How to copy references of elements of one vector to another vector?

Question

I have a vector:

std::vector<island> sea;

And now I want to hold pointers to all elements of this vector in another vector. But just for a personal exercise I want to do this in a fancy C++ style, so instead of std::vector<const island*> p_sea I type:

std::vector<std::reference_wrapper<const island>> r_sea;

And now I'd like to populate this new vector with references:

std::transform(sea.begin(), sea.end(),
               std::back_inserter(r_sea),
               std::cref<island>
);

The way I understand it, from cppreference articles, the fourth argument of transform should be a function that takes const references to elements from the source range and returns elements of the destination range; and this is exactly what std::cref<island> does: it takes const island& as arguments and returns std::reference_wrapper<const island>. So I believe this should work??

However, it doesn't:

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
#include <iterator>


struct island {
    long long x,y; //coords
};

int main() {
    std::vector<island> sea;
    std::vector<std::reference_wrapper<const island>> r_sea;

    std::transform(sea.begin(), sea.end(),
                   std::back_inserter(r_sea),
                   std::cref<island>
    );

    return 0;
}

This results in the following compilation errors:

prog.cpp: In function ‘int main()’:
prog.cpp:19:5: error: no matching function for call to ‘transform(std::vector<island>::iterator, std::vector<island>::iterator, std::back_insert_iterator<std::vector<std::reference_wrapper<const island> > >, <unresolved overloaded function type>)’
     );
     ^
In file included from /usr/include/c++/6/algorithm:62:0,
                 from prog.cpp:3:
/usr/include/c++/6/bits/stl_algo.h:4166:5: note: candidate: template<class _IIter, class _OIter, class _UnaryOperation> _OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation)
     transform(_InputIterator __first, _InputIterator __last,
     ^~~~~~~~~
/usr/include/c++/6/bits/stl_algo.h:4166:5: note:   template argument deduction/substitution failed:
prog.cpp:19:5: note:   could not resolve address from overloaded function ‘cref<island>’
     );
     ^
In file included from /usr/include/c++/6/algorithm:62:0,
                 from prog.cpp:3:
/usr/include/c++/6/bits/stl_algo.h:4203:5: note: candidate: template<class _IIter1, class _IIter2, class _OIter, class _BinaryOperation> _OIter std::transform(_IIter1, _IIter1, _IIter2, _OIter, _BinaryOperation)
     transform(_InputIterator1 __first1, _InputIterator1 __last1,
     ^~~~~~~~~
/usr/include/c++/6/bits/stl_algo.h:4203:5: note:   template argument deduction/substitution failed:
prog.cpp:19:5: note:   could not resolve address from overloaded function ‘cref<island>’
     );

https://ideone.com/E80WXH

What am I doing wrong?

...I'm back to evil C pointers.

Answer 1

std::cref is overloaded. Merely specifying the template argument is not enough to disambiguate between the overloads. You have two options:

1. Cast it

   static_cast<std::reference_wrapper<const island>(*)(const island&)>(std::cref<island>)
   

2. Lift the name up to a functor object (a lambda). like @Yakk proposed.