[PATCH] libstdc++: Fix vector(from_range_t, R&&) for exceptions [PR120367]

[Jonathan Wakely]

On Thu, 22 May 2025, 07:21 Tomasz Kaminski, <tkaminsk@redhat.com> wrote:

>
>
> On Wed, May 21, 2025 at 5:41 PM Jonathan Wakely <jwakely@redhat.com>
> wrote:
>
>> Because this constructor delegates to vector(a) the object has been
>> fully constructed and the destructor will run if an exception happens.
>> That means we need to set _M_finish == _M_start so that the destructor
>> doesn't try to destroy any elements.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         PR libstdc++/120367
>>         * include/bits/stl_vector.h (_M_range_initialize): Initialize
>>         _M_impl._M_finish.
>>         * testsuite/23_containers/vector/cons/from_range.cc: Check with
>>         a type that throws on construction.
>>         exceptions during construction.
>> ---
>>
>> Tested x86_64-linux.
>>
>>  libstdc++-v3/include/bits/stl_vector.h        |  1 +
>>  .../23_containers/vector/cons/from_range.cc   | 22 +++++++++++++++++++
>>  2 files changed, 23 insertions(+)
>>
>> diff --git a/libstdc++-v3/include/bits/stl_vector.h
>> b/libstdc++-v3/include/bits/stl_vector.h
>> index 57680b7bbcf3..43b913da778d 100644
>> --- a/libstdc++-v3/include/bits/stl_vector.h
>> +++ b/libstdc++-v3/include/bits/stl_vector.h
>> @@ -1971,6 +1971,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>>         {
>>           pointer __start = this->_M_impl._M_start =
>>
> Not required change, but I was a bit confused where _M_start is set.
> Maybe assign all pointers here?
>           pointer __start = this->_M_impl._M_start =
> this->_M_impl._M_finish =
>

That's how I wrote it at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120367#c4 but it seemed a bit
crowded so I did it on a separate line for the real patch.


            this->_M_allocate(_S_check_init_len(__n,
>> _M_get_Tp_allocator()));
>> +         this->_M_impl._M_finish = __start;
>>           this->_M_impl._M_end_of_storage = __start + __n;
>>           this->_M_impl._M_finish
>>               = std::__uninitialized_copy_a(_GLIBCXX_MOVE(__first),
>> __last,
>> diff --git
>> a/libstdc++-v3/testsuite/23_containers/vector/cons/from_range.cc
>> b/libstdc++-v3/testsuite/23_containers/vector/cons/from_range.cc
>> index 7a62645283d2..3784b9cd66ad 100644
>> --- a/libstdc++-v3/testsuite/23_containers/vector/cons/from_range.cc
>> +++ b/libstdc++-v3/testsuite/23_containers/vector/cons/from_range.cc
>> @@ -106,8 +106,30 @@ test_constexpr()
>>    return true;
>>  }
>>
>> +void
>> +test_pr120367()
>> +{
>> +#ifdef __cpp_exceptions
>> +  struct X
>> +  {
>> +    X(int) { throw 1; }     // Cannot successfully construct an X.
>> +    ~X() { VERIFY(false); } // So should never need to destroy one.
>> +  };
>> +
>> +  try
>> +  {
>> +    int i[1]{};
>> +    std::vector<X> v(std::from_range, i);
>> +  }
>> +  catch (int)
>> +  {
>> +  }
>> +#endif
>> +}
>> +
>>  int main()
>>  {
>>    test_ranges();
>>    static_assert( test_constexpr() );
>> +  test_pr120367();
>>  }
>> --
>> 2.49.0
>>
>>
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