Obtain void* pointer to content of boost::any

Question

I am using an external library that has a method which accepts a void*

I want this void* to point to an object contained within a boost::any object.

Is it possible to get at the address of the content of a boost::any object?

I'm trying to play with myAny.content but no luck so far! I'm hoping some combination of dynamic_cast or unsafe_any_cast will give me what I need.

Thanks!

Answer 1

This isn't possible, unfortunately; boost::any_cast will refuse the cast if the type is distinct from the contained type.

If you're willing to use an unsupported internal hack, the current version of the header has an undocumented and unsupported function boost::unsafe_any_cast which (as its name suggests) bypasses the type check performed by boost::any_cast:

boost::any any_value(value);
void *content = boost::unsafe_any_cast<void *>(&any_value);

The header has this to say about unsafe_any_cast:

// Note: The "unsafe" versions of any_cast are not part of the
// public interface and may be removed at any time. They are
// required where we know what type is stored in the any and can't
// use typeid() comparison, e.g., when our types may travel across
// different shared libraries.

Answer 2

You can use boost::any_cast to get a pointer to the underlying type (provided you know it at compile time).

boost::any any_i(5);

int* pi = boost::any_cast<int>(&any_i);
*pi = 6;

void* vpi = pi;