How to turn a class into an abstract class C++ code example

Example: abstract class in c++

struct Abstract {
    virtual void f() = 0; // pure virtual
}; // "Abstract" is abstract
 
struct Concrete : Abstract {
    void f() override {} // non-pure virtual
    virtual void g();     // non-pure virtual
}; // "Concrete" is non-abstract
 
struct Abstract2 : Concrete {
    void g() override = 0; // pure virtual overrider
}; // "Abstract2" is abstract
 
int main()
{
    // Abstract a; // Error: abstract class
    Concrete b; // OK
    Abstract& a = b; // OK to reference abstract base
    a.f(); // virtual dispatch to Concrete::f()
    // Abstract2 a2; // Error: abstract class (final overrider of g() is pure)
}