[cfe-dev] Questions/discussions about cast types in clang

[Stephen Kelly via cfe-dev]

On 20/06/2020 08:21, Ray Zhang via cfe-dev wrote:
> Hi all,
>
> I'm currently implementing a clang tool to replace C style casts to 
> C++ style casts, i.e. const/static/reinterpret_casts. I'm almost done, 
> but I've ran into a collection of issues/questions that I'd like to 
> raise here to make sure it's not a misunderstanding on my part. My 
> solutions are included in each bullet point:
>
> 1. (Question) For dependent types, it is unknown whether static_cast 
> or reinterpret_cast is appropriate for the situation, so I would like 
> to leave that as a C-style cast in the refactor tool. Example below:
>
> template <typename T> void foo() { int* ptr; (T*) ptr; }
> If we call foo<int>, it can be a const_cast, but if we call 
> foo<double>, it has to be a reinterpret_cast. In a situation where the 
> user is writing library code, it won't only be relevant to the current 
> translation unit but also for future purposes. Therefore, I propose 
> that we leave dependent type c-style casts as they are.


Makes sense. You shouldn't be trying to transform instantiations of 
templates. Perhaps issuing a warning as Arthur wrote makes sense though.


> 3. (Question) In general, should this tool for C++ also support OpenCL 
> and Objective C++? Technically, const/static/reinterpret_casts exist 
> in those languages (see https://godbolt.org/z/DEz8Rs for example). In 
> my opinion, I think the casting tool should strictly adhere to C++ as 
> the language of support and nothing more for the first iteration 
> because different coding standards may be held for different 
> languages(and I only have ample experience with C++). As a result, I 
> propose that the first iteration of the casting tool be placed outside 
> of clang-tidy as clang-tidy also supports Objective C (but not 
> OpenCL), as seen here: https://clang.llvm.org/extra/clang-tidy/#id2. 
> In the main driver I will explicitly check whether the program is in C++.


Clang-tidy has plenty of checks which are C++-specific.

You can make your check c++-specific too:


bool isLanguageVersionSupported(const LangOptions &LangOpts) const 
override {
   return LangOpts.CPlusPlus;
}

clang-tidy is still an appropriate place for your check.

Thanks,

Stephen.


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