boost::interprocess::interprocess_mutex is not an implicit lifetime type #249
Description
We've always known that accessing objects in shared memory were technically UB, but we did it anyway.
The standard has tried to close that hole, and now defines how objects can have their lifetimes managed implicitly, either by start_lifetime_as or certain blessed functions like malloc, and implementation provided ones like mmap.
However, this all mandates that the only objects who can have their lifetime implicitly managed by the abstract machine are those that are implicit lifetime types, i.e., std::is_implicit_lifetime<T>::value is true.
Unfortunately, this is not true for boost::interprocess::interprocess_mutex and most (all?) other Boost.Interprocess types.
For boost::interprocess::interprocess_mutex, it has a non-trivial constructor, and deleted copy and move constructors. This means that there is no way to trivially construct an instance. Thus, it cannot be an implicit lifetime type, and it is still UB to use one in shared memory, even with all the implicit lifetime mechanisms.
I understand this "works" today, but I don't think it should be ignored going forward.
We can make the default ctor trivial. But, this will be a breaking change for existing code that relies on the initialization semantics.
We could add a macro, maybe BOOST_INTERPROCESS_IMPLICIT_LIFETIME, which would be an explicit opt-in to make types satisfy the implicit lifetime guarantee. The user, having explicitly defined that macro, would take the responsibility to make sure they correctly used and initialized the types.
I'd be willing to work on this and submit pull requests, but wanted to submit the suggestion first. Maybe somebody else has already looked into this. I didn't see anything in the other issues though.