[QGIS-Developer] clang-tidy warning for narrowing to float

[David Koňařík]

Hi,
thanks for your input. We can, however, disable only some aspects of the 
warning, and the only ones I'd like to disable are warnings on floating 
point conversions.

The PR you linked fixed a bug with an unwanted integer-to-integer 
conversion. I'm convinced something like this couldn't happen with float 
conversions, but feel free to send me counterexamples.

David Koňařík

On 10/28/25 09:09, Julien Cabieces via QGIS-Developer wrote:
> 
> Hi,
> 
> I totally get your point, and I already faced such a situation where I
> have to introduce (too) many static_cast<>. On the other side, I already
> faced a situation where the use of this warning would have prevented a
> real issue [0].
> 
> So, I'm not really in favor of your proposal because it would disable
> the warning for the whole codebase, on some situation where it could be relevant. Could we not instead:
> - introduce float getters/setters to have the conversion at only one place
> - just have some converted variable "const float fvar =
> static_cast<float>( var )". I don't think it makes the code so difficult
> to read.
> 
> Regards,
> Julien
> 
> 
> [0] https://github.com/qgis/QGIS/pull/50016
> 
> 
>> Hi all,
>> I'd like to propose a change to QGIS's clang-tidy settings. Currently,
>> we have bugprone-narrowing-conversions [0] turned on by wildcard,
>> which by default warns on converting between integer types, between
>> floating point types, and from integers to floating points, if the
>> full range of the source type won't fit into the destination type
>> without loss of precision.
>>
>> I fully agree that conversions between different integer types should
>> be conscious decisions, since they can lead to security issues [1]. I
>> find the floating point warnings to have dubious usefulness, though.
>>
>> In QGIS' 3D code, we often need to use integer values (e.g. screen
>> size) in floating point calculations, with practically no chance that
>> the integer value won't be representable as a float, and zero risk
>> even if it is - creating a security issue would require a very
>> creative abuse of floats. Fixing the warning leads to a visual spam of
>> static_cast<float>()s everywhere, obscuring any actual bugs.
>>
>> Converting doubles to floats can actually cause issues, but it's also
>> inevitable in many places, seeing as Qt3D uses floats, but we need to
>> use doubles ourselves for precision reasons. Given this, I also think
>> the warning isn't very useful here.
>>
>> I propose turning off WarnOnIntegerToFloatingPointNarrowingConversion
>> and possibly WarnOnFloatingPointNarrowingConversion, based on your
>> feedback.
>>
>> David Koňařík
>>
>> [1]:
>> https://clang.llvm.org/extra/clang-tidy/checks/bugprone/narrowing-conversions.html
>> [0]: e.g. char *copy_string(char *str, uint64_t str_size) {
>>    uint32_t size_plus_null = str_size + 1;
>>    char *copy = malloc(size_plus_null);
>>    memcpy(copy, str, str_size);
>>    copy[str_size] = '\0';
>>    return copy;
>> }
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