Cannot expand vector in place using interprocess::allocator and interprocess::rbtree_best_fit

[camden.mannett@…]

When using reserve() on a interprocess::vector (synomyn for container::vector) inside a interprocess::basic_managed_mapped_file (using the default interprocess::rbtree_best_fit memory algorithm), it cannot in place expand forward.

I noticed this when getting help on ​this SO question. Using the code in that question, put a breakpoint at \boost\interprocess\mem_algo\rbtree_best_fit.hpp:964, and then trace the call stack back to vector::priv_reserve_no_capacity(size_type new_cap, version_2):

pointer reuse = 0;
pointer const ret(this->m_holder.allocation_command(allocate_new | expand_fwd | expand_bwd, new_cap, real_cap = new_cap, reuse));

The reuse pointer is set to null, resumably so it can be overridden by the allocator. The pointer is passed up the stack to interprocess::allocator::allocation_command(boost::interprocess::allocation_type command, size_type limit_size, size_type &prefer_in_recvd_out_size, pointer &reuse) where the raw pointer is retrieved:

value_type *reuse_raw = ipcdetail::to_raw_pointer(reuse);
pointer const p = mp_mngr->allocation_command(command, limit_size, prefer_in_recvd_out_size, reuse_raw);

reuse_raw continues to be passed up the stack to where our breakpoint is:

//Expand in place
prefer_in_recvd_out_size = preferred_size;
if(reuse_ptr && (command & (boost::interprocess::expand_fwd | boost::interprocess::expand_bwd))){
  void *ret = priv_expand_both_sides
     (command, limit_size, prefer_in_recvd_out_size, reuse_ptr, true, backwards_multiple);
  if(ret)
     return ret;
}

reuse_ptr is our raw pointer and is as null as it was when created, causing the expand in place block to be skipped. In short there appears to be no way for a vector to be in place expanded when used inside a memory mapped file.

[Ion Gaztañaga]

Thanks for the report, but this issue was fixed in:

​https://github.com/boostorg/container/commit/0b75df992019cd3ace25912f74b6d416926f4a22

and released in boost 1.62.