[webkit-dev] Should we ever use std::function instead of WTF::Function?

Filip Pizlo fpizlo at apple.com
Tue Jun 13 12:51:39 PDT 2017

> On Jun 13, 2017, at 12:37 PM, Alex Christensen <achristensen at apple.com> wrote:
> Ok, maybe we can get rid of std::function, then!  I hadn’t used BlockPtr as much as Chris.  I’d be opposed to adding a copy constructor to WTF::Function because the non-copyability of WTF::Function is why we made it, and it has prevented many bugs.

I agree that the copy semantics of std::function are strange - each copy gets its own view of the state of the closure.  This gets super weird when you implement an algorithm that accidentally copies the function - the act of copying invokes copy constructors on all of the lambda-lifted state, which is probably not what you wanted.  So I’m not surprised that moving to WTF::Function avoided bugs.  What I’m proposing also prevents many of the same bugs because the lambda-lifted state never gets copied in my world.

Do you think that code that uses ObjC blocks encounters the kind of bugs that you saw WTF::Function preventing?  Or are the bugs that Function prevents more specific to std::function?  I guess I’d never heard of a need to change block semantics to avoid bugs, so I have a hunch that the bugs you guys prevented were specific to the fact that std::function copies instead of sharing.

> I’ve also seen many cases where I have a WTF::Function that I want to make sure is called once and only once before destruction.  I wouldn’t mind adding a WTF::Callback subclass that just asserts that it has been called once.  That would’ve prevented some bugs, too, but not every use of WTF::Function has such a requirement.
>> On Jun 13, 2017, at 12:31 PM, Chris Dumez <cdumez at apple.com <mailto:cdumez at apple.com>> wrote:
>> We already have BlockPtr for passing a Function as a lambda block.
>> Chris Dumez
>> On Jun 13, 2017, at 12:29 PM, Alex Christensen <achristensen at apple.com <mailto:achristensen at apple.com>> wrote:
>>> std::function, c++ lambda, and objc blocks are all interchangeable.  WTF::Functions cannot be used as objc blocks because the latter must be copyable.  Until that changes or we stop using objc, we cannot completely eliminate std::function from WebKit.
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