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doc: formalize non-const reference usage in C++ style guide #23155

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38 changes: 38 additions & 0 deletions CPP_STYLE_GUIDE.md
Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@
* [Memory allocation](#memory-allocation)
* [Use `nullptr` instead of `NULL` or `0`](#use-nullptr-instead-of-null-or-0)
* [Ownership and Smart Pointers](#ownership-and-smart-pointers)
* [Avoid non-const references](#avoid-non-const-references)
* [Others](#others)
* [Type casting](#type-casting)
* [Do not include `*.h` if `*-inl.h` has already been included](#do-not-include-h-if--inlh-has-already-been-included)
Expand Down Expand Up @@ -200,6 +201,43 @@ void FooConsumer(std::unique_ptr<Foo> ptr);

Never use `std::auto_ptr`. Instead, use `std::unique_ptr`.

### Avoid non-const references

Using non-const references often obscures which values are changed by an
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assignment. Consider using a pointer instead, which requires more explicit
syntax to indicate that modifications take place.

```c++
class ExampleClass {
public:
explicit ExampleClass(OtherClass* other_ptr) : pointer_to_other_(other_ptr) {}

void SomeMethod(const std::string& input_param,
std::string* in_out_param); // Pointer instead of reference

const std::string& get_foo() const { return foo_string_; }
void set_foo(const std::string& new_value) { foo_string_ = new_value; }

void ReplaceCharacterInFoo(char from, char to) {
// A non-const reference is okay here, because the method name already tells
// users that this modifies 'foo_string_' -- if that is not the case,
// it can still be better to use an indexed for loop, or leave appropriate
// comments.
for (char& character : foo_string_) {
if (character == from)
character = to;
}
}

private:
std::string foo_string_;
// Pointer instead of reference. If this object 'owns' the other object,
// this should be a `std::unique_ptr<OtherClass>`; a
// `std::shared_ptr<OtherClass>` can also be a better choice.
OtherClass* pointer_to_other_;
};
```

## Others

### Type casting
Expand Down