diff --git a/cpp/map.cpp b/cpp/map.cpp
index d92b13b..5c1b520 100644
--- a/cpp/map.cpp
+++ b/cpp/map.cpp
@@ -15,9 +15,9 @@
 
     There seems to be no explicit hashmap container, only a generic map interface,
 
-    See map.
+    However unordered_map is likely to be hashmap based.
 
-    Nonstandard `hash_map` already provided with gcc and msvc++.
+    A nonstandard `hash_map` already provided with gcc and msvc++.
     It is placed in the `std::` namespace, but it is *not* ISO.
 */
 
diff --git a/cpp/set.cpp b/cpp/set.cpp
index a84d495..fab1b65 100644
--- a/cpp/set.cpp
+++ b/cpp/set.cpp
@@ -102,8 +102,8 @@ int main() {
         assert(ret.first == s.find(0));
         assert(ret.second == true);
 
-        //item already present:
-        //nothing is done and returns false on the pair
+        // Item already present:
+        // nothing is done and returns false on the pair.
         ret = s.insert(1);
         assert(ret.first == s.find(1));
         assert(ret.second == false);
diff --git a/cpp/unique_ptr.cpp b/cpp/unique_ptr.cpp
index 65b5979..6621940 100644
--- a/cpp/unique_ptr.cpp
+++ b/cpp/unique_ptr.cpp
@@ -74,8 +74,8 @@ int main() {
     manual_ptr_test();
     assert(Base::count == 10);
 
-    // Convert to raw pointer.
-    // Not possible.
+    // ERROR: Convert to raw pointer.
+    // Not possible, the cast operator is not defined.
     {
         std::unique_ptr<int> p(new int);
         //int *raw = p;
diff --git a/cpp/unordered_set.cpp b/cpp/unordered_set.cpp
index baabce6..ec71904 100644
--- a/cpp/unordered_set.cpp
+++ b/cpp/unordered_set.cpp
@@ -22,5 +22,32 @@
 
 #include "common.hpp"
 
-int main() {
+class MyClass {
+    public:
+        int i;
+        int j;
+        MyClass(int i, int j) : i(i), j(j) {}
+};
+
+bool operator==(const MyClass& lhs, const MyClass& rhs) {
+    return lhs.i == rhs.i;
+}
+
+namespace std {
+  template <>
+  struct hash<MyClass> {
+    std::size_t operator()(const MyClass& k) const {
+        return k.i;
+    }
+  };
+}
+
+int main() {
+    std::unordered_set<MyClass> mySet;
+    assert(mySet.size() == 0);
+    mySet.insert(MyClass(1, 2));
+    assert(mySet.size() == 1);
+    mySet.insert(MyClass(1, 3));
+    assert(mySet.size() == 1);
+    assert(mySet.find(MyClass(1, -1))->j == 2);
 }