[nmeofthestate]

A pointer parameter can be null and it doesn't make sense for this parameter to be null, so IMO a reference is the better choice here.

A non-const reference is just as clear a signal that the parameter may be modified as a non-const pointer. If there's no modification const ref should be used.

[jdashg]

It's about clarity of intent at the call site. Passing by mutable ref looks like `foo`, same as passing by value, but passing mutability of a value by pointer is textually readably different: `&foo`. That's the purpose of the pass by pointer style.

You could choose to textually "tag" passing by mutable ref by passing `&foo` but this can rub people the wrong way, just like chaining pointer outvars with `&out_foo`.

[jcelerier]

If you want clarity of intent define dummy in and out macros but please don't make clean reference-taking APIs a mess by turning them into pointers for no good reason

[gpderetta]

In theory a from_char with an optional output parameter could be useful to validate that the next field is a number and/or discard it without needing to parse it; it might even be worth optimizing for that case.