Comment by lemmyg | Hacker News Reader

Comment by lemmyg | Hacker News Reader

In general this looks great!

I assume the reason why many functions take non-const pointers as arguments (rather than non-const references) is because it follows the google style guide? https://google.github.io/styleguide/cppguide.html#Reference_...

Cheeky question: have you Googlers thought about revising this guideline? It seemed weird to me when I read it years ago and it seems to be getting more and more unconventional. When I see a pointer argument in most C++ code now I would assume that passing a null ptr is not an error, but I see quite a lot of the code in this library doesn't check for null pointers inside the function body and would explode if you passed one in.

This doesn't mean I don't sympathize with the justification in the style guide: "References can be confusing, as they have value syntax but pointer semantics". If C++ had a non-null, non-reassignable pointer it might do a lot of what references do and be clearer, particularly for generic code. I don't really know, but references are what we have, they suit indicating the expectation of non-nullity, and it seems to me that the benefit of clearly communicating that expectation gets you more of a benefit than the confusion around semantics takes away.

This argument comes up a lot within Google. The style guide is revisited frequently (for example =delete in the public section is now the standard for disabling copy and/or move/assignment vs. the old macros in the private section).

Non-nullable types are helpful for implying pre-conditions. However, readability at the call site (e.g. Foo(&bar) might mutate bar, whereas Foo(bar) should not) is still considered more valuable in a large scale codebase. Passing nullptr as a pointer argument is generally assumed to not be okay unless explicitly documented as permitted -- this is opposite the assumption that you stated.

There are places exceptions are made, where the use of pointers is deemed more confusing than non-const references (e.g. move-maybe semantics in very specific cases). Ultimately, most code follows the default style guide.

Besides, nullptr dereferences are pretty easy to diagnose in a library like this. And more often than not everywhere else too.

> However, readability at the call site (e.g. Foo(&bar) might mutate bar, whereas Foo(bar) should not) is still considered more valuable in a large scale codebase

but does it make it any more readable ? if you use the pointer anywhere else and have it as a variable then suddenly you don't distinguish anymore between a pointer and a reference. It would frankly make more sense to have empty `#define in` and `#define out` macros and make a small clang plug-in that checks correct usage in your codebase - e.g.

    int foo(int x, const foobar& my_foobar, boo& my_boo);
    foo(x, in fb, out b); // ok
    foo(x, out fb, out b); // compile error

In most cases the context provides enough information:

  void Baz(Bar*);
  void Baz(const Bar&);

  void Foo(Bar* mutable_bar) {
    Baz(*mutable_bar); // Not mutated.
  }

  void Foo(Bar* mutable_bar) {
    Baz(mutable_bar); // Possibly mutated.
  }

  void Foo(const Bar& bar) {
    Baz(bar); // Not mutated.
  }

  void Foo(const Bar& bar) {
    Baz(&bar); // Compiler error.
  }

The rule doesn't perfectly eliminate ambiguity, but it does a pretty good job overall. The point is to address the general use cases with familiar constructs that work across multiple toolchains.

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