RE: [Boost-Users] boost::any and operator ==

From: Jim.Hyslop (jim.hyslop_at_[hidden])
Date: 2002-08-27 08:48:18

> How do I compare two boost::any values for equality?  How to I compare
> two containers of boost::any's for equality?
> 
> Why doesn't boost::any define this operator?  There has to be 
> a reason.
There is. 

boost::any can, quite literally, hold *anything*. In order to compare the
objects being held, you must first know if they _can_ be compared. Consider:

const char * initialMessage = "Snowflakes keep falling on my head";
void f()
{
 boost::any multiA(42L);
 boost::any multiB(initialMessage);
//...

It doesn't make sense to compare the two 'any' objects. I suppose boost::any
could coerce the pointer into a long, and then apply a comparison operator,
but the results of that comparison would be meaningless.

In order to compare the objects, you'll have to extract them from the any
container using an any_cast. Which, of course, means you'll have to know the
exact type of the object being held (or which type of a known set of types).

-- 
Jim