Question

Suppose some data structure:

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

If I use the field data.s as key when adding instances of data in a map of type std::map<std::string&, data>, do the string gets copied? Is it safe to erase an element of the map because the reference will become invalid?

Also do the answers to these questions also apply to an unordered_map?

EDIT:

This is my current solution… but adding iterator to the map is UGLY:

typedef struct {
    const std::string* s;
    int i;
} data;

std::map<std::string, data> map;
typedef std::map<std::string, data>::iterator iterator;

// add an element to the map
iterator add_element(const std::string& s) {
    std::pair<iterator, bool> p = states.insert(std::make_pair(s, data()));
    iterator i = p.first;
    if(p.second) {
        data& d = (*i).second;
        d.s = &(*i).first;
    }
    return i;
}

Answer 1

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>