Signed/unsigned comparisons - c++

Question

I'm trying to understand why the following code doesn't issue a warning at the indicated place.
//from limits.h
#define UINT_MAX 0xffffffff /* maximum unsigned int value */
#define INT_MAX 2147483647 /* maximum (signed) int value */
/* = 0x7fffffff */
int a = INT_MAX;
//_int64 a = INT_MAX; // makes all warnings go away
unsigned int b = UINT_MAX;
bool c = false;
if(a < b) // warning C4018: '<' : signed/unsigned mismatch
c = true;
if(a > b) // warning C4018: '<' : signed/unsigned mismatch
c = true;
if(a <= b) // warning C4018: '<' : signed/unsigned mismatch
c = true;
if(a >= b) // warning C4018: '<' : signed/unsigned mismatch
c = true;
if(a == b) // no warning <--- warning expected here
c = true;
if(((unsigned int)a) == b) // no warning (as expected)
c = true;
if(a == ((int)b)) // no warning (as expected)
c = true;
I thought it was to do with background promotion, but the last two seem to say otherwise.
To my mind, the first == comparison is just as much a signed/unsigned mismatch as the others?

Answer 1

When comparing signed with unsigned, the compiler converts the signed value to unsigned. For equality, this doesn't matter, -1 == (unsigned) -1. For other comparisons it matters, e.g. the following is true: -1 > 2U.
EDIT: References:
5/9: (Expressions)
Many binary operators that expect
operands of arithmetic or enumeration
type cause conversions and yield
result types in a similar way. The
purpose is to yield a common type,
which is also the type of the result.
This pattern is called the usual
arithmetic conversions, which are
defined as follows:
If either
operand is of type long double, the
other shall be converted to long
double.
Otherwise, if either operand
is double, the other shall be
converted to double.
Otherwise, if
either operand is float, the other
shall be converted to float.
Otherwise, the integral promotions
(4.5) shall be performed on both
operands.54)
Then, if either operand
is unsigned long the other shall be
converted to unsigned long.
Otherwise, if one operand is a long
int and the other unsigned int, then
if a long int can represent all the
values of an unsigned int, the
unsigned int shall be converted to a
long int; otherwise both operands
shall be converted to unsigned long
int.
Otherwise, if either operand is
long, the other shall be converted to
long.
Otherwise, if either operand
is unsigned, the other shall be
converted to unsigned.
4.7/2: (Integral conversions)
If the destination type is unsigned,
the resulting value is the least
unsigned integer congruent to the
source integer (modulo 2n where n is
the number of bits used to represent
the unsigned type). [Note: In a two’s
complement representation, this
conversion is conceptual and there is
no change in the bit pattern (if there
is no truncation). ]
EDIT2: MSVC warning levels
What is warned about on the different warning levels of MSVC is, of course, choices made by the developers. As I see it, their choices in relation to signed/unsigned equality vs greater/less comparisons make sense, this is entirely subjective of course:
-1 == -1 means the same as -1 == (unsigned) -1 - I find that an intuitive result.
-1 < 2 does not mean the same as -1 < (unsigned) 2 - This is less intuitive at first glance, and IMO deserves an "earlier" warning.

Answer 2

Why signed/unsigned warnings are important and programmers must pay heed to them, is demonstrated by the following example.
Guess the output of this code?
#include <iostream>
int main() {
int i = -1;
unsigned int j = 1;
if ( i < j )
std::cout << " i is less than j";
else
std::cout << " i is greater than j";
return 0;
}
Output:
i is greater than j
Surprised? Online Demo : http://www.ideone.com/5iCxY
Bottomline: in comparison, if one operand is unsigned, then the other operand is implicitly converted into unsigned if its type is signed!

Answer 3

The == operator just does a bitwise comparison (by simple division to see if it is 0).
The smaller/bigger than comparisons rely much more on the sign of the number.
4 bit Example:
1111 = 15 ? or -1 ?
so if you have 1111 < 0001 ... it's ambiguous...
but if you have 1111 == 1111 ... It's the same thing although you didn't mean it to be.

Answer 4

Starting from C++20 we have special functions for correct comparing signed-unsigned values
https://en.cppreference.com/w/cpp/utility/intcmp

Answer 5

In a system that represents the values using 2-complement (most modern processors) they are equal even in their binary form. This may be why compiler doesn't complain about a == b.
And to me it's strange compiler doesn't warn you on a == ((int)b). I think it should give you an integer truncation warning or something.

Answer 6

The line of code in question does not generate a C4018 warning because Microsoft have used a different warning number (i.e. C4389) to handle that case, and C4389 is not enabled by default (i.e. at level 3).
From the Microsoft docs for C4389:
// C4389.cpp
// compile with: /W4
#pragma warning(default: 4389)
int main()
{
int a = 9;
unsigned int b = 10;
if (a == b) // C4389
return 0;
else
return 0;
};
The other answers have explained quite well why Microsoft might have decided to make a special case out of the equality operator, but I find those answers are not super helpful without mentioning C4389 or how to enable it in Visual Studio.
I should also mention that if you are going to enable C4389, you might also consider enabling C4388. Unfortunately there is no official documentation for C4388 but it seems to pop up in expressions like the following:
int a = 9;
unsigned int b = 10;
bool equal = (a == b); // C4388