add another polymorphism example

.gitignore

@@ -21,3 +21,4 @@ rtti
 future
 return-type-deduction
 division
+poly

CMakeLists.txt

@@ -18,6 +18,7 @@ add_executable(classes classes.cpp)
 add_executable(lvalues lvalues.cpp)
 add_executable(bad_alloc bad_alloc.cpp)
 add_executable(rtti rtti.cpp)
+add_executable(poly poly.cpp)
 add_executable(division division.cpp)
 
 add_executable(future future.cpp)

poly.cpp

@@ -0,0 +1,37 @@
+#include <iostream>
+#include <string>
+
+/* abstract */
+class Animal {
+  public:
+    // virtual std::string talk() = 0; /* pure virtual */
+    virtual std::string talk() { ; /* or an implementation */
+      return "<sound>";
+    }
+};
+
+class Cat : public Animal {
+  std::string talk() {
+    return "Meow!";
+  }
+};
+
+class Dog : public Animal {
+  std::string talk() {
+    return "Woof!";
+  }
+};
+
+/* Note: need pointer or ref here! */
+void letsHear(Animal& a) {
+  std::cout << a.talk() << std::endl;
+}
+
+int main() {
+  Cat c;
+  Dog d;
+  letsHear(c);
+  letsHear(d);
+
+  return 0;
+}