It was possible with copy on write strings (e.g. libstdc++'s strings until C++11), but C++11 added complexity requirements that outlawed COW strings, because th

It was possible with copy on write strings (e.g. libstdc++'s strings until C++11), but C++11 added complexity requirements that outlawed COW strings, because they're only really useful in very specific circumstances, at the cost of making the common case slow.