RE: [boost] Proposal --- A type-safe union

From: Yitzhak Sapir (yitzhaks_at_[hidden])
Date: 2002-06-12 05:02:35

> -----Original Message-----
> From: Douglas Gregor [mailto:gregod_at_[hidden]]

> When does one prefer the boost::any storage method instead of 
> the on-stack 
> method? The on-stack approach is a lightweight approach that closely 
> resembles C-style unions; in the cases where variant objects 
> are too large to 
> put on the stack, the variant objects can easily be allocated 
> on the heap 
> (just like we would do with a C-style union). The only cases 
> where on-stack 
> allocation isn't feasible occur with recursive or incomplete 
> types (discussed 
> later...). So, I would say that we should only use on-stack 
> allocation but 
> allow an 'escape' to heap allocation for recursion and 
> incomplete types.

boost::any also allows an infinite range of types that may not necessarily be incomplete.  For example, you could have a union of all types that inherit from some common base type (Shape), or a union of any instantiation of a templated type (I wonder if it is possible to check the existence of a class as part of a templated type at compile time -- perhaps using the "has member" compile time test).