[bodyfour]

In the early days of std::string multi-threading was much rarer, so reference counting was cheaper. As CPU count grows the relative cost of needing an atomic refcount on every string also grew.

The COW strings also were a constant source of surprising behavior. As with any COW object you have to make sure that the copy happens before and write. Simple enough, but the STL-style interface does not support this well. All you had to do is call operator[] or begin() on a non-const std::string and it would force a copy.

This meant that something that to the programmer clearly looked like a read-only operation like:

    if (str[2] == 'x') { ...}

...could end up copying the string. Worse, it also meant that any previously taken references are now invalidated. This lead to horrible bugs where you would hold a pointer at the previous copy of the string (which you no longer own a refcount on) and it would almost always work... but every so often another thread would come by and reuse the memory behind your back.

So COW std::string was a long festering mess in C++. Their removal in C++11 was completely warranted.