Unique_ptr and forward declaration

The constructor needs access to the deleter in just the same way the destructor does: exception safety requires that the constructor be able to roll-back initialisation of all the members in the case that your constructor's body throws:

[C++14: 12.6.2/10]: In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked (12.4). [ Note: This provision ensures that destructors can be called for fully-constructed sub-objects in case an exception is thrown (15.2). —end note ]

Related:

• https://stackoverflow.com/a/26995534/560648