CODEWITHSUNDEEP
c++g++gcc-warningg++4.8
anni
anni

Reputation: 331

g++ doesn't issue -Wsign-compare in template code

I recently noticed that g++ doesn't issue signed/unsigned comparison warning when the offending code is in a function template. Here is a sample:

// signed_unsigned.cc
#include <cassert>
#include <string>

template<typename T, typename U>
bool compare(T t, U u) {
    return t >= u;
}

int main(int argc, char** argv)
{
    size_t x = strtoul(argv[1], 0, 0);
    int y = strtol(argv[2], 0, 0);
    // bool chk = (x >= y);   // if I use this statement instead, it throws [-Wsign-compare] warning
    bool chk = compare(x, y);
    assert(chk);
    return 0;
}

And I'm compiling and executing it like this:

$ g++ -std=gnu++11 signed_unsigned.cc -Wall -Wsign-compare
$ ./a.out 0 -5
a.out: signed_unsigned.cc:15: int main(int, char**): Assertion `chk' failed.
Aborted (core dumped)

The assertion failure is expected as the integer promotion will convert -5 to really large unsigned value. But the compilation should have issued warning about this comparison, No?

I might be missing something basic here but I searched online and couldn't find anything relevant. Does anybody know why the template version of comparison doesn't throw warning?

GCC version used:

$ g++ --version
g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Upvotes: 0

Views: 110

Answers (1)

eerorika
eerorika

Reputation: 238351

Does anybody know why the template version of comparison doesn't throw warning?

It is probably a bug (quality of implementation issue) in that version of GCC. GCC 5.5.0 for example does not fail to issue a diagnostic for the example program, so the issue seems to have been fixed in later versions.

The assertion failure is expected as the integer promotion

To be pedantic, this conversion is not classified as integer promotion.

Upvotes: 1

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