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Subject: Re: [std-proposals] Explicit using
From: Yves Bailly (yves.bailly_at_[hidden])
Date: 2021-01-19 03:22:15


From: Std-Proposals <std-proposals-bounces@lists.isocpp.org> On Behalf Of Arthur O'Dwyer via Std-Proposals
Sent: Friday, January 15, 2021 17:11
To: Std-Proposals <std-proposals@lists.isocpp.org>
Cc: Arthur O'Dwyer <arthur.j.odwyer@gmail.com>
Subject: Re: [std-proposals] Explicit using

On Fri, Jan 15, 2021 at 10:54 AM Yves Bailly via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

First, the "stronger type alias":
using U = new T;

 As you've realized (good!), the problems are going to be with T's existing customization points:

- std::swap(u, u)
- std::hash<T>(u)

            using Name = new std::string;
            void Store(std::string the_name); // (1)
            void Store(Name the_name);        // (2)
/* error */ Store("Your name here"); // ambiguous, ill-formed

 FWIW, this is mildly surprising; this would be a place where `std::is_base_of_v<T, U>` and yet the signature `void(U)` is not considered more-specialized-than `void(T)`.

In fact it's precisely the point and goal of this proposal: to avoid potential surprises.

As there's an implicit conversion from char const* to std::string and to Name (this one "inherited" from std::string), then as a mere code reader I'm not sure which one Store() would be called here. As a mere code writer I may have an idea which may well be wrong. So here the suggestion is to declare the line ill-formed as ambiguous, because we have here a kind of "two-levels" implicit conversion, which is getting difficult to track.

Therefore the writer would have to make the intent explicit. After all it's the goal of strong typing: make things explicit. It seems to me it's also a general trend in C++ last years.

From what I can see, a well-known (to the "initiated") situation like this:

void foo(bool b);
void foo(std::string s);
foo("bar"); // which one is called?

...is really confusing for many developers, even experieced ones. Most developers are "just" language users, not language lawyers :-)

But I think this is just an indication that your intuition about std::is_base_of is wrong. Types with no inheritance relationships (no base classes) definitely should not claim to have is_base_of relationships.

I see the point here (I think). So indeed std::is_base_of_v<T,U> should be false. Probably std::underlying_type<> is enough to be able to query the relationship between T and U.

/* ok    */ using Names_Income = std::unordered_map<Name, double>;
            // std::hash<std::string> used because of
            // implicit cast from Name to std::string

If there's an implicit conversion from Name to std::string, then why did you say

    std::string s;
    s = name;

didn't work?

I don't think I said that, on the contrary... or did I miss something?

Or are you refering to the second form called "strict type alias"?


Nice page I saw but forgot about :-)

Let me try to answer your questions, giving an answer for both suggested cases ("new T" and "explicit T"):

A: yes / yes
B: yes / no
C: yes / no
D: yes / no
E: yes / no
F: yes / yes
G: yes / yes
H: yes / no
I: yes / no
J: "make_gadget" as an identifier is not defined anywhere, so no in both cases. If the return type was part of a function's signature, then maybe it could be defined from "make_widget" with a "= default", but as far as I know it's currently not possible.
K: "gset" is not defined as an identifier, but what about
   auto gset = static_cast<std::unordered_set<Gadget>&>(wset);
  Explicit casting, no ambiguity, no surprise.


(o< | Yves Bailly             | -o) 
//\ | Software architect      | //\ 
\_/ | http://kafka.fr.free.fr | \_/`

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