c++ - 是否有任何 C++ 编译器可以对悬空引用发出警告？

Question

给定以下代码，其中 x 是对已消失对象的悬空 const 引用，因此是未定义的行为。

auto get_vec() { return std::vector<int>{1,2,3,4,5}; }
const auto& x = get_vec().back();

似乎 GCC 7.3、Clang 6.0 和 MSVC 都无法发出警告，即使启用了所有警告。有谁知道在这些情况下是否可以发出警告？在这些情况下，const auto& 和 auto&& 之间有什么区别吗？

请注意，如果 back() 将按值返回，则它不会是未定义行为，因为生命周期临时对象 x 已扩展为函数 scoop。

长话短说:我有一个代码库，其中 const auto& 被用作初始化变量的默认方式，出于某些奇怪的原因，这些情况使用 MSVC 正确执行，但是当使用Clang for android，每次出现都会导致错误分配的值。目前，该解决方案似乎调查了整个代码库中的每个 const auto&。此外，在许多情况下，const auto& 指的是通过引用返回的重对象，因此简单地删除 & 并不是解决方案。

还有一件事，我要为 const auto& 的误用负责:)

Answer 1

我现在唯一能想到的就是使用带有 -fsanitize=address 的 CLANG。但是当然这只会在运行时有所帮助，但是你会得到像这样的好东西:

==102554==ERROR: AddressSanitizer: heap-use-after-free on address 0x603000000020 at pc 0x00000050db71 bp 0x7ffdd3a5b770 sp 0x7ffdd3a5b768
READ of size 4 at 0x603000000020 thread T0
    #0 0x50db70 in main (/home/user/testDang+0x50db70)
    #1 0x1470fb404889 in __libc_start_main (/lib64/libc.so.6+0x20889)
    #2 0x41a019 in _start (/home/user/testDang+0x41a019)

0x603000000020 is located 16 bytes inside of 20-byte region [0x603000000010,0x603000000024)
freed by thread T0 here:
    #0 0x50a290 in operator delete(void*) (/home/user/testDang+0x50a290)
    #1 0x50eccf in __gnu_cxx::new_allocator<int>::deallocate(int*, unsigned long) (/home/user/testDang+0x50eccf)
    #2 0x50ec9f in std::allocator_traits<std::allocator<int> >::deallocate(std::allocator<int>&, int*, unsigned long) (/home/user/testDang+0x50ec9f)
    #3 0x50ec2a in std::_Vector_base<int, std::allocator<int> >::_M_deallocate(int*, unsigned long) (/home/user/testDang+0x50ec2a)
    #4 0x50e577 in std::_Vector_base<int, std::allocator<int> >::~_Vector_base() (/home/user/testDang+0x50e577)
    #5 0x50e210 in std::vector<int, std::allocator<int> >::~vector() (/home/user/testDang+0x50e210)
    #6 0x50db16 in main (/home/user/testDang+0x50db16)
    #7 0x1470fb404889 in __libc_start_main (/lib64/libc.so.6+0x20889)

previously allocated by thread T0 here:
    #0 0x509590 in operator new(unsigned long) (/home/user/testDang+0x509590)
    #1 0x50e9ab in __gnu_cxx::new_allocator<int>::allocate(unsigned long, void const*) (/home/user/testDang+0x50e9ab)
    #2 0x50e94b in std::allocator_traits<std::allocator<int> >::allocate(std::allocator<int>&, unsigned long) (/home/user/testDang+0x50e94b)
    #3 0x50e872 in std::_Vector_base<int, std::allocator<int> >::_M_allocate(unsigned long) (/home/user/testDang+0x50e872)
    #4 0x50e2ff in void std::vector<int, std::allocator<int> >::_M_range_initialize<int const*>(int const*, int const*, std::forward_iterator_tag) (/home/user/testDang+0x50e2ff)
    #5 0x50deb7 in std::vector<int, std::allocator<int> >::vector(std::initializer_list<int>, std::allocator<int> const&) (/home/user/testDang+0x50deb7)
    #6 0x50dafb in main (/home/user/testDang+0x50dafb)
    #7 0x1470fb404889 in __libc_start_main (/lib64/libc.so.6+0x20889)

SUMMARY: AddressSanitizer: heap-use-after-free (/home/user/testDang+0x50db70) in main
Shadow bytes around the buggy address:
  0x0c067fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c067fff8000: fa fa fd fd[fd]fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb

也许您有自动化的单元测试，您可以轻松地将其作为“sanizizer”构建运行。