Closed #108.

CppCoreGuidelines.md

@@ -9383,15 +9383,39 @@ See also:
 It is hard to be certain that concurrency isn't used now or sometime in the future.
 Code gets re-used.
 Libraries using threads my be used from some other part of the program.
+Note that this applies most urgently to library code and least urgently to stand-alone applications.
 
 ##### Example
 
-    ???
+    double cached_computation(double x)
+    {
+        static double cached_x = 0.0;
+        static double cached_result = COMPUTATION_OF_ZERO;
+        double result;
+ 
+        if (cached_x = x)
+            return cached_result;
+        result = computation(x);
+        cached_x = x;
+        cached_result = result;
+        return result;
+    }
+
+Although `cached_computation` works perfectly in a single-threaded environment, in a multi-threaded environment the two `static` variables result in data races and thus undefined behavior.
+
+There are several ways that this example could be made safe for a multi-threaded environment:
+* Delegate concurrency concerns upwards to the caller.
+* Mark the `static` variables as `thread_local` (which might make caching less effective).
+* Implement concurrency control, for example, protecting the two `static` variables with a `static` lock (which might reduce performance).
+* Have the caller provide the memory to be used for the cache, thereby delegating both memory allocation and concurrency concerns upwards to the caller.
+* Refuse to build and/or run in a multi-threaded environment.
+* Provide two implementations, one which is used in single-threaded environments and another which is used in multi-threaded environments.
 
 **Exception**: There are examples where code will never be run in a multi-threaded environment.
 However, there are also many examples where code that was "known" to never run in a multi-threaded program
 was run as part of a multi-threaded program. Often years later.
 Typically, such programs lead to a painful effort to remove data races.
+Therefore, code that is never intended to run in a multi-threaded environment should be clearly labeled as such.
 
 ### <a name="Rconc-races"></a> CP.2: Avoid data races