Question

What is the alternative if I need to use a reference, and the data I am passing I cannot change the type of, hence I cannot really store a pointer to it?

Code:

    #include <map>     
    #include<iostream>
    #include<string>     

    using namespace std;

    int main()
    {
       string test;
       pair<string, string> p=pair<string, string>("Foo","Bar");
       map<pair<string, string>, string&> m;
       m[make_pair("aa","bb")]=test;

       return 0;
}

Error:

$ g++ MapPair.cpp
/usr/include/c++/3.2.3/bits/stl_map.h:
In instantiation of
std::map<std::pair<std::string,
std::string>, std::string&,
std::less<std::pair<std::string,
std::string> >,
std::allocator<std::pair<const
std::pair<std::string, std::string>,
std::string&> > >': MapPair.cpp:15:
instantiated from here
/usr/include/c++/3.2.3/bits/stl_map.h:221:
forming reference to reference type
std::string&’ MapPair.cpp: In
function int main()': MapPair.cpp:16:
no match for
std::map, std::string&,
std::less >,
std::allocator,
std::string&> > >& [std::pair]’ operator
/usr/include/c++/3.2.3/bits/stl_pair.h:
At global scope:
/usr/include/c++/3.2.3/bits/stl_pair.h:
In instantiation of std::pair<const
std::pair<std::string, std::string>,
std::string&>':
/usr/include/c++/3.2.3/bits/stl_tree.h:122:
instantiated from
std::_Rb_tree_node

What am I doing wrong to cause this errror?

Answer 1

You cannot store references. References are just aliases to another variable.

The map needs a copy of the string to store:

map<pair<string, string>, string> m;

The reason you are getting that particular error is because somewhere in map, it’s going to do an operation on the mapped_type which in your case is string&. One of those operations (like in operator[], for example) will return a reference to the mapped_type:

mapped_type& operator[](const key_type&)

Which, with your mapped_type, would be:

string&& operator[](const key_type& _Keyval)

And you cannot have a reference to a reference:

Standard 8.3.4:

There shall be no references to references, no arrays of references, and no pointers to references.

---

On a side note, I would recommend you use typedef‘s so your code is easier to read:

int main()
{
    typedef pair<string, string> StringPair;
    typedef map<StringPair, string> StringPairMap;

    string test;

    StringPair p("Foo","Bar");
    StringPairMap m;
    m[make_pair("aa","bb")] = test;

   return 0;

}