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The “unsigned” Conundrum

October 16, 2016 25 comments

A few weeks ago, CppCon16 conference organizer Jon Kalb gave a great little lightning talk titled “unsigned: A Guideline For Better Code“. Right up front, he asked the audience what they thought this code would print out to the standard console:

mixed-mode-math

Even though -1 is obviously less 1, the program prints out “a is not less than b“. WTF?

The reason for the apparently erroneous result is due to the convoluted type conversion rules inherited from C regarding unsigned/signed types.

Before evaluating the (a < b) expression, the rules dictate that the signed int object, a, gets implicitly converted to an unsigned int type. For an 8 bit CPU, the figure below shows how the bit pattern 0xFF is interpreted differently by C/C++ compilers depending upon how it is declared:

 

eightbits

 

Thus, after the implicit type conversion of a from -1 to 255, the comparison expression becomes (255 < 1) –  which produces the “a is not less than b” output.

Since it’s unreasonable to expect most C++ programmers to remember the entire arcane rule set for implicit conversions/promotions, what heuristic should programmers use to prevent nasty unsigned surprises like Mr. Kalb’s example?  Here is his list of initial candidates:

 

unsignedguidlines

If you’re trolling this post and you’re a C++ hater, then the first guideline is undoubtedly your choice :). If you’re a C++ programmer, the second two are pretty much impractical – especially since unsigned (in the form of size_t) is used liberally throughout the C++ standard library. (By the way, I once heard Bjarne Stroustrup say in a video talk that requiring size_t to be unsigned was a mistake). The third and fourth guidelines are reasonable suggestions; and those are the ones I use in writing my own code and reviewing the code of others.

At the end of his interesting talk, Mr. Kalb presented his own guideline:

kalbguideline

I think Jon’s guideline is a nice, thoughtful addition to the last two guidelines on the previous chart. I would like to say that “Don’t use “unsigned” for quantities” subsumes those two, but I’m not sure it does. What do you think?

Categories: C++ Tags: , ,
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