Default std::vector<bool> default and move constructor
Jonathan Wakely
jwakely@redhat.com
Thu Jun 1 13:34:00 GMT 2017
On 31/05/17 22:28 +0200, François Dumont wrote:
>Unless I made a mistake it revealed that restoring explicit call to
>_Bit_alloc_type() in default constructor was not enough. G++ doesn't
>transform it into a value-init if needed. I don't know if it is a
>compiler bug but I had to do just like presented in the Standard to
>achieve the expected behavior.
That really shouldn't be necessary (see blow).
>This value-init is specific to post-C++11 right ? Maybe I could remove
>the useless explicit call to _Bit_alloc_type() in pre-C++11 mode ?
No, because C++03 also requires the allocator to be value-initialized.
>Now I wonder if I really introduced a regression in rb_tree...
Yes, I think you did. Could you try to verify that using the new
default_init_allocator?
>+ struct _Bvector_impl
>+ : public _Bit_alloc_type, public _Bvector_impl_data
>+ {
>+ public:
>+#if __cplusplus >= 201103L
>+ _Bvector_impl()
>+ noexcept( noexcept(_Bit_alloc_type())
>+ && noexcept(_Bvector_impl(declval<const _Bit_alloc_type&>())) )
This second condition is not needed, because that constructor should
be noexcept (see below).
>+ : _Bvector_impl(_Bit_alloc_type())
This should not be necessary...
>+ { }
>+#else
> _Bvector_impl()
>- : _Bit_alloc_type(), _M_start(), _M_finish(), _M_end_of_storage()
>+ : _Bit_alloc_type()
> { }
>+#endif
I would expect the constructor to look like this:
_Bvector_impl()
_GLIBCXX_NOEXCEPT_IF( noexcept(_Bit_alloc_type()) )
: _Bit_alloc_type()
{ }
What happens when you do that?
> _Bvector_impl(const _Bit_alloc_type& __a)
>- : _Bit_alloc_type(__a), _M_start(), _M_finish(), _M_end_of_storage()
>+ _GLIBCXX_NOEXCEPT_IF( noexcept(_Bit_alloc_type(__a)) )
Copying the allocator is not allowed to throw. You can use simply
_GLIBCXX_NOEXCEPT here.
>+void test01()
>+{
>+ typedef default_init_allocator<T> alloc_type;
>+ typedef std::vector<T, alloc_type> test_type;
>+
>+ test_type v1;
>+ v1.push_back(T());
>+
>+ VERIFY( !v1.empty() );
>+ VERIFY( !v1.get_allocator().state );
This is unlikely to ever fail, because the stack is probably full of
zeros anyway. Did you confirm whether the test fails without your
fixes to value-initialize the allocator?
One possible way to make it fail would be to construct the
vector<bool> using placement new, into a buffer filled with non-zero
values. (Valgrind or a sanitizer should also tell us, but we can't
rely on them in the testsuite).
More information about the Gcc-patches
mailing list