How do I initialize boost::any with a reference to an object?
Question
I want to store a reference to an object in a boost::any
object. How do I initialize the boost::any object? I tried std::ref()
, but boost::any
gets initialized with std::reference_wrapper<>
. For example, the following
#include <boost/any.hpp>
#include <cxxabi.h>
#include <iostream>
int main(void)
{
int s;
int i = 0;
boost::any x(std::ref(i));
std::cout << abi::__cxa_demangle(x.type().name(), 0, 0, &s) << "\n";
return 0;
}
prints
std::reference_wrapper<int>
I want the boost::any
to contain int&
instead.
Solution
The boost::any
class doesn't have an interface allowing something like this: you would need to specify the type of the reference with the constructor. I don't think that you can explicitly specify the type of templated constructor because I don't see any place you could stick it. Even if you can explicitly specify the template parameter, it wouldn't work in C++2003 bcause there is no reference collapsing available and the parameter is declared as taking a T const&
: you'd be trying to create a T& const&
which won't fly.
I think your best option is to either use std::reference_wrapper<T>
if you insist on something looking remotely reference like or just to use T*
.
That said, it would be generally possible to have a templatized static factor method of a type similar to boost::any
which would be used to explicitly specify the template argument. However, since boost::any
is deliberately designed to deal with value types this isn't done. I'm a bit dubious whether it should be done as well: using a pointer is perfectly good alternative. If you really need a reference type you'll probably have to implement it yourself.
OTHER TIPS
The behaviour is correct, expected and appropriate. std::ref
is a helper function that creates an object of type std::reference_wrapper<T>
, and the reference wrapper is a class with value semantics that holds a reference -- that's exactly the sort of thing you want to put into a container if you want the container to track outside references.
So just go with the solution you have.
If you will, you cannot have a container of direct, naked references, much like you cannot have an array of references. The wrapper is designed precisely to accommodate such needs.