#include <cassert>
#include <type_traits>

class Job {
public:
    virtual int cost() const = 0;
    virtual ~Job() = 0;
};

int Job::cost() const { return 5; }
Job::~Job() = default;

class ConcreteJob final : public Job {
public:
    int cost() const override { return Job::cost() + 3; }
};

int main() {
    static_assert(std::is_abstract<Job>::value, "abstract interface");
    static_assert(!std::is_abstract<ConcreteJob>::value, "complete implementation");
    ConcreteJob job;
    const Job& interface = job;
    assert(interface.cost() == 8);
    assert(job.Job::cost() == 5);
}