How does OpenMP 3.0 work better with C++ (Part 3)?
Michael_Wong 120000M1EH Visits (6030)
Let's get to the good stuff. OpenMP 2.5 did not really specify what constructors should be called with various priv
In some cases, it did not even specify that they should apply to non-PODs (Plain'ol Data, i.e. C structs).
OpenMP 3.0 changed that. Beside specifying non-PODs, it also specified precise rules for the constructor sequence that is in line with what the semantics would require.
For instance, it would specify that a firstprivate for a class type variable should expect an accessible copy constructor, since it is required to initialize each of the one or more list items private to a thread with the value that the corresponding original item has when the construct is encountered.
For a class type variable, it requires an accessible, unambiguous copy assignment operator for the class type. And it requires an accessible, unambiguous default constructor for the class type unless the variable is also specified in a firstprivate clause.
This is the most interesting as it differentiates three kinds of initializaton in C++.
1. Without initialization: Object1 o;
The semantics of this is that for the master thread, global static objects and static class members are constructed before main() is entered in an undefined order.
For the slave threads, the exact point in time of object construction is unspecified, but is has to happen before the thread references it the first time
These changes were a long time coming. It causes what used to be vaguely implied by the 2.5 specification, now to be clearly specified so all compilers can conform. It also allows the users of C++ with OpenMP to have more consistent behavior.