[webkit-dev] Watch out for std::optional's move constructor

[Saam Barati]

> On Dec 14, 2018, at 1:54 PM, Saam Barati <sbarati at apple.com> wrote:
> 
> 
> 
>> On Dec 14, 2018, at 1:37 PM, Chris Dumez <cdumez at apple.com <mailto:cdumez at apple.com>> wrote:
>> 
>> Hi,
>> 
>> I have now been caught twice by std::optional’s move constructor. It turns out that it leaves the std::optional being moved-out *engaged*, it merely moves its value.
>> 
>> For example, testOptional.cpp:
>> #include <iostream>
>> #include <optional>
>> 
>> int main()
>> {
>>     std::optional<int> a = 1;
>>     std::optional<int> b = std::move(a);
>>     std::cout << "a is engaged? " << !!a << std::endl;
>>     std::cout << "b is engaged? " << !!b << std::endl;
>>     return 0;
>> }
>> 
>> $ clang++ testOptional.cpp -o testOptional -std=c++17
>> $ ./testOptional
>> a is engaged? 1
>> b is engaged? 1
>> 
>> I would have expected:
>> a is engaged? 0
>> b is engaged? 1
> I would have expected this too.
> 
>> 
>> This impacts the standard std::optional implementation on my machine as well as the local copy in WebKit’s wtf/Optional.h.
>> 
>> As far as I know, our convention in WebKit so far for our types has been that types getting moved-out are left in a valid “empty” state.
> I believe it's UB to use an object after it has been moved.
I'm wrong here.
Apparently objects are left in a "valid but unspecified state".

https://stackoverflow.com/questions/32346143/undefined-behavior-with-stdmove <https://stackoverflow.com/questions/32346143/undefined-behavior-with-stdmove>

- Saam
> 
> - Saam
> 
>> As such, I find that std::optional’s move constructor behavior is error-prone.
>> 
>> I’d like to know how do other feel about this behavior? If enough people agree this is error-prone, would we consider having our
>> own optional type in WTF which resets the engaged flag (and never allow the std::optional)?
>> 
>> Thanks,
>> --
>>  Chris Dumez
>> 
>> 
>> 
>> 
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> 
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