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Difficulty-MediumA false positive or false negative report which is expected to take 1-5 days effort to addressA false positive or false negative report which is expected to take 1-5 days effort to addressImpact-HighStandard-AUTOSARfalse positive/false negativeAn issue related to observed false positives or false negatives.An issue related to observed false positives or false negatives.top-25-fpsuser-reportIssue reported by an end user of CodeQL Coding StandardsIssue reported by an end user of CodeQL Coding Standards
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A15-4-2
Description
Copy elision allows the compiler to omit copy and move construction in certain circumstances. Some of these circumstances are considered "mandatory" - i.e. the compiler must omit the explicit construction - and in those circumstances copy and move constructors may never be called.
If you have move/copy constructors which are never expected to be called because they are always elided, you may decide to throw an exception from the copy/move constructor to enforce that it is only used in circumstances where the elision occurs. As our call graph does not currently reflect the mandatory elision of functions, this can cause false positives.
Example
// `foo` is only used in circumstances where it is elided, so this exception is never thrown in practice
constexpr foo(const foo&) : dummy() {
throw some_bad_exception();
};
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Difficulty-MediumA false positive or false negative report which is expected to take 1-5 days effort to addressA false positive or false negative report which is expected to take 1-5 days effort to addressImpact-HighStandard-AUTOSARfalse positive/false negativeAn issue related to observed false positives or false negatives.An issue related to observed false positives or false negatives.top-25-fpsuser-reportIssue reported by an end user of CodeQL Coding StandardsIssue reported by an end user of CodeQL Coding Standards
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lcartey commentedon Jan 6, 2023
See also:
#20
As it appears we do sometimes elide the copy/move constructors.
lcartey commentedon Mar 17, 2023
Modify https://github.com/github/codeql-coding-standards/blob/main/cpp/common/src/codingstandards/cpp/exceptions/ExceptionFlow.qll#LL315C49-L315C97 to exclude calls to copy or move constructors we think are elided.
lcartey commentedon Jan 20, 2025
Adding a reference to these related changes:
#641
This eliminates unused compiler generated elided functions. However, because the user in the above case explicitly added the elided function, I believe that's why we still add a call in this case. We should confirm by creating a test case.