diff --git a/ch25/design_pattern.cc b/ch25/design_pattern.cc
new file mode 100644
index 0000000..ef0b092
--- /dev/null
+++ b/ch25/design_pattern.cc
@@ -0,0 +1,62 @@
+#include <cstdlib>
+
+using namespace std;
+
+class IntIterator {     
+ public:
+  virtual bool hasNext() = 0;
+  virtual int next() = 0;
+};
+
+class IntSet {     
+ public:
+  virtual void add(int k) = 0;
+  virtual IntIterator *interator() const = 0;
+};
+
+int size(const InSet &set) {
+  int s = 0;
+  IntIterator *i = set.interator();
+  while (i->hasNext) {
+	i->next();
+    s++;
+  }
+  delete i;
+  return s;
+}
+
+class IntSetArrayIterator : public IntIterator {     
+ public:
+  IntSetArrayIterator(int *v, int s) : values(v), current(0), size(s) {}
+  bool hasNext() {
+    return current < size;
+  }
+  int next() {
+    return values[current++];
+  }
+ private:
+  int *values;
+  int current, size;
+};
+
+class IntSetArray : public IntSet {     
+ public:
+  IntSetArray(int sm) : values(new int[sm]), size(0), sizemax(sm) {}
+  ~IntSetArray() {
+    delete values;
+  }
+  void add(int k) {
+    if (size >= sizemx) {
+      abort();
+      values[size++] = k;
+    }
+  }
+  IntIterator *iterator() const {
+    return new IntSetArrayIterator(values, size);
+  }
+ private:
+  int *values;
+  int size, sizemax;
+};
+
+