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Re: Structured binding: referencing an existing variable

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On 2/16/2022 7:46 AM, Paavo Helde wrote:
> 
> I was cleaning up some code and getting rid of an obscure in-out 
> reference parameter by replacing it by a tuple return. E.g. with a 
> simplified example, instead of former
> 
> A foo(int& pos_in_out);

I'm not sure I get how a byref parameter is obscure, but anyway...

> 
> we now have
> 
> std::pair<A, int> foo(int pos_in);
> 
> where the updated pos is returned explicitly.
> 
> Now how do I capture these results when calling it? In the old call it was:
> 
> int pos = 0;
> while(pos < N) {
>     A a = foo(pos);
>     // ...
> }
> 
> How should I replace it? The thing is A does not have a default ctor and 
> should be created afresh each time when foo is called, while pos should 
> be updated instead.
> 
> std::tie(a, pos) = foo(pos);  // Does not work for A as it cannot be 
> pre-constructed.
> 
> An alternative is to use C++17 structured binding:
> 
> auto [a, pos] = foo(pos); // Does not work for pos as it creates a new 
> variable, instead of updating the old one.
> 
> In short, I would want to use structured binding for A and effectively 
> std::tie() for the other member. Is that somehow possible?
> 
> For now I resorted to:
> 
> auto [a, newPos] = foo(pos);
> pos = newPos;
> 
> but this introduces an unneeded extra variable and does not look like so 
> great improvement over the old code.

Hmm.
So far it looks like the original "problem"

> A foo(int& pos_in_out);

is not worse than the solution ;)

More to the point, if you really don't want the reference argument in 
foo(), I'd probably look closer at two options:

1) See if std::pair<A, int> is a type that may actually belong in the 
calling context, and use that directly [*], not as a temp to be split 
after the call.

2) Look closer at why an A() ctor is not acceptable, which is what stops 
std::tie from working . I'm not sure that std::tie makes for clearer 
code than the reference argument, though.


[*] it might be argued that the number and type of the variables being 
used are what constitutes the state of a system, so changing two 
separate int and A variables into a combined tuple does have some impact 
on the model of the system. Then the question becomes if this tuple 
yields a better model of the system at hand than two separate variables.

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Structured binding: referencing an existing variable Paavo Helde <eesnimi@osa.pri.ee> - 2022-02-16 08:46 +0200
  Re: Structured binding: referencing an existing variable Manfred <noname@add.invalid> - 2022-02-16 21:43 +0100
  Re: Structured binding: referencing an existing variable Vir Campestris <vir.campestris@invalid.invalid> - 2022-02-16 22:00 +0000
    Re: Structured binding: referencing an existing variable Paavo Helde <eesnimi@osa.pri.ee> - 2022-02-17 16:00 +0200

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