Error(s):

source_file.cpp:16:7: error: deleted function ‘virtual DerLocker::~DerLocker()’
 class DerLocker : public Locker
       ^
source_file.cpp:12:7: error: overriding non-deleted function ‘virtual Locker::~Locker()’
 class Locker: protected virtual Base
       ^
source_file.cpp:16:7: note: ‘virtual DerLocker::~DerLocker()’ is implicitly deleted because the default definition would be ill-formed:
 class DerLocker : public Locker
       ^
source_file.cpp:10:1: error: ‘virtual Base::~Base()’ is private
 Base::~Base() {}
 ^
source_file.cpp:16:7: error: within this context
 class DerLocker : public Locker
       ^