new file: ch21

ch21/AA.cc

@@ -0,0 +1,38 @@
+#include <iostream>
+
+using namespace std;
+
+class AA {     
+ public:
+  virtual void something() {
+    std::cout << "AA::something()" << std::endl;
+  }
+};
+
+class BB : public AA {     
+ public:
+  void something() {
+    std::cout << "BB::something()" << std::endl;
+  }
+};
+
+void byvalue(AA x) {
+  x.something();
+}
+
+void byref(AA &x) {
+  x.something();
+}
+
+void byporinter(AA *x) {
+  x->something();
+}
+
+int main(int argc, char **argv) {
+  BB b;
+  byvalue(b);
+  byref(b);
+  byporinter(&b);
+
+  return 0;
+}