Re: [boost] [conversion] ADL and templates

Subject: Re: [boost] [conversion] ADL and templates
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-05-30 17:05:38

On Mon, May 30, 2011 at 1:51 PM, Vicente BOTET <vicente.botet_at_[hidden]>wrote:

> Hi,
>
> I've tried to add a configuration point in Boost.Conversion via overloading
> and ADL, but while the internal call of the library can find them using a
> trick that adds a parameter that contains the type of the return type, the
> user is not able to find them without using the same this trick, but this is
> too dark to be natural for user code.
>
> Next follows a simplified code of the problem. The following
>
> namespace N {
> struct S{};
>
> template (*typename T*)
> T fct1(S const& s) { return T(s); }
> template (*typename T*)
> void fct2(T &t, S const& s) { t=s; }
> }
>
> void test ()
> {
> N::S s1;
> N::S s2;
> fct2(s2, s1);
> s2=fct1(*N::S*)(s1); // (1)
> }
>
> Results in a compile error in (1)
>
> error: ‘fct1’ was not declared in this scope
>
> Is it standard behavior that fct1 can not be found by ADL?
>
> Thanks,
> Vicente
>
> P.S. Please replace (* and *) by angle brackets.
>

AFAIK, this is standard behavior.  Explicitly providing template parameters
to a function call (as well as namespace qualification and putting the
function name in parentheses) disables ADL.  I don't think you can provide
an extension framework for Boost.Conversion without the result type
appearing somewhere in the customization function's function parameters.

I don't really view that as any "darker" than, say, using Boost.EnableIf, or
any of a number of other TMP hacks...they are all constrained from a
(perhaps) more ideal syntax and form by the constraints of the language.

- Jeff