Error(s):

source_file.cpp: In constructor ‘DerLocker::DerLocker()’:
source_file.cpp:9:1: error: ‘Base::~Base()’ is private
 Base::~Base() {}
 ^
source_file.cpp:17:26: error: within this context
     DerLocker() : Locker() {}
                          ^
source_file.cpp: In destructor ‘virtual DerLocker::~DerLocker()’:
source_file.cpp:9:1: error: ‘Base::~Base()’ is private
 Base::~Base() {}
 ^
source_file.cpp:15:7: error: within this context
 class DerLocker : public Locker
       ^
source_file.cpp: At global scope:
source_file.cpp:25:1: note: synthesized method ‘virtual DerLocker::~DerLocker()’ first required here 
 }
 ^