Reference as key in std::map

Question

Suppose some data structure:

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

If I use the field data.s as key when adding i

Answer 1

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

map  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;
};