[boost] boost::hash - different values for the same input?

Subject: [boost] boost::hash - different values for the same input?
From: trafdev (trafdev_at_[hidden])
Date: 2015-12-09 15:13:51

Hi.

As per hash_range docs 
(http://www.boost.org/doc/libs/1_59_0/doc/html/hash/reference.html#boost.hash_range_idp93681104):

"This hash function is not intended for general use, and isn't 
guaranteed to be equal during separate runs of a program - so please 
don't use it for any persistent storage or communication."

As per code 
(http://www.boost.org/doc/libs/1_59_0/boost/functional/hash/hash.hpp) it 
uses hash_combine_impl which doesn't use any random, process_id etc 
values which might change during new process launches.

At the same time, in an example describing container usage in the shared 
memory 
(http://www.boost.org/doc/libs/1_59_0/doc/html/interprocess/allocators_containers.html#interprocess.allocators_containers.additional_containers),
boost::hash is being used:

typedef boost::unordered_map
       < KeyType               , MappedType
       , boost::hash<KeyType>  ,std::equal_to<KeyType>
       , ShmemAllocator>
    MyHashMap;

My question is: is it safe to use boost::hash in a shared (persistent) 
storage or it's safe only for a particular (non-pointer) types?

In this case, boost::hash docs should be changed to mention this 
exceptions, because otherwise no one could use boost::hash in a shared 
memory containers.