Re: [Boost-users] [boost-users] enable_if and checking if class derives from boost::operators does not work.

From: David Abrahams (dave_at_[hidden])
Date: 2007-10-15 15:52:11

on Fri Oct 12 2007, "chun ping wang" <cablepuff-AT-gmail.com> wrote:

> namespace boost
> {
>        template <class T, class U>
>         char (& is_op_test(boost::operators<T,U> const&) )[2];
>
>         template <class T>
>         char is_op_test(T const&);
>
>         template<class T>
>         T const& make();
>
>         template <class T>
>         struct is_operators
>          : boost::mpl::bool_<(sizeof(is_op_test(make<T>())) == 2)>
>         {};
> }

You should not be creating definitions in namespace boost.  Also,
I'll advise against checking for derivation from operators<T,U>;
There's probably a much better way to do what you are trying to do,
whatever it is.

> and this function declaration.
> template <typename T,
>      template <typename ELEM, typename = std::allocator<ELEM> >
>      class CONT
>>
> typename boost::enable_if<boost::mpl::or_<boost::is_integral<T>, boost:: is_operators<T> >, CONT
> <T> >::type
> primelist(const T& MAX_BOUND, const T& K, boost::function<bool(const T&, const T&)> f);
>
> BigInteger class declaration looks like this..
>
> class BigInteger : public BigUnsigned, boost::operators<BigInteger>,
> boost::operators<BigInteger, int>
> {
> };
>
> I compile with T = int and it works.
>
> When i compile with T = BigInteger I get the following errors.
>
> on visual c++.
> error C2770: invalid explicit template argument(s) for 'boost::enable_if<boost::mpl::or_
> <boost::is_integral<T>,boost::is_operators<T> >,CONT<T>>::type primelist(const T &,const T
> &,boost::function<bool(const T &,const T &)>)'
>         ./cs512/c++/primeMain.cpp(24) : see declaration of 'primelist'"
>
> on mingw dev-cpp i get this error.
> unable to find function....
>
> In both cases passing BigInteger does not work.

You haven't shown the call that produces the error, and you haven't
shown the full error.

I suggest reducing your problem to a *minimal* complete example (one
.cpp file) and posting that.

The most obvious problem you have is that instantiating
is_operator<BigInteger> is likely to cause an overloading ambiguity
because BigInteger is derived from two different operator<T,U>
specializations.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com