"Scoping" C ++ compilation Error

Question

I can't seem to figure out what is causing the following error:

trainLine.cpp:108: error: name lookup of ‘i’ changed for new ISO ‘for’ scoping

trainLine.cpp:98: error: using obsolete binding at ‘i’

Here is the code that is causing the error:

int main()
{
  pthread_t tidArray[NUM_TRAINS];
  Train* trainArray[NUM_TRAINS];

  for (int i = 0; i < NUM_TRAINS;  i++)
    trainArray[i] = new Train(TRAIN_NAME_ARRAY[i], TRAIN_TRANSIT_TIME_MSECS[i]);

  int trainInd = 0;

  pthread_create(&tidArray[i], NULL, initiallyNorthernly,(void*)&trainArray[i]);

  pthread_create(&tidArray[i], NULL, initiallySouthernly,(void*)&trainArray[i]);

  for  (int i = 0; i < NUM_TRAINS;  i++)
    trainArray[i] = NULL;

  pthread_join(tidArray[i], (void**)&trainInd);

  return(EXIT_SUCCESS);
}

I apologize in advance, but I am very new to C/C++ and nothing is popping out as wrong to me. Possibly not placing brackets around for loop statements? Thanks in advance for any help.

you're probably forgot braces that create compount statement: `for(...) { trainArr[i] = ...; pthread_create(...); }` Also, why do you create two threads in the same object `tidArray[i]`, it is incorrect — myaut, May 22 '15 at 11:33
Yes, "not placing brackets around for loop statements", as you suspect. (But didn't attempt to do anything about, apparently.) — molbdnilo, May 22 '15 at 11:40
http://stackoverflow.com/questions/359732/why-is-it-considered-a-bad-practice-to-omit-curly-braces?lq=1 — Barmar, May 22 '15 at 11:44
It strikes me as interesting that you're new enough to C to not know about enclosing statements in `{`/`}` compound blocks for loops (and other control structures, I guess), but you're diving into pthreads already? It seems that you would have done at least one example program between "hello world" and multi-threaded C. — Michael Burr, May 23 '15 at 17:03

Emil Laine · Answer 1 · 2015-05-22T11:34:58.443

1

Your variable i exists only inside the for-loop, but you're trying to use it outside the loop.

Something like this could solve the problem, without altering the effect of the code:

  // ...
  int i = 0; // i is now accessible outside the loop as well
  for  (;  i < NUM_TRAINS;  i++)
    trainArray[i] = new     Train(TRAIN_NAME_ARRAY[i],TRAIN_TRANSIT_TIME_MSECS[i]);

  // use i, which is equal to NUM_TRAINS at this point

edited May 22 '15 at 11:34

answered May 22 '15 at 11:32

Emil Laine

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3

It's certainly not what he wants, though, as then his subsequent access are out-of-bounds. – Lightness Races in Orbit May 22 '15 at 11:34

score 1 · Answer 2 · answered May 22 '15 at 11:32

In the line:

pthread_join(tidArray[i], (void**)&trainInd);

you're referencing i, which is out of scope. I'm guessing you actually wanted this to be part of a loop, e.g.

for (int i = 0; i < NUM_TRAINS; ++i)
    pthread_join(tidArray[i], (void**)&trainInd);

"Scoping" C ++ compilation Error

2 Answers2