boost::any confusion with pointers vs values

It took me a while to figure this out, but the semantics of boost::any are confusing.

With value types, you use it like so:

int value = 100;
boost::any something;
something = value;
//...later...
int value = boost::any_cast<int>(&something);

This code is clear and makes sense, but stores value internally as a copy. This means for larger objects I place inside boost::any, they will be copied. Also any functions that I replace void* with this will expect that the value outside the function is modified when I modify the value contained in the boost::any object (which won't happen, since it copied it).

So if I put pointers into it, things get weird:

int value = 100;
boost::any something;
something = &value;
//...later...
int* value = *boost::any_cast<int*>(&something);

I have to dereference the return value in this case because boost::any_cast returns int**! I also haven't checked but I think this may crash if something.empty() == true. This is just not straightforward at all.

I do not want to store values in my boost::any, I want it to only function on pointers and behave semantically closer to void*. Pointers in, pointers out, with some type-safety mixed in. Essentially what I want is boost::any_pointer, or something like that. Is there a way to prohibit boost::any from accepting anything except pointers? And if not, is there an alternative to boost::any that can give me the semantics I'm looking for?

Solution

You are using any_cast wrong:

There are essentially two (three) flavors.

Taking a reference to any and returning a value or reference to the content
Taking a pointer to any and returning a pointer to the content

Examples:

#include <boost/any.hpp>
int main()
{
    // Any holding a value
    {
        boost::any any_value(1);
        // Throws bad_any_cast if the content is not 'int' (in this case):
        int  value = boost::any_cast<int>(any_value);
        // Throws bad_any_cast if the content is not 'int' (in this case):
        int& reference = boost::any_cast<int&>(any_value);
        // Returns a null pointer if the content is not 'int' (in this case):
        int* pointer = boost::any_cast<int>(&any_value);
    }

    // Any holding a pointer (which is nothing else but a value)
    {
        int integer = 0;
        boost::any any_ptr(&integer);
        // Throws bad_any_cast if the content is not 'int*' (in this case):
        int * pointer = boost::any_cast<int*>(any_ptr);
        // Throws bad_any_cast if the content is not 'int*' (in this case):
        int*& pointer_reference = boost::any_cast<int*&>(any_ptr);
        // Returns a null pointer if the content is not 'int*' (in this case):
        int** pointer_pointer = boost::any_cast<int*>(&any_ptr);
    }
}