Reference as key in std::map

C++11

Since C++11 reference wrapper is part of standard.

#include <functional> 

std::map<std::reference_wrapper<std::string>, data>

Using Boost

You may want to take a look at boost.ref. It provides a wrapper that enables references to be used in STL-containers like this:

std::map<boost::reference_wrapper<std::string>, data>

You can't store references in Standard Library containers - your map should look like:

map <string,data> mymap;

The map will manage both the key string and the struct instances, which will be copies, for you. Both map and unordered_map work in the same way in this regard, as do all other Standard Library containers.

Note that in C++, you don't need typedefs to declare structs:

struct data {
    std::string s;
    int i;
};