Re: auto_ptr of incomplete type not allowed?
On 3 Feb., 00:08, "Leigh Johnston" <le...@i42.co.uk> wrote:
N3000 states that an incomplete type is allowed for shared_ptr and
unique_ptr but it doesn't say the same for auto_ptr. Is this omission a
mistake or deliberate?
An incomplete type auto_ptr member variable should be allowed as long as
parent class destructor is not declared inline. This is useful when
implemented pimpl idiom:
struct opaque_type;
struct foo
{
~foo(); // dtor defined elsewhere
std::auto_ptr<opaque_type> pimpl;
};
std::auto_ptr is a deprecated component from
C++0x on, it's suggested replacement is
std::unique_ptr.
The design of std::auto_ptr bases essentially
on some obscure rule in C++03, and has some
inherent defects. It seemed to be the best
compromise not to change std::auto_ptr at all
- a correct fix would have removed
std::auto_ptr_ref but this change would have
caused a potential backward-compatibility problem
for those who explicitly did depend on this
template.
The fact that std::auto_ptr_ref was specified
as a normal component (and not kept unspecified)
was another problem of the original spec. This
will be changed for C++0x.
Adding support for incomplete types for auto_ptr
was considered by the committee but the idea
was rejected, because of it's deprecated state.
Any change would have required that existing
implementations need to be changed and in regard
to existing alternatives this seemed not worth
the effort, such a correct specification would
require.
Greetings from Bremen,
Daniel Kr?gler
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