[llvm-dev] When to use auto instead of iterator/const_iterator?

[Matthias Braun via llvm-dev]

As for:
- range based for: Use them wherever possible instead of the iterators. There are some cases however where you cannot avoid the iterators (for example if you want to call `iter = container.erase(iter)` in a loop)
- Avoid `auto` unless the type is obviously clear from the context:

> On May 2, 2017, at 6:36 AM, 陳韋任 via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi All,
> 
>   While reading LLVM source code, sometimes I am wondering when should we use auto instead of iterator/const_iterator.
> I want to use the patch [1] I sent before as an example. Could someone give me advice/guideline here? Also, I have another
> question. Sometimes the for-loop uses const_iterator, say
These are topics discussed in the developer policy: http://llvm.org/docs/DeveloperPolicy.html <http://llvm.org/docs/DeveloperPolicy.html>

While reading LLVM sourcecode keep in mind that sometimes the sourcecode is older than some rules in the policy and might violate them. If you write new code however it should adhere to the policy.

>   for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
> 
>          I != E; ++I)
> 
>     if (I->isCtrl())
> 
>       NumberDeps++;
> 
> 
This should work just as well with a range based for, so you should prefer that over the iterator!
> Can we rewrite above code as,
> 
>   for (auto &Succ : SU->Succs)
> 
>     if (Succ.isCtrl())
> 
> 
>       NumberDeps++;
> 
> Or do we need to use `const auto &` instead, like this
>   for (
> 
> ​const ​auto &Succ : SU->Succs)
> 
>     if (Succ.isCtrl())
> 
> 
>       NumberDeps++; 

This should be written as:

for (const SDep &Succ : SU->Succs) {
  if (Succ.isCtrl())
    ++NUmberDeps;
}

And I think the LLVM community never agreed whether you need {} around the for in a case like this. Some add the {} here, some people don't. Attempts to standardize this as in https://reviews.llvm.org/D26943 <https://reviews.llvm.org/D26943> did not reach agreement.

- Matthias
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