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

Chris Dumez cdumez at apple.com
Fri Dec 14 13:37:03 PST 2018


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

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.
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)?

 Chris Dumez

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