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

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::result_type is "int const&"; that is, a reference.

and:

numeric::converter::argument_type is also "int const&"

<snip>

A "fix" on your side would be to use ::boost::numeric::converter::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 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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