Question

Its known that Qt widgets use implicit sharing. So I am interested if stl containers std::vector, std::string use implicit sharing too.

If no, why? Since it is very useful.

And if the answer is yes, how we can ascertain in it? I need simple C++ stl program which shows that stl containers use implicit sharing. It doesn’t do deep copy when is copied.

Answer 1

No. They cannot. When you try to modify the contents of the container, or even calling a mutable begin() on it, it would imply a potential copy-on-write and thus invalidate all references and iterators to the container. This would be a hard to debug situation, and it is prohibited.

Although std::string is technically not a container, it is still prohibited to do copy-on-write since C++11:

References, pointers, and iterators referring to the elements of a basic_string sequence may be invalidated by the following uses of that basic_string object:
…
— Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.

[string.require]

… Since it is very useful.

Heh, what for? Passing by reference almost always solves all ‘performance problems’. Atomic ref-counts are inherently non-scalable on multi-processors machines.