Assume I have a template class like this:
template<typename T> class Foo {}; And a class like:
class PossibleArg { typedef int required_type; } Is it possible to write a static_assert in class Foo which checks if T::required_type is defined?
- I'm search for a solution which doesn't require third-party libs like boost.cytrinox– cytrinox2011-03-14 21:18:37 +00:00Commented Mar 14, 2011 at 21:18
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4 Answers
Maybe something looks like:
template <typename T> class Foo { static_assert(sizeof(T::required_type) > 0, "Never shows up"); }; EDIT: Other way: SFINAE
template <typename T> struct has_required_type { typedef char yes[1]; typedef char no[2]; template <typename C> static yes& test(typename C::required_type*); template <typename> static no& test(...); static const bool value = sizeof(test<T>(0)) == sizeof(yes); }; template <typename T> class Foo { static_assert(has_required_type<T>::value, "Hasn't required type"); }; 
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You can use BOOST_MPL_HAS_XXX_TRAIT_DEF, in Boost.MPL:
BOOST_MPL_HAS_XXX_TRAIT_DEF( required_type ) BOOST_MPL_ASSERT(( has_required_type< PossibleArg > )); BOOST_MPL_HAS_XXX_TRAIT_DEF is a macro, taking a name xxx as parameter, which generates a metafunction has_xxx< T > that evaluates to true if T define a nested type named xxx.
(Note that an MPL metafunction is a compile-time function whose result can be accessed using ::type. The result in this case is a compile-time boolean constant(i.e. a bool_.)
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If your goal is to get a compiler error if T doesn't have a required_type you could just typedef it in Foo. Or am I missing something?
template<typename T> class Foo { typedef typename T::required_type T_required_type; }; 
1 Comment
cytrinox
Yes, thats how I solve the problem for now. But since C++0x defines static_assert, using it seems a nicer soltion.
If you're actually looking for static_assert, there's something similar over here - Static assert without boost or C++0x
