Ok! Thanks! I got it... The problem is, that I thought that SFINAE would work from the template class in the template function which does not depend on the argument.
is_const is a member of the traits_class which this type is derived from... Pretty much like:
template<class T>
struct traits_class
{
// some type calcs here
typedef ... result_type;
enum { is_const = false };
};
template<class T>
struct traits_class<const T>
{
// some type calcs here
typedef ... result_type;
enum { is_const = true };
};
template<class T>
struct some_class : traits_class<T>
{
template<class LookUpType_>
typename boost::disable_if_c<is_const, result_type>::type
at(LookUpType_ const& key)
{
return has_key(view_, key); //has key is some external function, which receives the result_type from the view_
}
};
My idea was to enable only
result_type at(LookUpType_ const& key) const;
if T is const
OR
enable non-const version.
result_type at(LookUpType_ const& key);
if T is not const
On the other hand Compiler does this for me automatically if T is const (but if T is non-const I would like to disable the const access).
Anyway in that case I thought SFINAE should work, but it did not. Do I really need a type to be a template param of the member function, even if the class is a template itself?
Thanks for your help!
Ovanes
On 10/31/07, David Abrahams <dave@boost-consulting.com> wrote:
on Tue Oct 30 2007, "Ovanes Markarian" <om_boost-AT-keywallet.com> wrote:
> Hello!
>
> Does anybody else experience a problem using enable_if templates in Visual C++ Express 2005 SP1?
> I get compiler errors in this code:
>
> template<class LookUpType_>
> typename boost::enable_if_c<is_const, result_type>::type
> at(LookUpType_ const& key)const
> {
> return has_key(view_, key);
> }
>
> template<class LookUpType_>
> typename boost::disable_if_c<is_const, result_type>::type
> at(LookUpType_ const& key)
> {
> return has_key(view_, key);
> }
>
> It blames that type is not a member of enable_if_c or disable_if_c?
> This is really strance, since this should not be considered by
> compiler as a possible template instantiation. is_const is an enum
> value containing either true or false.
In order for SFINAE to work, at least one argument to [dis|en]able_if
has to depend on a template argument (in this case, LookUpType_ is the
only one). As written, none of them do.
I don't know what you're doing with is_const, but at least in the
Boost world that is a template that requires a template argument in
that context. For example,
template<class LookUpType_>
typename boost::disable_if_c<boost::is_const<LookUpType_>, result_type>::type
at(LookUpType_ const& key)
{
return has_key(view_, key);
}
HTH,
--
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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