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?

What's the difference between set<pair> and map in C++?

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>

set<pair<key_class,value_class> >

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