1

I am writing a simple C++ Program which allocates dynamic memory to my Program and then deletes this memory. Here is my Program:

#include <iostream>
#include <new>
using namespace std;

int main ()
{
  int i,n;
  int * p;
  cout << "How many numbers would you like to type? ";
  cin >> i;
  p= new (nothrow) int[i];
  if (p == nullptr)
    cout << "Error: memory could not be allocated";
  else
  {
    for (n=0; n<i; n++)
    {
      cout << "Enter number: ";
      cin >> p[n];
    }
    cout << "You have entered: ";
    for (n=0; n<i; n++)
      cout << p[n] << ", ";
    delete[] p;
  }
  return 0;
}

In the above program when I input the value of i (Number of inputs) equal to or less than 2 Billion than this program works as expected. However when I enter anything above 2 Billion like 3 Billion or higher, this program goes in an infinite loop without taking a number input in my for loop.

I am expecting this program to fail when I enter a very high value for i by saying it could not allocate the memory.

As per my understanding, I think when I enter a very high value of int i, I am going out of bound for integer data type but still in this case, it should take number input from me in for loop as I have a cin statement there instead of going in for loop or memory allocation should fail simply.

When I changed type of i from int to long then it works but I am curious to know for i of type int, why it goes in infinite loop instead of taking values when it sees cin in for loop?

I am running this program on Mac OS X and compiling it using g++ compiler.

Improve this question
15
  • 1
    This isn't an answer, but since you only want the user to enter positiv numbers you should use unsigned int or perhaps unsigned long instead of int. Commented Feb 15, 2015 at 23:43
  • 1
    What you're missing is that cin is an object that has a state. That state can be good, bad, fail, or eof. You should add error checking to make sure after you read user input that your cin state is still good before invoking >> on it again. A good C++ tutorial will explain how. The suggestions here to change data types are completely missing the point and you will still have this problem, just with different values of user input. Commented Feb 15, 2015 at 23:47
  • 1
    @NeilKirk I can not find precise information but size_t has only 4 bytes on my computer. It is an alias to unsigned int. Even if other compilers give it a bigger size, this solution is not portable. Commented Feb 16, 2015 at 0:05
  • 1
    @NO_NAME size_t is guaranteed to be large enough to hold the size of the largest possible contiguous array. This program is allocating contigous arrays. Why is that not portable? Commented Feb 16, 2015 at 0:07
  • 1
    @NO_NAME: That's right, size_t doesn't solve the original problem. But on the other hand, suggesting long long int or whatever doesn't work either in this program because the prototype to malloc is void *malloc(size_t size);. You'd just be truncating whatever massive number the user typed. Commented Feb 16, 2015 at 0:09

2 Answers 2

Reset to default
1

1) You are trying assign to int value bigger than 2147483647, which is usually maximum value for this type. Generally, if you want to handle such a large numbers, you should use long long int (or something from <cstdint> for better portability).

2) You don't clear state of cin after it fails. The code bollow generate infinite loop:

int i = 0;
while (i <= 0)
{
    std::cout << "Enter a number greater than 10..." << std::endl;
    std::cin >> i;
}

You can solve it this way:

int i = 0;
while (i <= 0)
{
    std::cout << "Enter a number greater than 10..." << std::endl;
    if (!(std::cin >> i))
    {
        std::cin.clear();  // Clear error flag
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');  // Remove incorrect data from buffer
    }
}

3) You are trying to create a really big array. You need a few GiB of contiguous memory to this. Even if you succeed allocate the array, it is still a design issue. You should use many of smaller arrays or use/create a suitable container.

Improve this answer
Sign up to request clarification or add additional context in comments.

6 Comments

You mean maximum value of int for this particular compiler. You should use std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); to clear the entire buffer. Why not the smaller if (! (std::cin >> i))? #3 depends. I've allocated multi-gigabyte arrays before for specialist purposes.
Good observations. I'll change it.
@NeilKirk The third point is not corrected, because I still believe that the allocation of such a large array is a design issue. It not so hard to write own container, which behave like ordinary array.
What happens if my machine has dozens of gigabytes of ram and I need to load 10gb of data and processing speed is important? (ie minimize indirections)
@NeilKirk OK, you have right. But I won't change my answer :P Beginners, who will be reading this, they should not take on bad habits.
|
0

The maximum value of a 32 bit signed integer is 2^31-1, namely 2147483647. Now, if you assign a higher value to an Int variable the behaviour is quite unexpected, try and see what i actually contains.

BTW, what do you mean by "infinite loop"? Does the program crash or does it never end executing?

Improve this answer

7 Comments

It doesn't crash. I keeps printing "Enter Number: " string infinitely without terminating or waiting for cin input.
I printed the value of i and it's shown as 2147483647 which is the highest possible value for a signed int. But still it should not go in infinite loop. Seems there is something wrong with my cin object as pointed out by @indiv above.
@PrabhashRathore Are you sure it's in a truly infinite loop, and not just a loop of a few billion?
@PrabhashRathore: I found a good explanation for you: stackoverflow.com/questions/5864540/infinite-loop-with-cin
@Neil Kirk: I think you are right. It could be a loop of billion count. I tried to let my program run for a few minutes for it to crash or terminate but it kept running so thought it's running infinitely but it could just be a billion count loop. Let me verify the count of the loop.
|

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.