Infinite loop without accepting input

问题:

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.

回答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.

回答2:

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?