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c++ccomparisonunsignedsigned

Would it break the language or existing code if we'd add safe signed/unsigned compares to C/C++?

After reading this question on signed/unsigned compares (they come up every couple of days I'd say):

I wondered why we don't have proper signed unsigned compares and instead this horrible mess? Take the output from this small program: 

#include <stdio.h>
#define C(T1,T2)\
 {signed   T1 a=-1;\
 unsigned T2 b=1;\
  printf("(signed %5s)%d < (unsigned %5s)%d = %d\n",#T1,(int)a,#T2,(int)b,(a<b));}\

 #define C1(T) printf("%s:%d\n",#T,(int)sizeof(T)); C(T,char);C(T,short);C(T,int);C(T,long);
int main()
{
 C1(char); C1(short); C1(int); C1(long); 
}

Compiled with my standard compiler (gcc, 64bit), I get this:

char:1
(signed  char)-1 < (unsigned  char)1 = 1
(signed  char)-1 < (unsigned short)1 = 1
(signed  char)-1 < (unsigned   int)1 = 0
(signed  char)-1 < (unsigned  long)1 = 0
short:2
(signed short)-1 < (unsigned  char)1 = 1
(signed short)-1 < (unsigned short)1 = 1
(signed short)-1 < (unsigned   int)1 = 0
(signed short)-1 < (unsigned  long)1 = 0
int:4
(signed   int)-1 < (unsigned  char)1 = 1
(signed   int)-1 < (unsigned short)1 = 1
(signed   int)-1 < (unsigned   int)1 = 0
(signed   int)-1 < (unsigned  long)1 = 0
long:8
(signed  long)-1 < (unsigned  char)1 = 1
(signed  long)-1 < (unsigned short)1 = 1
(signed  long)-1 < (unsigned   int)1 = 1
(signed  long)-1 < (unsigned  long)1 = 0

If I compile for 32 bit, the result is the same except that: 

long:4
(signed  long)-1 < (unsigned   int)1 = 0

The "How?" of all this is easy to find: Just goto section 6.3 of the C99 standard or chapter 4 of C++ and dig up the clauses which describe how the operands are converted to a common type and this can break if the common type reinterprets negative values.

But what about the "Why?". As we can see, the '<' fails in 50% of all cases, also it depends on the concrete sizes of the types, so it is platform dependent. Here are some points to consider:

  • The convert & compare process is not really a prime example for the Rule of Least Surprise

  • I don't believe that there is code out there, which relies on the proposition that (short)-1 > (unsigned)1 and is not written by terrorists.

  • This is all terrible when you're in C++ with template code, because you need type trait magic to knit a correct "<".

After all, comparing signed and unsigned value of different types is easy to implement: 

signed X < unsigned Y -> (a<(X)0) || ((Z)a<(Z)b) where Z=X|Y

The pre-check is cheap and can also be optimized away by the compiler if a>=0 can statically be proven.

So here's my question:

Would it break the language or existing code if we'd add safe signed/unsigned compares to C/C++?

("Would it break the language" means would we need to make massive changes to different parts of the language to accommodate this change)

UPDATE: I've ran this on my good old Turbo-C++ 3.0 and got this output: 

char:1
(signed  char)-1 < (unsigned  char)1 = 0

Why is (signed char)-1 < (unsigned char) == 0 here?

Solution

  • Yes it would break the language/existing code. The language, as you have noted, carefully specifies the behavior when signed and unsigned operands are used together. This behavior with comparison operators is essential for some important idioms, like:

    if (x-'0' < 10U)

    Not to mention things like (equality comparison):

    size_t l = mbrtowc(&wc, s, n, &state);
if (l==-1) ... /* Note that mbrtowc returns (size_t)-1 on failure */

    As an aside, specifying "natural" behavior for mixed signed/unsigned comparisons would also incur a significant performance penalty, even in programs which are presently using such comparisons in safe ways where they already have their "natural" behavior due to constraints on the input which the compiler would have a hard time determining (or might not be able to determine at all). In writing your own code to handle these tests, I'm sure you've already seen what the performance penalty would look like, and it's not pretty.