Re: [Boost-users] Concept check vs enable_if

11 Aug 2008

      On Mon, Aug 11, 2008 at 9:33 AM, joel falcou  wrote:
...
I was trying to use it as a simple traits to see if a given type fullfilled
the result_of protocol
whatever the arguments used. It works ok for the result_type variant but now
I see it's still incomplete. The problem is that I can't see, for an
arbitrary result structure, how I can see if it actually has at least one
overload that has a proper nested type typedef.
I  can't either :-)
...
Typical use is :
For F a given type,  support_result_of_protocol<F>::type evaluates as a
compile time boolean indicating if either F has a nested result structure or
result_type typedef. It doesn't make any assumptions on wether or nor result
is correcly implemented.
As for testing if a particuliar set of arguments produces a valid result_of,
one can easily do :
template< class F
        , template<class> class T = boost::result_of<F>::type
        > struct valid_impl
{
  typedef void type;
};
template
struct produce_valid_result_of : boost::false_type {};
template<class F>
struct produce_valid_result_of :
boost::true_type {};
or did I miss something in your question ?
I think you got my question, but that won't work.  If you fix it to:

template< class F
        , typename T = typename boost::result_of<F>::type
        > struct valid_impl
{
  typedef void type;
};

template
struct produce_valid_result_of : boost::false_type {};

template<class F>
struct produce_valid_result_of :
boost::true_type {};

...and then try:

struct x
{};

std::cout << produce_valid_result_of::type::value << std::endl;

... you will get an error.  I get:
/Development/boost/boost/utility/result_of.hpp: In instantiation of
'boost::detail::result_of_nested_result':
/Development/boost/boost/utility/result_of.hpp:38:   instantiated from
'boost::detail::result_of_impl'
/Development/boost/boost/utility/result_of.hpp:31:   instantiated from
'boost::result_of'
../../libs/patterns/example/video_gesture_recognition.cpp:224:
instantiated from here
/Development/boost/boost/utility/result_of.hpp:73: error: no class
template named 'result' in 'struct x'

With the way boost::result_of is implemented, I couldn't find a way to
make an approach like the above work.  The best I could do was the
result_of_defined code.

Kind regards,

Stjepan

Re: [Boost-users] Concept check vs enable_if

Stjepan Rajko