是内存问题吗？ c ++ [代码块] [Windows]

Question

My code blocks of able to compile the code below and worked well:

unsigned long int a=100000000000;

My code blocks of able to compile the code below But my program crashed immediately after i enter the same number(100000000000) :

unsigned long int a;
 cin>>a;

how to fix? Why the fist one is working !why not the second case?

if i entered a number more than max unsigned long int size! then why did the first work?

Answer 1

There is no reason for crash here.

The initialization causes a warning on my compiler because of truncation of an unsigned long long value, and the read attempt simply result in an failed read and does not change the value of the variable.

Code demonstrating it (on a 32 bit machine):

#include <iostream>
#include <iomanip>

using std::cin;
using std::cout;
using std::endl;

int main() {
    unsigned long int b=100000000000; // warning here

    unsigned long int a = 0;
    cin>>a;

    cout << "b:" << b << "(" << std::hex << b << ")" << " a:" << std::dec << a << endl;

    return 0;
}

The output is as expected

b:1215752192(4876e800) a:0

because as an ULL on my architecture 100000000000 is 0x174876E800 and the high order bytes have been truncated...