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From: Ian Collins <ian-news@hotmail.com>
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Date: Thu, 29 Aug 2019 23:35:05 +1200
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Xref: csiph.com comp.lang.c++:67996

On 29/08/2019 23:24, David Brown wrote:
> On 29/08/2019 12:21, Ian Collins wrote:
>> On 29/08/2019 21:02, David Brown wrote:
>>>
>>> One thing that I gather Rust does well, that C++ has trouble with, is
>>> that when you do the Rust equivalent of std::move'ing something, any
>>> attempt to use the moved-from object is flagged by the compiler.  I
>>> think that would be good to spot in C++.
>>
>> clang-tidy does a pretty good job with "bugprone-use-after-move"
>> <https://clang.llvm.org/extra/clang-tidy/checks/bugprone-use-after-move.html>
>>
> 
> Yes, I saw that after googling a bit.
> 
> I would have thought, however, that pretty much any use (other than
> assignment of some sort) of a named object after it is moved, is likely
> to be a bug.  And it is therefore something that could be warning about
> by default.
> 
> 
> #include <memory>
> 
> extern int a, b, c;
> 
> 
> void foo(void) {
>      auto p = std::make_unique<int>(3);
>      a = *p;
>      auto q = std::move(p);
>      b = *q;
>      c = *p;
> }
> 
> 
> Both clang and gcc are smart enough to move the value 3 directly into
> "a" and "b", and if the last line is omitted, they can remove the call
> to "new" (at least in their trunk versions).  Both are smart enough to
> know that "c = *p;" is undefined behaviour here, and generate "ud2"
> instructions on x86.
> 
> Yet neither compiler warns about this clearly wrong behaviour despite
> -Wall -Wextra.

Use a bigger hammer!

$ clang-tidy x.cc

1 warning generated.
/tmp/x.cc:11:9: warning: Dereference of null smart pointer 'p' of type 
'std::unique_ptr' [clang-analyzer-cplusplus.Move]
     c = *p;
         ^
/tmp/x.cc:9:14: note: Smart pointer 'p' of type 'std::unique_ptr' is 
reset to null when moved from
     auto q = std::move(p);
              ^
/tmp/x.cc:11:9: note: Dereference of null smart pointer 'p' of type 
'std::unique_ptr'
     c = *p;

-- 
Ian.