diff --git a/ch21/pure_virtual_function.c b/ch21/pure_virtual_function.c
new file mode 100644
index 0000000..a7cc37b
--- /dev/null
+++ b/ch21/pure_virtual_function.c
@@ -0,0 +1,39 @@
+#include <cmath>
+#include <iostream>
+
+using namespace std;
+
+class Function {     
+ public:
+  // This is pure virtual
+  virtual double eval(double x) = 0;
+
+  // This calls the pure one
+  double derivative(double x, double e) {
+    return (eval(x+e) - eval(x-e))/(2*e);
+  }
+};
+
+class Oscillating : public Function {     
+ public:
+  Oscillating(double aa, double bb, double cc)
+      :a(aa), b(bb), c(cc) {}
+  double eval(double x) {
+    return a*sin(b*x+c);
+  }
+ private:
+  double a, b, c;
+};
+
+int main(int argc, char **argv) {
+  Oscillating f(1, 1, 0);
+  cout << f.derivative(0, 0.1) << endl;
+  cout << f.derivative(0, 0.01) << endl;
+  cout << f.derivative(0, 0.001) << endl;
+  Function f1;
+  // let's mess the complier!
+  std::cout << f1.eval(3.0) << std::endl;
+  return 0;
+}
+
+
diff --git a/test.cc b/test.cc
index 4f9d1bd..1face39 100644
--- a/test.cc
+++ b/test.cc
@@ -7,5 +7,6 @@ int main(int argc, char **argv) {
   cout << "the patch test of git" << endl;
 
   std::cout << "test of commit --amend" << std::endl;
+  std::cout << "show diff" << std::endl;
   return 0;
 }