glr2.cc: make the example more C++

Currently the example really looks like C.  Instead of a union of
structs to implement the AST, use a hierarchy.  It would be nice to
feature a C++17 version with std variants.

* examples/c++/glr/c++-types.yy (Node, free_node, new_nterm)
(new_term): Move into...
* examples/c++/glr/ast.hh: here, a proper C++ hierarchy.

examples/c++/glr/ast.hh

@@ -0,0 +1,100 @@
+#include <iostream>
+
+#if __cplusplus < 201103L
+# define nullptr 0
+#endif
+
+class Node
+{
+public:
+  Node ()
+    : parents_ (0)
+  {}
+
+  virtual ~Node ()
+  {}
+
+  void free ()
+  {
+    parents_ -= 1;
+    /* Free only if 0 (last parent) or -1 (no parents).  */
+    if (parents_ <= 0)
+      delete this;
+  }
+
+  virtual std::ostream& print (std::ostream& o) const = 0;
+
+protected:
+  friend class Nterm;
+  friend class Term;
+  int parents_;
+};
+
+
+static std::ostream&
+operator<< (std::ostream& o, const Node &node)
+{
+  return node.print (o);
+}
+
+class Nterm : public Node
+{
+public:
+  Nterm (char const *form,
+         Node *child0 = nullptr, Node *child1 = nullptr, Node *child2 = nullptr)
+    : form_ (form)
+  {
+    children_[0] = child0;
+    if (child0)
+      child0->parents_ += 1;
+    children_[1] = child1;
+    if (child1)
+      child1->parents_ += 1;
+    children_[2] = child2;
+    if (child2)
+      child2->parents_ += 1;
+  }
+
+  ~Nterm ()
+  {
+    for (int i = 0; i < 3; ++i)
+      if (children_[i])
+        children_[i]->free ();
+  }
+
+  std::ostream& print (std::ostream& o) const
+  {
+    o << form_;
+    if (children_[0])
+      {
+        o << '(' << *children_[0];
+        if (children_[1])
+          o << ", " << *children_[1];
+        if (children_[2])
+            o << ", " << *children_[2];
+        o << ')';
+      }
+    return o;
+  }
+
+private:
+  char const *form_;
+  Node *children_[3];
+};
+
+class Term : public Node
+{
+public:
+  Term (const std::string &text)
+    : text_ (text)
+  {}
+
+  std::ostream& print (std::ostream& o) const
+  {
+    o << text_;
+    return o;
+  }
+
+private:
+  std::string text_;
+};