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doc: formalize `auto` usage in C++ style guide

We generally avoid using `auto` if not necessary. This
formalizes this rules by writing them down in the C++ style guide.

PR-URL: #23028
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
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addaleax committed Sep 22, 2018
1 parent 847037e commit aba7677e826065fc1f1025ee2db5faee035fd515
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  1. +19 −0 CPP_STYLE_GUIDE.md
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@@ -20,6 +20,7 @@
* [Ownership and Smart Pointers](#ownership-and-smart-pointers)
* [Others](#others)
* [Type casting](#type-casting)
* [Using `auto`](#using-auto)
* [Do not include `*.h` if `*-inl.h` has already been included](#do-not-include-h-if--inlh-has-already-been-included)
* [Avoid throwing JavaScript errors in C++ methods](#avoid-throwing-javascript-errors-in-c)
* [Avoid throwing JavaScript errors in nested C++ methods](#avoid-throwing-javascript-errors-in-nested-c-methods)
@@ -209,6 +210,24 @@ Never use `std::auto_ptr`. Instead, use `std::unique_ptr`.
- Use `static_cast` for casting whenever it works
- `reinterpret_cast` is okay if `static_cast` is not appropriate
### Using `auto`
Being explicit about types is usually preferred over using `auto`.
Use `auto` to avoid type names that are noisy, obvious, or unimportant. When
doing so, keep in mind that explicit types often help with readability and
verifying the correctness of code.
```cpp
for (const auto& item : some_map) {
const KeyType& key = item.first;
const ValType& value = item.second;
// The rest of the loop can now just refer to key and value,
// a reader can see the types in question, and we've avoided
// the too-common case of extra copies in this iteration.
}
```
### Do not include `*.h` if `*-inl.h` has already been included
Do

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