The primary reason when deciding whether a type should be mutable or not is psychological: mutable things are containers with independent identity from their co

The primary reason when deciding whether a type should be mutable or not is psychological: mutable things are containers with independent identity from their content; things that are identified by their value should be immutable. The prototypical mutable type is an array; the prototypical immutable types are numbers: if you change the imaginary part of a complex number, you don't have the same number with different content, you have a different number. When you think about strings as arrays of bytes like you do in C, it makes sense for them to be mutable; in a higher level language where they behave much more like atomic values, it makes much more sense for them to be immutable – it can be really jarring when some called code deep down in the guts of a program mutates a string and you see the results at the top level. In C++, which is somewhere between a low level and high level language, it's hard to say which way it should be, but the STL approach does seem to treat them more like values than containers, which implies that they probably should be immutable.