add original source

source/listing_3.11.cpp

@@ -0,0 +1,28 @@
+#include <memory>
+#include <mutex>
+
+struct some_resource
+{
+    void do_something()
+    {}
+    
+};
+
+
+std::shared_ptr<some_resource> resource_ptr;
+std::mutex resource_mutex;
+void foo()
+{
+    std::unique_lock<std::mutex> lk(resource_mutex);
+    if(!resource_ptr)
+    {
+        resource_ptr.reset(new some_resource);
+    }
+    lk.unlock();
+    resource_ptr->do_something();
+}
+
+int main()
+{
+    foo();
+}