Graham Nicholson / realtimeMMLib

lib for realteimMM funcs

callbacknotes.cpp@5:3906f93b8b39, 2018-02-16 (annotated)

Committer:
GTNicholson
Date:
Fri Feb 16 10:03:35 2018 +0000
Revision:
5:3906f93b8b39
Parent:
3:13301255d95a 
Changing Pin assignment

Who changed what in which revision?

UserRevisionLine numberNew contents of line
GTNicholson 3:13301255d95a 1 /*
GTNicholson 3:13301255d95a 2
GTNicholson 3:13301255d95a 3 Because a member function is meaningless without an object to invoke it on, you can’t do this directly (if The X Window System was rewritten in C++, it would probably pass references to objects around, not just pointers to functions; naturally the objects would embody the required function and probably a whole lot more).
GTNicholson 3:13301255d95a 4
GTNicholson 3:13301255d95a 5 As a patch for existing software, use a top-level (non-member) function as a wrapper which takes an object obtained through some other technique. Depending on the routine you’re calling, this “other technique” might be trivial or might require a little work on your part. The system call that starts a thread, for example, might require you to pass a function pointer along with a void*, so you can pass the object pointer in the void*. Many real-time operating systems do something similar for the function that starts a new task. Worst case you could store the object pointer in a global variable; this might be required for Unix signal handlers (but globals are, in general, undesired). In any case, the top-level function would call the desired member function on the object.
GTNicholson 3:13301255d95a 6
GTNicholson 3:13301255d95a 7 Here’s an example of the worst case (using a global). Suppose you want to call Fred::memberFn() on interrupt:
GTNicholson 3:13301255d95a 8
GTNicholson 3:13301255d95a 9 class Fred {
GTNicholson 3:13301255d95a 10 public:
GTNicholson 3:13301255d95a 11 void memberFn();
GTNicholson 3:13301255d95a 12 static void staticMemberFn(); // A static member function can usually handle it
GTNicholson 3:13301255d95a 13 // ...
GTNicholson 3:13301255d95a 14 };
GTNicholson 3:13301255d95a 15 // Wrapper function uses a global to remember the object:
GTNicholson 3:13301255d95a 16 Fred* object_which_will_handle_signal;
GTNicholson 3:13301255d95a 17 void Fred_memberFn_wrapper()
GTNicholson 3:13301255d95a 18 {
GTNicholson 3:13301255d95a 19 object_which_will_handle_signal->memberFn();
GTNicholson 3:13301255d95a 20 }
GTNicholson 3:13301255d95a 21 int main()
GTNicholson 3:13301255d95a 22 {
GTNicholson 3:13301255d95a 23 signal(SIGINT, Fred::memberFn); // Can NOT do this
GTNicholson 3:13301255d95a 24 signal(SIGINT, Fred_memberFn_wrapper); // Okay
GTNicholson 3:13301255d95a 25 signal(SIGINT, Fred::staticMemberFn); // Okay usually; see below
GTNicholson 3:13301255d95a 26 }
GTNicholson 3:13301255d95a 27 Note: static member functions do not require an actual object to be invoked, so pointers-to-static-member-functions are usually type-compatible with regular pointers-to-functions. However, although it probably works on most compilers, it actually would have to be an extern "C" non-member function to be correct, since “C linkage” doesn’t only cover things like name mangling, but also calling conventions, which might be different between C and C++.
GTNicholson 3:13301255d95a 28
GTNicholson 3:13301255d95a 29 Test
GTNicholson 3:13301255d95a 30 */