clang-tidy cppcoreguidelines-special-member-functions should encourage rule of zero

[peakschris]

The current wording in cppcoreguidelines-special-member-functions description and documentation encourages classes with a default or no-op destructor to be converted into rule of five classes.

Take:

myclass.hxx

class myclass
{
    myclass();
    ~myclass();
}

myclass.cxx

myclass::~myclass()
{
}

clang-tidy on cxx yields:
Use '= default' to define a trivial destructor (fix available)

clang-tidy on hxx yields:
Class 'myclass' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator

Neither message encourages the developer to remove the destructor hence creating a rule-of-zero class as per: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-zero

Both messages encourage a new developer to add special functions instead.

Perhaps this requires a custom rule-of-zero check

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: None (peakschris)

The current wording in cppcoreguidelines-special-member-functions description and documentation encourages classes with a default or no-op destructor to be converted into rule of five classes.

Take:

myclass.hxx

class myclass
{
    myclass();
    ~myclass();
}

myclass.cxx

myclass::~myclass()
{
}

clang-tidy on cxx yields:
Use '= default' to define a trivial destructor (fix available)

clang-tidy on hxx yields:
Class 'myclass' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator

Neither message encourages the developer to remove the destructor hence creating a rule-of-zero class as per: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-zero

Both messages encourage a new developer to add special functions instead.

Perhaps this requires a custom rule-of-zero check

[carlosgalvezp]

I agree that the user should be guided towards rule of 0, but I think it's best to have a separate check for that implementing the previous rule, C.20.

[vbvictor]

clang-tidy on hxx yields:
Class 'myclass' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator

As an alternative, we could extend the wording to:
Class 'myclass' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator; consider using rule of zero or defining missing methods
or
Class 'myclass' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator; consider removing custom destructor or defining missing methods

AFAIK, this would fix the issue, WDYT?

[peakschris]

For sure the existing check could be reworded to prefer rule of zero. Since characters are limited in UI space and often descriptions are truncated how about:

Class 'myclass' does not follow rule of zero or five (destructor defined but some special functions missing). Prefer rule of zero (remove destructor) but if not possible then follow rule of five (define all five special functions).

There could be another check when it is more likely that rule of zero is the correct direction; e.g. if no virtual functions and and no manual memory management this check could guide more assertively towards that outcome.

I don't know the internals of clang-tidy so don't know how much context is available when checking a header file alone.

[carlosgalvezp]

Rewording makes sense! I agree it would be good to start with "does not follow Rule of O nor Rule of 5" , and then provide the details (possibly as a note instead?)