large negative number multiplied by -1 gives output negative c++

Question

Why is the output of this program -2147483648?

#include <iostream>
using namespace std;

int main() {
    long long a=-2147483648;
    a=a*-1;
    cout<<a;
    return 0;
}

It should be 2147483648 because it is in range of long long. Why the sign is not changing? I have even tried abs() function but the result is the same.

Also more surprising is that this program outputs 2147483648:

#include <iostream>
using namespace std;

int main() {
    long long a=-2147483648;
    a=a*-1;
    a=a*-1;
    cout<<a;
    return 0;
} 

The second time, multiplying by -1 worked. If it matters, I'm using C++ 4.8.1.

Answer 1

Enable -Wall and you'll see the answer.

foo.C:5: warning: this decimal constant is unsigned only in ISO C90

Use -2147483648LL in place of your constant.

Answer 2

Add 'll' to the end of your declaration:

long long a=-2147483648ll;

I checked this on IDEONE and saw the problem and verified the fix. The problem is that your constant value exceeds that for a default int so you have to type it (as long long) to get the constant you expect.