cdmh

21.4.1 p6 states invalidation of iterators/references is only allowed for — as an argument to any standard library function taking a reference to non-const basic_string as an argument. — Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.

The non-const operator[] in a COW string requires making a copy of the string if the ref count > 1 which invalidates references and violates this paragraph.

The class is movable already. Reference counting will allow a copy constructor or copy assignment to avoid copying the string object, just increase the reference count.

COW is prohibited in C++11 Standard implementations of std::basic_string

That is accurate. The object reference by immutable msg is hello_world which is mutable. If the value of hello_world changes, then the value of msg changes too. In the simple example, it won't happen, but the point is that it could, especially when multi-threaded.

Hi Alex, you say I "claim[s] that objects accessed via reference to const can modify themselves". Which paragraph are you referring to? It don't see that? I'm happy to update the article to make it clearer. Thanks, Craig

I'm toying with implementing O(1) copies by using a reference counted pointer to basic_string instead of storing it by value within the class. It would mean a small memory overhead and another level of indirection, but probably worth doing. Thoughts?