[webkit-dev] Style guideline on initializing non-POD types via member initialization

[Alex Christensen]

A more specific example of why I object is that I want to do things like add a pointer to the thread an object was created on if ASSERT_DISABLED is false so I can assert if things are done on invalid threads.  If I do this in a class like RefCounted, this rule would make me add a guarded initializer to every RefCounted class. 

> On Mar 21, 2019, at 1:37 PM, Alex Christensen <achristensen at apple.com> wrote:
> 
> I object.  I don’t find using { *this } in a header disorienting at all.  I think it’s better than adding many duplicate lines in each constructor and risking forgetting one.  I think if we were to remove all the 
> m_attributeOwnerProxy initializers in WebKit it would add lots of duplication with little benefit. If it were a class with a default constructor we would have a high risk of forgetting a constructor somewhere.
> 
>> On Mar 20, 2019, at 9:22 AM, Simon Fraser <simon.fraser at apple.com> wrote:
>> 
>>> On Mar 14, 2019, at 1:06 PM, Filip Pizlo <fpizlo at apple.com> wrote:
>>> 
>>> I like to draw this distinction: is the initializer a constant?
>>> 
>>> It’s not a constant if it relies on arguments to the constructor. “This” is an argument to the constructor. 
>>> 
>>> It’s also not a constant if it involves reading the heap. 
>>> 
>>> So, like you, I would want to see this code done in the constructor. But I’m not sure that my general rule is the same as everyone’s. 
>> 
>> This seems like a reasonable proposal to me: only use initializers when their input is constant data.
>> 
>> Any objections?
>> 
>> Simon
>> 
>>> 
>>> -Filip
>>> 
>>>> On Mar 14, 2019, at 12:59 PM, Simon Fraser <simon.fraser at apple.com> wrote:
>>>> 
>>>> I've seen some code recently that initializes non-POD members via initializers. For example, SVGAElement has:
>>>> 
>>>> AttributeOwnerProxy m_attributeOwnerProxy { *this };
>>>> 
>>>> I find this a little disorientating, and would normally expect to see this in the constructor as m_attributeOwnerProxy(*this), as it makes it easier to find places to set breakpoints, and the ordering of initialization is easier to see.
>>>> 
>>>> Are people OK with this pattern, or should we discourage it via the style guidelines (and style checker)?
>>>> 
>>>> Simon
>>>> 
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