gistfile1.txt

class spacelike_impl {
public:
  virtual spacelike_impl * coalesce() = 0;
};

class spacelike
{
public:
  spacelike coalesce() { return p->coalesce(); }
protected:
  spacelike(spacelike_impl * p): p(p) {}
private:
  spacelike_impl * p;
};

// ------- //

class set_impl : public spacelike_impl
{
public:
  spacelike_impl * coalesce() override { return do_coalesce(); }
  set_impl * do_coalesce() { ... }
};

class set : public spacelike
{
public:
  set(): set(new set_impl) {}
  set coalesce() { return p->do_coalesce(); }
private:
  set(set_impl *p): spacelike(p), p(p) {}
  set_impl * p;
};

// ------- //

class map_impl : public spacelike_impl
{
public:
  spacelike_impl * coalesce() override { return do_coalesce(); }
  map_impl * do_coalesce() { ... }
};

class map : public spacelike
{
public:
  map(): map(new map_impl) {}
  map coalesce() { return p->do_coalesce(); }
private:
  map(map_impl *p): spacelike(p), p(p) {}
  map_impl * p;
};