launder_cast triggers warning about dropping const and volatile qualifiers #61
Description
I'm using Boost 1.88.0 on my Mac, and I get the following error when trying to use boost::container::static_vector:
In file included from ./src/my_source_file.cc:8:
In file included from ./inc/my_header_file.hh:12:
In file included from /opt/homebrew/Cellar/boost/1.88.0/include/boost/container/static_vector.hpp:25:
/opt/homebrew/Cellar/boost/1.88.0/include/boost/move/detail/launder.hpp:47:24: error: cast from 'const volatile void *' to 'MyClassName *' drops const and volatile qualifiers [-Werror,-Wcast-qual]
47 | return (launder)((T)p);
| ^
/opt/homebrew/Cellar/boost/1.88.0/include/boost/container/static_vector.hpp:67:27: note: in instantiation of function template specialization 'boost::move_detail::launder_cast<MyClassName *>' requested here
67 | { return move_detail::launder_cast<T*>(&storage); }
| ^
/opt/homebrew/Cellar/boost/1.88.0/include/boost/container/vector.hpp:736:30: note: in instantiation of member function 'boost::container::dtl::static_storage_allocator<MyClassName, 4, 0, true>::internal_storage' requested here
736 | { return allocator_type::internal_storage(); }
| ^
/opt/homebrew/Cellar/boost/1.88.0/include/boost/container/vector.hpp:3225:54: note: in instantiation of member function 'boost::container::vector_alloc_holder<boost::container::dtl::static_storage_allocator<MyClassName, 4, 0, true>, unsigned long>::start' requested here
3225 | { return boost::movelib::to_raw_pointer(m_holder.start()); }
| ^
/opt/homebrew/Cellar/boost/1.88.0/include/boost/container/vector.hpp:1216:47: note: in instantiation of member function 'boost::container::vector<MyClassName, boost::container::dtl::static_storage_allocator<MyClassName, 4, 0, true>>::priv_raw_begin' requested here
1216 | (this->get_stored_allocator(), this->priv_raw_begin(), this->m_holder.m_size);
| ^
/opt/homebrew/Cellar/boost/1.88.0/include/boost/container/static_vector.hpp:162:7: note: in instantiation of member function 'boost::container::vector<MyClassName, boost::container::dtl::static_storage_allocator<MyClassName, 4, 0, true>>::~vector' requested here
162 | class static_vector
| ^
1 error generated.
I traced it to this bit of code in move/detail/launder.hpp:
template<class T>
BOOST_MOVE_FORCEINLINE T launder_cast(const volatile void* p)
{
return (launder)((T)p);
}
I was able to work around it for now by adding a const_cast<void*>. For good measure, I converted the remainder of the conversion to a static_cast<T>, to avoid any new warnings about C-style casts.
template<class T>
BOOST_MOVE_FORCEINLINE T launder_cast(const volatile void* p)
{
return (launder)(static_cast<T>(const_cast<void*>(p)));
}
I don't think boost::container::static_vector is at fault here, as the helper above really is casting away the cv-qualifiers on its argument.
It's a safe cast, as long as this helper is only called—directly or indirectly—by a container or allocator that's managing its own pool of mutable raw storage. AFAICT, the purpose of the const volatile void * argument is for it to serve as a "universal pointer recipient" for all ordinary pointers, similar to delete. And, like delete, it should ignore cv-qualifiers on the supplied argument.
I don't know for certain whether this laundering approach is actually kosher; however, that's part of the larger discussion of how to do what std::launder attempts to do. I'm not going there. 😄