enable_if and templated operator() of member of class template

MM

1 Mar 2012 1 Mar '12

9:42 a.m.

Hello, I am using the enable_if construct to disable an operator() template member function of a templated variant visitor: template <typename iter_t> class iterator_compare_visitor_ : public boost::static_visitor<bool> { public: iterator_compare_visitor_(const iter_t& to_compare_to) : to_compare_to_(to_compare_to) {} template <typename variant_iter_t> typename std::enable_if< boost::mpl::contains::value, bool >::type operator()(const variant_iter_t& it) const { return std::is_same::value && it==to_compare_to_; } template <typename variant_iter_t> typename std::enable_if< boost::mpl::contains::value, bool >::type operator()(const variant_iter_t& it) const { return std::is_same::value && it==to_compare_to_.base(); } private: const iter_t& to_compare_to_; }; The compilation fails in msvc2010 with: error C2039: 'type' : is not a member of 'std::tr1::enable_if<_Test,_Type>' 1> with 1> [ 1> _Test=false, 1> _Type=bool 1> ] in this case, iter_t is not contained in const_reverse_iterators_t and so the 2nd operator() should not be enabled. Rds, MM

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Philipp Moeller

1 Mar 1 Mar

2:40 p.m.

New subject: enable_if and templated operator() of member of class template

"MM" writes:

...

Hello,

I am using the enable_if construct to disable an operator() template member function of a templated variant visitor:

template <typename iter_t> class iterator_compare_visitor_ : public boost::static_visitor<bool> { public: iterator_compare_visitor_(const iter_t& to_compare_to) : to_compare_to_(to_compare_to) {}

template <typename variant_iter_t> typename std::enable_if< boost::mpl::contains::value, bool >::type operator()(const variant_iter_t& it) const { return std::is_same::value && it==to_compare_to_; }

template <typename variant_iter_t> typename std::enable_if< boost::mpl::contains::value, bool >::type operator()(const variant_iter_t& it) const { return std::is_same::value && it==to_compare_to_.base(); }

private: const iter_t& to_compare_to_; };

The compilation fails in msvc2010 with:

error C2039: 'type' : is not a member of 'std::tr1::enable_if<_Test,_Type>' 1> with 1> [ 1> _Test=false, 1> _Type=bool 1> ]

in this case, iter_t is not contained in const_reverse_iterators_t and so the 2nd operator() should not be enabled.

Rds,

MM

What you are trying cannot work as this is not a valid way to overload operator(). Consider using enable_if as an argument to your functions: http://codepad.org/nduBfhT6 As for the actual error: It looks like your compiler is using std::tr1::enable_if even if you specify `std::enable_if`. Those should be equivalent. You might want to use boost::enable_if. Also, please try to provide snippets that don't require modifications to show the bug (e.g. includes). HTH, Philipp Moeller

Jeremiah Willcock

4:48 p.m.

New subject: enable_if and templated operator() of member of class template

On Thu, 1 Mar 2012, MM wrote:

...

Hello,

I am using the enable_if construct to disable an operator() template member function of a templated variant visitor:

template <typename iter_t> class iterator_compare_visitor_ : public boost::static_visitor<bool> { public: iterator_compare_visitor_(const iter_t& to_compare_to) : to_compare_to_(to_compare_to) {}

template <typename variant_iter_t> typename std::enable_if< boost::mpl::contains::value, bool >::type operator()(const variant_iter_t& it) const { return std::is_same::value && it==to_compare_to_; }

template <typename variant_iter_t> typename std::enable_if< boost::mpl::contains::value, bool >::type operator()(const variant_iter_t& it) const { return std::is_same::value && it==to_compare_to_.base(); }

private: const iter_t& to_compare_to_; };

The compilation fails in msvc2010 with:

error C2039: 'type' : is not a member of 'std::tr1::enable_if<_Test,_Type>' 1> with 1> [ 1> _Test=false, 1> _Type=bool 1> ]

The main issue here is that the condition in enable_if does not depend on the template parameter to operator(); see the version in http://codepad.org/6AORo7tU, in which I've used boost::enable_if and added some tests. One fix is to make the enable_if condition dependent, as in http://codepad.org/916Tl6W8 (there were a number of other issues in the code; note that the is_same condition needs to be in the enable_if since the body of the function will not always compile when that is false). Please see if that last paste works for you. -- Jeremiah Willcock

MM

2 Mar 2 Mar

6:27 p.m.

New subject: enable_if and templated operator() of member of class template

...

-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Jeremiah Willcock Sent: 01 March 2012 16:48 To: boost-users@lists.boost.org Subject: Re: [Boost-users] enable_if and templated operator() of member of class template

On Thu, 1 Mar 2012, MM wrote: The main issue here is that the condition in enable_if does not depend on the template parameter to operator(); see the version in http://codepad.org/6AORo7tU, in which I've used boost::enable_if and added some tests. One fix is to make the enable_if condition dependent, as in http://codepad.org/916Tl6W8 (there were a number of other issues in the code; note that the is_same condition needs to be in the enable_if since the body of the function will not always compile when that is false). Please see if that last paste works for you.

-- Jeremiah Willcock

Thank you, http://codepad.org/916Tl6W8 compiled. Using std::enable_if as well. (msvc10 uses their TR1 impl). I need an extra check for the 2nd operator() of the visitor, that variant_iter_t is actually identical to iter_t::Iterator embedded type. Basically, it treats the case where iter_t is a reverse_iterator of the iterators in my variant, and we need to check that we're applying visitation on the proper iterator. So line 43: is_same should be the problem is that this operator is instantiated even for the case where iter_t is a const_iterator and not a const_reverse_iterator and compilation fails, because const_iterator has no embedded type Iterator. How to fix? rds, MM

Jeremiah Willcock

7:29 p.m.

New subject: enable_if and templated operator() of member of class template

On Fri, 2 Mar 2012, MM wrote:

...

...
-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Jeremiah Willcock Sent: 01 March 2012 16:48 To: boost-users@lists.boost.org Subject: Re: [Boost-users] enable_if and templated operator() of member of class template

On Thu, 1 Mar 2012, MM wrote: The main issue here is that the condition in enable_if does not depend on the template parameter to operator(); see the version in http://codepad.org/6AORo7tU, in which I've used boost::enable_if and added some tests. One fix is to make the enable_if condition dependent, as in http://codepad.org/916Tl6W8 (there were a number of other issues in the code; note that the is_same condition needs to be in the enable_if since the body of the function will not always compile when that is false). Please see if that last paste works for you.

-- Jeremiah Willcock

Thank you, http://codepad.org/916Tl6W8 compiled. Using std::enable_if as well. (msvc10 uses their TR1 impl).

I need an extra check for the 2nd operator() of the visitor, that variant_iter_t is actually identical to iter_t::Iterator embedded type. Basically, it treats the case where iter_t is a reverse_iterator of the iterators in my variant, and we need to check that we're applying visitation on the proper iterator. So line 43: is_same should be

the problem is that this operator is instantiated even for the case where iter_t is a const_iterator and not a const_reverse_iterator and compilation fails, because const_iterator has no embedded type Iterator.

How to fix?

Try this version instead: http://codepad.org/uA7oflvk. It uses a helper template to do the dispatching among the overloads. -- Jeremiah Willcock

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Jeremiah Willcock
MM
Philipp Moeller