Re: [boost] Variant (aka Discriminated Union) implementation

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-06-24 19:26:02

On Monday 24 June 2002 06:02 pm, Eric Friedman wrote:
> Despite having received no feedback <g>, I continued to work on my
> boost::variant implementation over the weekend. The product of my labors
> is a variant that has been tested successfully with GCC 2.9.5 and MSVC
> 7.

I meant to send feedback to the first version, but time went on and it never 
happened... so here's some feedback on the newer version. There are a few 
points I'm concerned about:

1) Why is it that the variant can be unbounded? I'm having a hard time 
justifying a variant type that contains a value that is probably one of the 
specified types, but might be something else. Here's a nightmare scenario:

  typedef boost::variant<int, float, string>::unbounded<true>::type MyVariant;
  // ...

  MyVariant v(3.14159); // think it's holding a float? nope...
  
  Bounding the variant completely has the wonderful property that many uses of 
the variant can check for completeness of decoding. So I guess there are two 
questions: why do we need the ability to unbound a variant, and how would 
this differ from the existing boost::any?

2) Why use the vtable idiom when the variant is completely bounded? The 
vtables are quite a bit larger than the single byte needed to specify which 
type of the variant is selected, and I don't see any benefit to the vtable 
approach.

3) I'd really like to see the ability to construct from a type convertible to 
some value that the variant can store, e.g.,

  boost::variant<int, float, string> v(3.14159); // convert double to float

I'm glad to see work on a variant type. It's been discussed a million times, 
but we've never had a variant type come up for review (and I've needed one 
for a LONG time).

        Doug