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

[Filip Pizlo]

I don’t like style rules that prevent doing useful things, and this seems like a useful thing.

-Filip


> On Mar 21, 2019, at 2:12 PM, Alex Christensen <achristensen at apple.com> wrote:
> 
> 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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