Why do I get an error in "forming reference to reference type" map?

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

Essentially, the question is if you can use references in containers. Of course, you can, IF you properly prepare your class AND your container. I demonstrate it below with two simple vector containers: vectoref which modifies std::vector<> and the other, vec, which is implemented from scratch.

#include <iostream>
#include <vector>

// requires compilation with --std=c++11 (at least)

using namespace std;

class A {
  int _a; // this is our true data
  A *_p; // this is to cheat the compiler

  public:
  A(int n = 0) : _a(n), _p(0)
  { cout << "A constructor (" << this << "," << _a << ")\n"; }
  // constructor used by the initializer_list (cheating the compiler)
  A(const A& r) : _p(const_cast<A *>(&r))
  { cout << "A copy constructor (" << this << "<-" << &r << ")\n"; }
  void print() const {cout << "A instance: " << this << "," << _a << "\n";}
  ~A() {cout << "A(" << this << "," << _a << ") destructor.\n";}
  // just to see what is copied implicitly
  A& operator=(const A& r) {
    cout << "A instance copied (" << this << "," << _a << ")\n";
    _a = r._a; _p = r._p;
    return *this;
  }
  // just in case you want to check if instance is pure or fake
  bool is_fake() const {return _p != 0;}
  A *ptr() const {return _p;}
};

template<typename T, int sz>
class vec { // vector class using initializer_list of A-references!!
  public:
  const T *a[sz]; // store as pointers, retrieve as references
  // because asignment to a reference causes copy operator to be invoked
  int cur;
  vec() : cur(0) {}
  vec(std::initializer_list<T> l) : cur(0) {
    cout << "construct using initializer list.\n";
    for (auto& t : l) // expecting fake elements
      a[cur++] = t.ptr();
  }
  const T& operator[](int i) {return *a[i];}
  // expecting pure elements
  vec& push_back(const T& r) {a[cur++] = &r; return *this;}
  void copy_from(vec&& r) {
    for (int i = 0; i < r.cur; ++i)
      push_back(r[i]);
  }
};

template<typename T>
class vectoref : public vector<T *> { // similar to vec but extending std::vector<>
  using size_type = typename vector<T*>::size_type;
  public:
  vectoref() {}
  vectoref(std::initializer_list<T> l) {
    cout << "construct using initializer list.\n";
    for (auto& t : l) // expecting fake elements
      vector<T*>::push_back(t.ptr());
  }
  const T& operator[](size_type i) {return *vector<T*>::at(i);}
  // expecting pure elements
  vectoref& push_back(const T& r)
  { vector<T*>::push_back(&r); return *this; }
  void copy_from(const vectoref&& r) {
    for (size_type i = 0; i < r.size(); ++i)
      vectoref<T>::push_back(r[i]);
  }
};

class X { // user of initializer_list of A
  public:
  X() {}
  void f(initializer_list<A> l) const {
    cout << "In f({...}):\n";
    for (auto& a : l)
      a.ptr()->print();
  }
};

int main()
{
  A a(7), b(24), c(80);
  cout << "----------------------------------\n";
  vectoref<A> w{a,a,b,c}; // alternatively, use next line
  // vec<A,5> w{a,a,b,c}; // 5-th element undefined
  w[0].print();
  w[3].print();
  cout << "----------------------------------\n";
  X x;
  x.f({a,b,c,a,b,c,b,a});
  cout << "==================================\n";
  return 0;
}

Answer 2

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;

}

Answer 3

The previous answers here are outdated. Today we have std::reference_wrapper as part of the C++11 standard:

#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>, reference_wrapper<string>> m;
    m[make_pair("aa", "bb")] = test;

    return 0;
}

A std::reference_wrapper will convert implicitly to a reference to its internal type, but this doesn't work in some contexts, in which case you call .get() for access.

http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper

Answer 4

You can use boost::reference_wrapper to store references in STL containers. Here is your example modified (not tested, and definitely not very well written, just illustrates a point)

#include <map>     
#include<iostream>
#include<string>   
#include <boost/ref.hpp>



int main()
{
   typedef std::pair< std::string, std::string> PairType;
   typedef std::map< PairType, boost::reference_wrapper<std::string> > MapType;
   std::string test = "Hello there!!";
   MapType m;
   PairType pp =  std::make_pair("aa","bb");
   m.insert(std::make_pair(pp , boost::ref(test) ) );

   MapType::iterator it (m.find( pp ) );
   if(it != m.end())
   {
       std::cout << it->second.get() << std::endl;
   }

   //change test
   test = "I am different now";
   std::cout << it->second.get() << std::endl;

   return 0;
}

Answer 5

You cannot use references as the val, due to how the template is built. You could also use pointer instead.