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Sun, 16 Dec 2018 23:58:34 -0800


> On Dec 16, 2018, at 3:47 PM, Daniel Bates <dba...@webkit.org> wrote:
> 
> 
> 
>> On Dec 14, 2018, at 3:52 PM, Chris Dumez <cdu...@apple.com 
>> <mailto:cdu...@apple.com>> wrote:
>> 
>> So to be clear, it is often not truly about using the value after it is 
>> moved. It is about expecting that the variable / member has been nulled out 
>> after moving it.
>> If I WTFMove() out a data member m_dataMember, I expect later on `if 
>> (m_dataMember)` to be false.
>> 
> 
> I do not mean to derail the discussion about whether or not we should have 
> our own std::optional that disengages on move. I am all for convenience. I 
> think expecting a valid ¡°empty¡± state (*) when moving out a data member is an 
> anti-pattern and I propose that we should be using std::exchange() instead 
> (or any code analogous to doing std::exchange()) for these cases, including 
> the case in your original email and especially with data members as in 
> <https://trac.webkit.org/changeset/239228 
> <https://trac.webkit.org/changeset/239228>>.

Expectations can¡¯t be an anti-pattern, only code can be an anti-pattern. At the 
point of checking whether a value is ¡°empty¡± by some definition, you may not 
know that someone has std::move()d it, and shouldn¡¯t have to. So to deal with 
the bug-prone nature of check after std::move(), we can do one of these:

(1) Ban all use of std::move() unless it¡¯s possible for some sort of static 
check to know that no one will examine the moved value
(2) Make sure we only use types that leave themselves in a state that meets 
expectations post-move().
(3) Try real hard not to make mistakes and hope for the best

Maybe there is an option I am missing? Out of these, I think (2) is likely the 
best, on the whole.

> 
> Why do I consider it an anti-pattern? See (*) and because the concept of 
> ¡°moving" is not spec'ed to behave like this. Here¡¯s one reference to the 
> spec¡¯ed behavior and an example comparable to the one in your first email 
> (highlighting is my own for emphasis):
> 
> [[
> Unless otherwise specified, all standard library objects that have been moved 
> from are placed in a valid but unspecified state. That is, only the functions 
> without preconditions, such as the assignment operator, can be safely used on 
> the object after it was moved from:

Note that this is specified for ¡°standard library objects¡±. There¡¯s nothing 
wrong with choosing to give a stronger API contract for our own objects, and 
then to rely on it.

> std::vector <free online bettinghttp://en.cppreference.com/w/cpp/container/vector><std::string 
> <free online bettinghttp://en.cppreference.com/w/cpp/string/basic_string>> v;
> std::string <http://en.cppreference.com/w/cpp/string/basic_string> str = 
> "example";
> v.push_back(std::move(str)); // str is now valid but unspecified
> str.back(); // undefined behavior if size() == 0: back() has a precondition 
> !empty()
> str.clear(); // OK, clear() has no preconditions
> 
> ]]
> <https://en.cppreference.com/w/cpp/utility/move 
> <https://en.cppreference.com/w/cpp/utility/move>>
> 
> (*) What is the valid ¡°empty¡± state for a type without a default constructor?
> 
> Dan
> 
>> --
>> ? Chris Dumez
>> 
>> 
>> 
>> 
>>> On Dec 14, 2018, at 3:45 PM, Chris Dumez <cdu...@apple.com 
>>> <mailto:cdu...@apple.com>> wrote:
>>> 
>>> 
>>>> On Dec 14, 2018, at 3:39 PM, Fujii Hironori <fujii.hiron...@gmail.com 
>>>> <mailto:fujii.hiron...@gmail.com>> wrote:
>>>> 
>>>> 
>>>> On Sat, Dec 15, 2018 at 6:38 AM Chris Dumez <cdu...@apple.com 
>>>> <mailto:cdu...@apple.com>> wrote:
>>>> 
>>>> I have now been caught twice by std::optional¡¯s move constructor. 
>>>> 
>>>>  I don't understand how this could be useful? Do you want to use the value 
>>>> after it is moved? I'd like to see these your code. Could you show me 
>>>> these two patches?
>>> 
>>> This is the latest one: https://trac.webkit.org/changeset/239228/webkit 
>>> <https://trac.webkit.org/changeset/239228/webkit>
>>> 
>>> 
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>> 
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