Storing references in a map

Question

I tried to store a foo object into a std::reference_wrapper, but I end up with a compiler error I don't understand.

#include <functional>
#include <map>

struct foo
{
};

int main()
{
    std::map< int, std::reference_wrapper< foo > > my_map;
    foo a;
    my_map[ 0 ] = std::ref( a );
}

The compiler error is pretty lengthy, but it boils down to this:

error: no matching function for call to ‘std::reference_wrapper<foo>::reference_wrapper()’

What exactly am I doing wrong?

Answer 1

std::reference_wrapper is not default-constructible (otherwise it would be a pointer).

my_map[0]

creates, if 0 is not already a key in the map, a new object of the mapped type, and for this the mapped type needs a default constructor. If your mapped type is not default-constructible, use insert():

my_map.insert(std::make_pair(0, std::ref(a)));

or emplace():

my_map.emplace(0, std::ref(a));