Execute a piece of code in a function from the second invocation onwards

Question

If I desire to run a piece of code in a function, only from the second invocation of the function onwards,

Questions:

Is there something wrong to do that?
How can I possibly achieve this ? Is using a static variable to do this a good idea ?

A regular static is fine, unless your function is called from multiple threads. — Peter, Sep 05 '13 at 13:56

Bernhard Barker · Answer 1 · 2013-09-05T14:34:09.313

1

Multi-threading will be a problem. To prevent this, if required, you'll probably need something like a mutex.

Like this:

void someFunction()
{
   static bool firstRun = true;
   if (!firstRun)
   {
      // code to execute from the second time onwards
   }
   else
   {
      firstRun = false;
   }
   // other code
}

edited Sep 05 '13 at 14:34

answered Sep 05 '13 at 14:06

Bernhard Barker

54,589
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1

move 'firstRun = false;' into the conditional to save a minor amount of CPU time. (edit: I mean as an else part) – Speed8ump Sep 05 '13 at 14:21
Sorry, I wasn't clear, I meant as an else. That way it only gets run the first pass. – Speed8ump Sep 05 '13 at 14:25
@Speed8ump Ok, cool. Yeah, that will probably be slightly better (though I'd say for readability). – Bernhard Barker Sep 05 '13 at 14:39

score 1 · Accepted Answer · answered Sep 05 '13 at 14:44

There's two answers to this question, depending on whether you have to deal with multi-threaded serialization or not.

No threading:

void doSomething() {
    static bool firstTime = true;
    if (firstTime) {
       // do code specific to first pass
       firstTime = false;
    } else {
       // do code specific to 2nd+ pass
    }
    // do any code that is common
}

With threading: I'll write the generic boilerplate, but this code is system specific (requiring some variant of an atomic compareAndSet).

void doSomethingThreadSafe() {
    static volatile atomic<int> passState = 0;

    do {
        if ( passState == 2 ) {
            //perform pass 2+ code
            break;
        } else
        if ( passState.compareAndSet(0,1) ) { // if passState==0 set passState=1 return true else return false
            //perform pass 1 initialization code
            passState = 2;
            break;
        } else {
            //loser in setup collision, delay (wait for init code to finish) then retry
            sleep(1);
        }
    } while(1);

    //perform code common to all passes
}

Arun · Answer 3 · 2017-05-02T21:45:42.177

-1

Add a global counter. eg:-

static int counter = 0;

public void testFunc(){

    if(counter==1){

       ........
       <Execute the functionality>
       ........

     }
    counter++;

}

edited May 02 '17 at 21:45

answered Sep 05 '13 at 13:52

Arun

3,701
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This is not the only way, plus it leaves the counter out there for others to tamper with. The counter could be a `static` variable inside the function instead. Oh, and the question is tagged C++. – juanchopanza Sep 05 '13 at 13:54
then do it the c++ way :) – Arun Sep 05 '13 at 13:55
2

The is wrong in so many ways: 1) public voud? 2) doesn't deal with the multi-threaded question, 3) executes only on the second call, which wasn't the OP's request, 4) the counter will eventually roll over, 5) why initialize counter at 1 rather than 0? initialized at 1 it's hard to say what it's counting. 6) polluting the global namespace with names like 'counter' is a bad idea. – Speed8ump Sep 05 '13 at 14:19

Execute a piece of code in a function from the second invocation onwards

3 Answers3