CPP: Mysterious error for array initialization and crash?

Question

My program seems to always produce ridiculous errors. Please provide directions for me. The following code segment cutout all irrelevant parts. Thanks.

Part A of the code segment seems failed to initialize the array correctly, how to debug? Part B of the code segment always crash, is there anything i missed?

typedef unsigned long T_PSIZE;
int main()
{
   int AG_TOTAL = 6 ;
   /* part A1 */
   T_PSIZE* cntPeopleByAge = new T_PSIZE[AG_TOTAL + 1];
   /* part A2 - originally i use static array like this, but it also fails */
   //T_PSIZE cntPeopleByAge T_PSIZE[AG_TOTAL + 1];
   for (int i = 0; i < (AG_TOTAL + 1); i++)
   {
     std::cout << i << ":" << cntPeopleByAge[i] << "\t";
     cntPeopleByAge[i] = 0;
     std::cout << cntPeopleByAge[i] << "\n";
   }
   std::cout << "cntPeopleByAge:" << cntPeopleByAge[ AG_TOTAL + 1 ] << "\n";
   /* part B */
   delete [] cntPeopleByAge;
   return 0; // <---  crash here!
}

Sample Output

0:200320        0
1:201581        0
2:201582        0
3:201583        0
4:0     0
5:0     0
cntPeopleByAge:1799119387:0:0

Platform: win 7 x64
Compiler: TDM-GCC x64

Array indices are numbered starting from 0. So cntPeopleByAge[ AG_TOTAL + 1] is a non-existing element. — jogojapan
– jogojapan, Commented Sep 16, 2012 at 7:55

jogojapan · Accepted Answer · 2012-09-16 07:57:41Z

5

for (int i = 0; i < (AG_TOTAL + 1); i++)
   {
     std::cout << i << ":" << cntPeopleByAge[i] << "\t";
     //                       ^^^^^^^^^^^^^^^^
     // You're reading uninitialized memory here

     cntPeopleByAge[i] = 0;
     std::cout << cntPeopleByAge[i] << "\n";
   }

And here

std::cout << "cntPeopleByAge:" << cntPeopleByAge[ AG_TOTAL + 1 ] << "\n";

you're going out of bounds. The last valid index is AG_TOTAL.

You've got undefined behaviour (UB). The errors are only as ridiculous as UB can be.

edited Sep 16, 2012 at 7:57

jogojapan

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answered Sep 16, 2012 at 7:55

jrok

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2 Comments

YamHon.CHAN Over a year ago

Thanks a lot. I'm so dump for this extremely newbie question.

Jeff D. Over a year ago

This is not correct! The array is declared a length and the loop limit correctly test for < length. Please see my post below.

Jeff D. · Accepted Answer · 2012-09-16 09:38:50Z

-1

/* Sorry. but that earlier answer is not correct. The loop correctly starts at zero and ends at < the limit. The problem is that you are declaring an array of pointers, but never allocating memory to the objects that they point to. Your output is showing you addresses not numbers. One way is to allocate the objects as you use them (you must delete them individually as well) */

T_PSIZE* cntPeopleByAge = new T_PSIZE[AG_TOTAL + 1];    
for (int i = 0; i < (AG_TOTAL + 1); i++)    
{ 
cntPeopleByAge[i] = new T_PSIZE();
}

What you really want to use is the vector class in the standard library which handles all of this for you:

#include <vector>

std:vector<T_PSIZE *> cntPeopleByAge;
cntPeopleByAgex.resize(AG_TOTAL + 1);

Good luck ...

answered Sep 16, 2012 at 9:38

Jeff D.

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1 Comment

jrok Over a year ago

No, you've got this all wrong. OP is allocating an array of unsigned longs, not pointers. The loop condition is correct, sure, and I never said it isn't. The problem is that he's reading those unsigned longs uninitialized (new doesn't zero out memory or anything). The values are unspecified (but surely they're not adresses) and reading them is undefined behaviour.

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