0

Say I have two C++ functions foo1() and foo2(), and I want to minimize the likelihood that that foo1() starts execution but foo2() is not called due to some external event. I don't mind if neither is called, but foo2() must execute if foo1() was called. Both functions can be called consecutively and do not throw exceptions.

Is there any benefit / drawback to wrapping the functions in an object and calling both in the destructor? Would things change if the application was multi-threaded (say the parent thread crashes)? Are there any other options for ensuring foo2() is called so long as foo1() is called?

I thought having them in a destructor might help with e.g. SIGINT, though I learned SIGINT will stop execution immediately, even in the middle of the destructor.

Edit:

To clarify: both foo1() and foo2() will be abstracted away, so I'm not concerned about someone else calling them in the wrong order. My concern is solely related to crashes, exceptions, or other interruptions during the execution of the application (e.g. someone pressing SIGINT, another thread crashing, etc.).

dontarius
  • 99
  • 1
  • 8
  • 2
    Make the public interface `bar() { foo1(); foo2(); }` and keep `foo1` private? – Kerrek SB Dec 09 '15 at 22:34
  • 1
    If you are dealing with virtual functions (ie, if `foo1()` or `foo2()` are virtual) you should avoid calling them in constructors or destructors. The vtable might not be what you expect during either construction or destruction, and this can lead to nearly impossible to track errors. However, if you are not using virtuals, I can't see any major issues in single-threaded programs. You could always use a bool flag variable to make sure functions are run in the proper order/run at all. –  Dec 09 '15 at 22:57
  • Thanks for the comments. I added a brief clarification to my question. Neither function is virtual, but that's a good thing to look out for in the future. – dontarius Dec 09 '15 at 23:08

1 Answers1

1

If another thread crashes (without relevant signal handler -> the whole application exits), there is not much you can do to guarantee that your application does something - it's up to what the OS does. And there are ALWAYS cases where the system will kill your app without your actual knowledge (e.g. a bug that causes "all" memory being used by your app and the OS "out of memory killer" killing your process).

The only time your destructor is guaranteed to be executed is if the object is constructed and a C++ exception is thrown. All signals and such, make no such guarantees, and contininuing to execute [in the same thread] after for example SIGSEGV or SIGBUS is well into the "undefined" parts of the world - nothing much you can do about that, since the SEGV typically means "you tried to do something to memory that doesn't exist [or that you can't access in the way you tried, e.g. write to code-memory]", and the processor would have aborted the current instruction. Attempting to continue where you were will either lead to the same instruction being executed again, or the instruction being skipped [if you continue at the next instruction - and I'm ignoring the trouble of determining where that is for now]. And of course, there are situations where it's IMPOSSIBLE to continue even if you wanted to - say for example the stack pointer has been corrupted [restored from memory that was overwritten, etc].

In short, don't spend much time trying to come up with something that tries to avoid these sort of scenarios, because it's unlikely to work. Spend your time trying to come up with schemes where you don't need to know if you completed something or not [for example transaction based programming, or "commit-based" programming (not sure if that's the right term, but basically you do some steps, and then "commit" the stuff done so far, and then do some further steps, etc - only stuff that has been "committed" is sure to be complete, uncommitted work is discarded next time around) , where something is either completely done, or completely discarded, depending on if it completed or not].

Separating "sensitive" and "not sensitive" parts of your application into separate processes can be another way to achieve some more safety.

Mats Petersson
  • 126,704
  • 14
  • 140
  • 227
  • Thanks for your response. It turns out foo1() and foo2() are used to push changes to a database that does not support ACID transactions, so I guess the only way around this would be to store / log what was supposed to happen, and if things go south, compare the db's state with what was supposed to happen. – dontarius Dec 10 '15 at 01:55