Re: [boost] [parameter] Binding type.

From: Rene Rivera (grafik.list_at_[hidden])
Date: 2006-03-06 20:15:12

Daniel Wallin wrote:
> I don't think the rationale for this is completely expressed in the
> docs. The reason binding<> works like this is because of dangling
> references to defaults. Consider from your example above:
> 
>   something_t const & q = args[something | 0];
> 
> When binding<> returns the default type here you will get a dangling
> reference here:
> 
>   int const& = args[something | 0]
> 
> Our design handles this by letting binding<> return a reference type
> when an argument is bound to the keyword, and otherwise return the
> default type unchanged.

OK I understand what you are saying, but I don't see it :-) As far as I 
understand from the docs the binding<>::type has nothing to do with the 
default value as above as it happens before one gets to extracting the 
value. I wasn't talking about changing the types that the operator[] 
return. And also from the docs, the binding<> function has it's own way 
of declaring what the default type should be, which could be different 
than the default value AFAICT.

Are you saying that operator[] uses the binding<>::type, instead of 
using the internal "reference" type. Looking at code again... Ah yes it 
does, but not for all compilers, in some it returns the reference type. 
I think I would consider that a bug ;-)

But given that, would not what I suggested of having 
binding<>::value_type, binding<>::const_reference, binding<>::reference, 
etc. And then either having the operator[] use the binding<>::reference. 
  Or another less incompatible change would be to just make those extra 
binding types available. And the users could use the value_type in most 
cases.

> I would consider adding a binding_value<> metafunction though.

Adding types to the binding<> function seems easier to me. But then I 
didn't write the code :-)

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