Question

There are two ways in which I can easily make a key,value attribution in C++ STL: maps and sets of pairs. For instance, I might have

map<key_class,value_class>

or

set<pair<key_class,value_class> >

In terms of algorithm complexity and coding style, what are the differences between these usages?

Answer 1

Set elements cannot be modified while they are in the set. set‘s iterator and const_iterator are equivalent. Therefore, with set<pair<key_class,value_class> >, you cannot modify the value_class in-place. You must remove the old value from the set and add the new value. However, if value_class is a pointer, this doesn’t prevent you from modifying the object it points to.

With map<key_class,value_class>, you can modify the value_class in-place, assuming you have a non-const reference to the map.