问派生类中的成员不可用的C++保护的ctor

EN

class Base { protected: Base() {} };

class Derived1 : public Base
  {
    Derived1()  {}
  };

class Derived2 : public Base
  {
    Derived2()  {}
    Base    other_base;
  };

test.cpp: In constructor ‘Derived2::Derived2()’:
test.cpp:3:25: error: ‘Base::Base()’ is protected
 class Base { protected: Base() {} };
                         ^
test.cpp:12:14: error: within this context
  Derived2()  {}