Add unused_storage optimization function

include/boost/container/devector.hpp

@@ -2170,6 +2170,28 @@ class devector
                                  || allocator_traits_type::is_always_equal::value)
    {   x.swap(y);   }
 
+   #ifndef BOOST_CONTAINER_DOXYGEN_INVOKED
+
+   //Functions for optimizations, not for users
+   T *unused_storage(size_type &size)
+   {
+      T *const storage_addr = boost::movelib::to_raw_pointer(m_.buffer);
+      if(this->empty()){
+         size = m_.capacity;
+         return storage_addr;
+      }
+      else if(this->back_free_capacity() > this->front_free_capacity()){
+         size = this->back_free_capacity();
+         return storage_addr + m_.back_idx;
+      }
+      else{
+         size = this->front_free_capacity();
+         return storage_addr;
+      }
+   }
+
+   #endif
+
    private:
 
    void priv_move_assign(BOOST_RV_REF(devector) x, dtl::bool_<true> /*steal_resources*/)

include/boost/container/vector.hpp

@@ -2394,6 +2394,14 @@ private:
       boost::movelib::adaptive_merge( this->begin(), pos, e, comp
                                     , this->priv_raw_end(), this->capacity() - this->size());
    }
+
+   //Function for optimizations, not for users
+   T *unused_storage(size_type &size)
+   {
+      size = static_cast<size_type>(this->capacity() - this->size());
+      return this->priv_raw_end();
+   }
+  
    #endif
 
    private: