[Bug c++/88050] Add a warning for breaking the "Rule of Three"

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88050

felix <felix.von.s at posteo dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |felix.von.s at posteo dot de

--- Comment #4 from felix <felix.von.s at posteo dot de> ---
Created attachment 52401
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52401&action=edit
Basic patch (no tests)

Attached is a simple patch adding a warning checks for violations of the Rule
of Three and the Rule of Five (Core Guideline C.21) and enabling it at the
-Wextra level.

Not included is a check for the rule-of-four-and-a-half variant: a swapping
by-value assignment operator overload, paired with the appropriate constructor,
is enough to implement copy- or move-assignment.

Rule of Zero would be much harder to check for, but it may be possible to
diagnose certain violations of it, by looking at the code inside the
destructor: if the delete operator, or a single function is invoked more than
once in the destructor, it probably means the class manages more than one
resource.