CODEWITHSUNDEEP
c++gcc
mkstrong
mkstrong

Reputation: 31

gcc warning -Wsign-compare doesn't seem to work when comparing const

I seem to not be getting sign compare errors on my g++ gcc version 5.1.1 20150618 (Red Hat 5.1.1-4) (GCC).

When I compile the following with the options I don't get an error - but when I get rid of the const - it shows the warning.

Question: Is this expected behavior?

g++ typecompat.cxx -Wsign-compare -std=c++14

This is the code

#include <stddef.h>
#include <stdio.h>
#include <assert.h>
#include <vector>

 int main() {
    std::vector<int> v {1,2,3};
    const int b = 2;
    assert(v.size()>b);
    return 0;
}

Upvotes: 1

Views: 470

Answers (1)

Shafik Yaghmour
Shafik Yaghmour

Reputation: 158469

From a godbolt session of this code it looks like gcc is performing constant folding:

cmpq    $2, %rax

So then it knows for sure the comparison is ok, any attempting to change a constant variable is undefined behavior and the compiler will assume no undefined behavior. If we look at the documents it does say:

Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned

This gcc bug report Bogus warning with -Wsign-compare looks like it covers this kind of false positive detection:

To be useful, these warnings are meant to be smart - not warning if it can be proved that the signed value is nonnegative [...]

Upvotes: 3

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