The analyzer had discovered the use of 'std::async' function, which can behave differently from what the developer had expected. The 'std::async' function receives the following arguments: the function to be executed, its arguments and an optional flag which influences the execution policy of 'std::async'. The function returns an 'std::future' instance, the value of which will be assigned after the function finishes its execution.
The behavior of 'std::async' depends upon the flags it receives in the following way:
1) 'std::launch::async' - an instance of the 'thread' class will be created immediately, using the function and its arguments as the arguments of new thread. It means that 'std::async' will encapsulate the creation of a thread and 'std::future' and provides a way to execute such these actions in a single line of code.
2) 'std::launch::deferred' – the behavior of the function will change - there will be no asynchronous execution. Instead of executing the function in a different thread, it will be saved, together with all of its arguments, to the 'std::future' instance, so it can be called later. This call will happen when someone invokes 'get' or 'wait' methods of this 'future' instance returned by 'std::async'. The execution will be performed in the same thread that called get\wait. This behavior is, in fact, a the deferred execution.
3) The flag is not specified (std::launch::async | std::launch::deferred) - in this case, one of the two execution policies specified above will be selected automatically. Which one? It is unspecified and depends on the implementation.
If the execution policy is not specified when executing the 'std::async' function, the third case is used. To help avoid possible uncertainty in the behavior of this function, analyzer detects such cases.
Future<int> foo = std::async(MyFunction, args...);After this call, there is a possibility that different systems with different implementations of standard libraries will have different behavior.
We recommend taking this into account and solving this potential behavioral uncertainty by explicitly specifying the execution policy with the function's first parameter. Reliable way of explicitly specifying the execution policy is the following:
Future<int> foo = std::async(launch::async, MyFunction, args...)