Re: [Boost-users] Re: numeric/conversion library question

From: Toon Knapen (toon.knapen_at_[hidden])
Date: 2005-05-30 05:45:02

Fernando Cacciola wrote:

<snip>

> int const& convert( int const& s ) { return s ; }
> 
> Notice that it takes a reference and returns a reference.
> 
> As such,
> 
> numeric::converter<int,int>::result_type is "int const&"; that is, a 
> reference.
> 
> and:
> 
> numeric::converter<int,int>::argument_type is also "int const&"
> 

<snip>

> 
> A "fix" on your side would be to use 
> ::boost::numeric::converter<Target,Source>::argument_type;
> but this isn't practical because users are forced to specify "Source" 
> explicitely.

but in your example above you say that both 'result_type' and 
'argument_type' result in a 'int const&' ? So replacing it would'nt make 
  sens? I suppose 'argument_type' results in 'int', right ?

> 
> So you need "Source" to be the source type, but then, you also _need_ 
> "Target" to be the result type:
> 
> template<typename Target, typename Source>
> inline Target convert2 ( Source arg )
> {
>   return ::boost::numeric::converter< Target, Source >::convert( arg ) ;
> }
> 
> Incidentally, boost::numeric_cast<> (the new one) is equivalent to your code 
> so is just as equally buggy.
> 
> I'm afraid I have to patch it not to use result_type anymore.
> 
> P.S: One could thing that the traits class, in the case of a trivial 
> conversion, could define bare types instead of references (that is, making 
> result_type being just "T" instead of "T const&). Altough this OK for 
> builtin types, it isn't for the UDT, and it is precisly in the case of UDTs 
> were the "reduction to a no-op in the trivial case" becomes really 
> important. This means though that numeric_cast<> is better not used with 
> UDTs in generic context when T==S is possible (because the optimization is 
> then lost)
> 
> P.S.2: Of course the alternative is to use "Source const&" instead of 
> "Source"... I don't really remember the details but I seem to recall that 
> the original numeric_cast<> used a non-reference argument for some good 
> reason; and so I just kept that.
> 
> 
> Thanks for the report!
> 
> Fernando Cacciola
> SciSoft
> 
> 
> 
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