boost::any has a perfect forward constructor declared as:
template<typename ValueType>
any(ValueType&& value
, typename boost::disable_if<boost::is_same<any&, ValueType> >::type* = 0 // disable if value has type `any&`
, typename boost::disable_if<boost::is_const<ValueType> >::type* = 0) // disable if value has type `const ValueType&&`
: content(new holder< typename decay<ValueType>::type >(static_cast<ValueType&&>(value)))
{}
The is_const<> SFINAE exclusion forces const types to the regular copy constructor:
template<typename ValueType>
any(const ValueType & value)
: content(new holder<
BOOST_DEDUCED_TYPENAME remove_cv<BOOST_DEDUCED_TYPENAME decay<const ValueType>::type>::type
>(value))
{}
What is different about how the regular copy constructor treats a const value than what the perfect forward constructor would do if the is_const<> exclusion were removed?
