Re: SFINAE help

From:

Tim Hockin <thockin@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 18 Aug 2009 12:35:24 -0700 (PDT)

Message-ID:

<cf3fd0ed-1bf0-42ec-9c3e-2b5aa603c4cf@y4g2000prf.googlegroups.com>

On Aug 18, 10:18 am, Maxim Yegorushkin <maxim.yegorush...@gmail.com>
wrote:

Tim Hockin wrote:

On Aug 18, 6:08 am, Maxim Yegorushkin <maxim.yegorush...@gmail.com>
wrote:

Tim H wrote:

I have a template class 'Property' that has a ValueType typedef in it=

I have a method of another class that looks like this:
template<class Tkey>
const typename Tkey::ValueType &GetProperty() const { ...=

}

I want to support Properties with void ValueType. I added a
specialization of Property and that works. what fails is when code
calls the above GetProperty<VoidProperty>() method. I get a "no
matching function" error (g++).
It looks like SFINAE is correctly determining that a 'const void &' i=

an error. The error message is not very helpful, though.
How can I use SFINAE (or other tricks) to make calls to
GetProperty<VoidProperty>() blow up in a more obvious manner?

There is no SFINAE involved because you instantiate the function templ=

ate

_explicitly_.

SFINAE works while the compiler is building a set of viable function o=

verloads.

In doing so it also tries to _implicitly_ instantiate function templat=

es. If

function template instantiation fails when the template arguments are
substituted with particular types, that Substitution Failure Is Not An=

Error,

the function template is simply disregarded and no compiler error prod=

uced.

Your example can be made to work without SFINAE. Something like that:

     template<class T>
     struct MakeRefToConst
     {
         typedef T const& type;
     };

     template<>
     struct MakeRefToConst<void>
     {
         typedef void type; // don't make references to void
     };

cute!

template<class Tkey>
typename MakeRefToConst<typename Tkey::ValueType>::type Get=

Property();

This is a class method, in my situation, which may matter.

This is about the return value, so it does not make a difference whether =

it is a

standalone or member function.

> The

question I have is - what does the body of this function look like?

This is for you to decide.

The generic implementation of this ends with

return specific->GetValue();

on which the compiler complains about "return-statement with a value,
in function returning 'void'"

It means that GetProperty() return type is void, but the return type of
GetValue() is not void.

Ahh, that was the key. I missed one place where I cut-and-pasted.
Fixed and it works.

Thanks

> How can I provide an alternate body for void ValueType that does not
> try to return a value?

In this case an alternative body may be unnecessary, since static_cast<> =

can

cast any value to void. So, your last statement can do:

     return static_cast<
         typename MakeRefToConst<
             typename Tkey::ValueType
         >::type>(specific->GetValue());

If Tkey::ValueType happens to be void this cast just discards the return =

value

of specific->GetValue().

--
Max