writes to measure there is any performance gain, why not. But that's just a pessimization that may become an optimization in certain cases, it's not a general approach.