[boost] Re: boost::any: explicit constructor?

From: Kevlin Henney (kevlin_at_[hidden])
Date: 2004-02-28 14:39:57

In message <uptbzrvmf.fsf_at_[hidden]>, David Abrahams 
<dave_at_[hidden]> writes
>>
>> T * to void *, derived * to base *, int (32 bit) to double (64 bit), T
>> * to const T *, etc. Widening conversions are in the direction of the
>> supertype, hence they often have a subset of the interface of source
>> type. The corresponding narrowing of the interface with the widening
>> of the type is not always the case, eg an int to long is effectively
>> interface preserving -- the same set of operators is supported for
>> both types.
>
>I guess I don't understand the whole concept now.  T* -> void* sounds
>like "narrowing", since it loses information.  int -> long never loses
>information, so "widening" makes sense to me.  What is the criterion?

All widening conversions lose type (ie interface and usage) information 
as a trade for widening the range or number of things that they can 
refer to. In the case of int to long, you have lost the information that 
it was originally an int, and could fit into an int. In the case of 
derived * to base * you have lost the information that you were pointing 
to something that was at least a derived. In the case of T * to void * 
you have lost the information that you were dealing with something that 
was at least a T.

In both cases we are dealing with "individuals" and we are widening the 
range or number of individuals we can deal with or refer to. In the 
int-to-long case we are dealing with values, so the notion of set 
containment and its relationship to the widen/narrow metaphor is most 
obvious here. When dealing with pointers we tend to focus on the 
entities that we are referring to, rather than on the pointer value 
itself. A void * can refer to a larger set of objects than a given T *, 
a base * can refer to a larger set of objects than a derived *. However, 
the same conclusion can be reached by considering the pointers as values 
rather than as handles to entities: there are more legal values that a 
void * can take in a program than a T *.

Hope that clarifies the terminology.

Kevlin
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