[webkit-dev] Smart Pointers Usage Guidelines

[Ryosuke Niwa]

Hi all,

It has been a while since I last announced the plan to adopt smart pointers using clang static analyzer:
https://lists.webkit.org/pipermail/webkit-dev/2020-September/031386.html

Here are some updates.


1. We’ve made a progress in implementing all the rules including rules for local variables in clang static analyzer:
https://github.com/WebKit/WebKit/wiki/Smart-Pointer-Usage-Guidelines#rules-for-using-ref-counted-objects


2. We also have a new kind of smart pointers: CheckedRef <https://github.com/WebKit/WebKit/blob/main/Source/WTF/wtf/CheckedRef.h> / CheckedPtr <https://github.com/WebKit/WebKit/blob/main/Source/WTF/wtf/CheckedPtr.h>. These behave like Ref and RefPtr in that they increment & decrement a counter in an object but unlike them don’t extend the lifetime of the object. Instead, the destructor of the base object release asserts that there are no live CheckedRef / CheckedPtr left.

I added a new section in the guide describing when to use each smart pointer type:
https://github.com/WebKit/WebKit/wiki/Smart-Pointer-Usage-Guidelines#when-to-use-which-smart-pointer


3. I wanted to describe what applying these smart pointer rules mean. A lot of code changes needed for this work involves creating Ref / RefPtr / CheckedRef / CheckedPtr in stack:
https://commits.webkit.org/267082@main

One subtle thing is that even when a member variable is already Ref / RefPtr / CheckedRef / CheckedPtr, we must create another one in stack as seen here:
https://commits.webkit.org/267108@main

This is because these member variables can be cleared during the course of invoking a non-trivial function; or put it another way, it’s not immediately obvious from the code inspection that the object pointed to stays alive over the course of a non-trivial function call.

- R. Niwa

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