Re: Creating a std::map of references

"Victor Bazarov" <>
Fri, 27 Jul 2007 17:07:17 -0400
relain wrote:

I was hoping to use to replace a map of pointers to objects with a map
of references instead, since the pointers way of doing what i want to
do is proving to be a bit tiresome. I'm still pretty new to C++ but i
know that a reference itself is no object, and so you can't stuff it
into a container. I came upon the boost::reference_wrapper template
and although i had success with using this in a std::vector i have no
luck with a map. If i understand the compiler errors this is because
there's no default constructor for reference_wrapper<thing> which
seems to be sensible, how can you have a reference to a non thing?

So my questions are:

1 - Have i just boobed and not actually understood the output from the

No, you got it right.

2 - If i haven't should i inherit reference_wrapper and bodge it up to
make it work, with some kind of default container with something like
a NULL reference, this is bad because then really im just using
pointers. Is there a better way? Or should i just turn back to the C-
side and make my pointers work.

You don't need a default constructor for the type you store in 'map'
if you never use the indexing operator with keys that don't exist.

the test code i was using to play around with it is:


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

class test_content{
test_content(int my_id){id = my_id;};
int id;

struct ltstr{
bool operator()(int thing1, int thing2){
if (thing1 < thing2){
} else {


int main (void){
std::map<int, boost::reference_wrapper<test_content>, ltstr>
test_content test_thing(5);
the_map[3] = boost::reference_wrapper<test_content>(test_thing);

Don't do that. Do

    the_map.insert(make_pair(3, test_thing));



And g++ (4.0.1 darwin) says:


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