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#include <iostream>
 
struct Base
{
  virtual void foo() const = 0;
};
 
void Base::foo() const
{
  std::cout << "Base!\n";
}
 
struct Derived : Base
{
  // Uncomment following line to remove error:
  //virtual void foo() const override { std::cout << "Derived\n"; Base::foo(); }
};
 
int main()
{
  Derived d;
  d.foo();
}

prog.cpp: In function ‘int main()’:
prog.cpp:21:11: error: cannot declare variable ‘d’ to be of abstract type ‘Derived’
   Derived d;
           ^
prog.cpp:13:8: note:   because the following virtual functions are pure within ‘Derived’:
 struct Derived : Base
        ^
prog.cpp:8:6: note: 	virtual void Base::foo() const
 void Base::foo() const
      ^

Standard output is empty