c++ - Why it is different between -2147483648 and (int)-2147483648

Question

When I run the following code under Windows7 x64, compiled with GCC of MinGW, the result seems to be underflowed:

cout<<-2147483648 ;    //Output: 2147483648

but when I assigned it to a integer variable, or just simply convert it to the int type :

cout<<(int)-2147483648 ; //Output: -2147483648 

So, what's wrong with the previous version of my code? Istn't it the type int? or what the lower bound the Integer is exactly? Many thanks.

Answer 1

2147483648 doesn't fit into an int or a long on your system, so it's treated as a constant of type unsigned long. (Edit: as ouah pointed out in the comments, it's undefined behaviour in standard C++, but your compiler accepts it as an extension.) Negating an unsigned integer value is possible, but results in another unsigned integer value, never a negative number. Negating 2147483648UL produces 2147483648UL (assuming, as is the case on your system, that unsigned long is a 32 bit type).

Casting that to int produces an implementation-defined result, commonly the result you see, but not necessarily. You can get the result you want without any conversions by writing -2147483647 - 1.