Question

I have an abstract base class and want to implement a function in the derived class. Why do I have to declare the function in the derived class again?

class base {
public:
    virtual int foo(int) const = 0;
};

class derived : public base {
public:
    int foo(int) const; // Why is this required?
};

int derived::foo(int val) const { return 2*val; }

Answer 1

Consider that the derived-class definition might be in a header, whereas its implementation may be in a source file. The header typically gets included in multiple locations (“translation units”), each of which will be compiled independently. If you didn’t declare the override, then the compiler wouldn’t know about it in any of those other translation units.