[boost] Re: unions

From: Eric Friedman (ebf_at_[hidden])
Date: 2003-10-07 03:38:32

E. Gladyshev wrote:

> --- Eric Friedman <ebf_at_[hidden]> wrote:
> [...]
> 
>>To check each type Ti in variant<T1,...,TN> would add O(n) compile-time 
>>complexity. To check only the first type T1 adds only O(1) complexity.
> 
> 
> I thought that you have to check each type anyway 
> to find the largest type, isn't it true?

Yes, it's true that each type must be checked. So perhaps "complexity" 
is not the correct word to use. Nonetheless, compile-time *performance* 
will be hampered, as there is a difference between O(N) and O(2N) when 
waiting for compilation to finish, even if they are both in theory O(N).

Further, while I perhaps understand your complaint that variant checks 
only T1 for a nothrow default constructor, it would not really buy you 
anything in a generic context if variant behaved similarly for any Ti 
with nothrow default-construction. That is,

   template <typename U1, ..., typename UN>
   class my_class
   {
     variant<U1, ..., UN> var_;
   };

Unless a precondition for use of my_class is that one of its arguments 
Ui must be nothrow default-constructible, you'd still be in the same 
boat even if variant checked every type. That is, you would have no way 
to ensure heap allocation was avoided in general.

Eric