#include <iostream>

struct base
{
    void f() const
    {
        // check preconditions here
        f_impl();
        // check postconditions here
    }
private:
    virtual void f_impl() const = 0;
};

struct derived :
    base
{
    void f() const
    {
        // check preconditions here
        f_impl();
        // check postconditions here
    }
private:
    virtual void f_impl() const
    {
        std::cout << "derived::f_impl()" << std::endl;
    }
};

int main()
{ }