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Subject: Re: [std-proposals] operator ==/!= for empty struct
From: Oleg Fatkhiev (ogfatog_at_[hidden])
Date: 2019-11-03 06:08:42


Hello,
Looks like for this concrete example you just need to use holds_alternative
instead of equals operator. And you don't need to create comparison
operators at all!

Best regards,
Oleg

On Sun, Nov 3, 2019, 12:58 Dawid Kurek via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I'm using `std::variant` with structs as events. Structs are often empty.
>
> E.g
> #include <variant>
>
> struct Exit {
> bool operator== (Exit) const { return true; }
> bool operator!= (Exit) const { return false; }
> };
>
> struct Reload{
> bool operator== (Reload) const { return true; }
> bool operator!= (Reload) const { return false; }
> };
>
> using Msg = std::variant<Exit, Reload>;
>
> extern Msg Update();
>
> void foo() {
> Msg msg;
> do {
> msg = Update();
> // ...
> } while (msg != Msg{Exit{}});
> }
>
> This approach requires that all those tag structs in `variant` has
> `operator==` or `operator!=` (depending on the check).
>
> This is not a hard limitation but adds a lot of additional and repetitive
> code.
> It can be minimized with inheritance (base class with those operators)
> or macro.
>
> The idea is that operator ==/!= for empty structs could be generated
> automatically by the compiler.
>
> So the code would look like:
> #include <variant>
>
> struct Exit {};
> struct Reload {};
>
> using Msg = std::variant<Exit, Reload>;
>
> extern Msg Update();
>
> void foo() {
> Msg msg;
> do {
> msg = Update();
> // ...
> } while (msg != Msg{Exit{}});
> }
>
> [Sub]idea would be to generate complementary operator,
> if struct/class provides operator ==, generate not-equal by negating
> the result of
> equal and vice versa.
>
> Thanks for any comments!
> Dawid
> --
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>



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