What does the STL map[key] return if the key wasn’t a initialized key in the map? [duplicate]

A default constructed std::string ins inserted into the std::map with key 'b' and a reference to that is returned.

It is often useful to consult the documentation, which defines the behavior of operator[] as:

Returns a reference to the object that is associated with a particular key. If the map does not already contain such an object, operator[] inserts the default object data_type().

(The SGI STL documentation is not documentation for the C++ Standard Library, but it is still an invaluable resource as most of the behavior of the Standard Library containers is the same or very close to the behavior of the SGI STL containers.)