For functions in derived classes to actually override their base-class counterparts, some things have to be taken into account:

• The base-class method has to be declared “override"
• The function names must be identical (except for destructors)
• The parameter types of the base and derived functions must be identical (not only compatible).
• The const-ness of the base and derived functions must be identical.
• The return types and exception specifications of the base and derived functions must be compatible.
• The reference-qualifieres of the base and derived functions must be identical. 

Thus in this case, there is not a single method in the derived class that overrides its base-class counterpart:

class Base

{

public:

     virtual void f1() const;

     virtual void f2(int x);

     virtual void f3() &;

     void f4() const;

};

class Derived : public Base

{

public:

     virtual void f1();

     virtual void f2(unsigned int x);

     virtual void f3() &&;

     void f4() const;

};

1. wrong const-nesss
2. non-identical types
3. wrong reference-qualifiers
4. not virtual in base class

By using override after the functions, all of those errors would have shown!

class Base { public: virtual void f1() const; virtual void f2(int x); virtual void f3() &; void f4() const; }; class Derived : public Base { public: virtual void f1() override; // 'f1' marked 'override' but does not override any member functions virtual void f2(unsigned int x) override; // 'f2' marked 'override' but does not override any member functions virtual void f3() && override; // 'f3' marked 'override' but does not override any member functions void f4() const override; // Only virtual member functinos can be marked 'override'

};

Thus, use override!