template<class T>
class cereal::memory_detail::EnableSharedStateHelper< T >
A helper struct for saving and restoring the state of types that derive from std::enable_shared_from_this
This special struct is necessary because when a user uses load_and_construct, the weak_ptr (or whatever implementation defined variant) that allows enable_shared_from_this to function correctly will not be initialized properly.
This internal weak_ptr can also be modified by the shared_ptr that is created during the serialization of a polymorphic pointer, where cereal creates a wrapper shared_ptr out of a void pointer to the real data.
In the case of load_and_construct, this happens because it is the allocation of shared_ptr that perform this initialization, which we let happen on a buffer of memory (aligned_storage). This buffer is then used for placement new later on, effectively overwriting any initialized weak_ptr with a default initialized one, eventually leading to issues when the user calls shared_from_this.
To get around these issues, we will store the memory for the enable_shared_from_this portion of the class and replace it after whatever happens to modify it (e.g. the user performing construction or the wrapper shared_ptr in saving).
Note that this goes into undefined behavior territory, but as of the initial writing of this, all standard library implementations of std::enable_shared_from_this are compatible with this memory manipulation. It is entirely possible that this may someday break or may not work with convoluted use cases.
Example usage:
T * myActualPointer;
{
EnableSharedStateHelper<T> helper( myActualPointer );
std::shared_ptr<T> myPtr( myActualPointer );
}
When possible, this is designed to be used in an RAII fashion - it will save state on construction and restore it on destruction. The restore can be done at an earlier time (e.g. after construct() is called in load_and_construct) in which case the destructor will do nothing. Performing the restore immediately following construct() allows a user to call shared_from_this within their load_and_construct function.
- Template Parameters
-
| T | Type pointed to by shared_ptr |
