cpp: ex1: add solution

cpp/ex1.cc

@@ -0,0 +1,28 @@
+#ifdef USE_ATOMIC_N
+#include <atomic>
+#endif
+
+class CountInstances {
+public:
+    CountInstances() {
+        ++n_instances_;
+    }
+
+    ~CountInstances() {
+        --n_instances_;
+    }
+
+    int nobjs() const {
+        return n_instances_; // If using atomics, load could use a more relaxed
+                             // memory order, such as memory_order_acquire
+    }
+
+private:
+#ifdef USE_ATOMIC_N
+    // Can be used in a multi-thread program
+    static atomic<int> n_instances_;
+#else
+    // Using a bare int mean races if used in a multi-threaded context
+    static int n_instances_;
+#endif
+};