We came across a very strange behavior with a code like this (this is a simplified version; it took quite a time before we were able to identify a similar problem in our code):


Program was compiled with the Visual Studio 2008. Can anybody explain why 'b' is not -5? It seems like 'b' is treated as unsigned int instead of int.

1.
int a = ((int-1) * x; // this does not compile!!!!

2.
assert(a == -5); // Assert fails!!!!!

Does assert(b == -5); work??

int a = ((int-1) * x;

I suppose you meant int a = ((int)-1) * x;

assert(a == -5); // Assert fails!!!!!

I suppose you meant assert(b == -5); // Assert fails!!!!!

consider int b = ((int)-1) * x / 2; which is actually
int b =( ((int)-1) * x )/ 2; if you care for the left associativity of the multiplication operator.
((int)-1) yields a integer -1 (note that this cast is useless). Since it is being multiplied by a unsigned int, -1 is promoted to a unsigned int. Any negative int when promoted to int will yield smallest non-negative value that is congruent to that integer, modulo one more than the largest value that can be represented in the unsigned type. So it is a very large number(4294967286 on my machine)which when multiplied by 10 yields larger number which is again demoted to fit into a unsigned int. Now you divide that number by two and you get the result.(which is still a larger number)(2147483643 on my machine.
The funny part is: all this doesn't happens in the line previous to this. That is perhaps because of your compiler's smartness, it doesn't promote -1 to int but demotes x to a int. This is done as, there is no more operation to perform.
Anyways., the correct procedure would be ((int)-1) * (x / 2); which would yield the desired result.
Note that int(-1) is same as -1.

If I were you I would have chosen a statement like this: int b = ( int(x)*(-1) ) /2;
The code tags are :
[code=cplusplus]
//code goes here
[/code]

This is C++ not C, so I would have written:

The main advantage of this is the you have an explicit bracket around the section you are casting and it is a BIG flag that you are changing type. Something that in numerical code is often such a big area of problems that the old (type) casts are often forbidden.

It also avoids nasty stuff when you convert (accidentally) from a pointer to a numeric type. Easily done when you have a long expression with pointers and multiplications.

1. Probably you mean doesn't promote -1 to unsigned int...
2. All this happens in int a = (int)-1 * x . The funny part is: (int)-1 is the same as -1 because an integer literal already (and always) has int type. The compiler never takes into account left part type: it promotes -1 to unsigned int (max value) then multiply it to unsigned(10) (ignores integer overflow). After that the result is unsigned(4294967286) or int(-10) - the same bit pattern...