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digitalmars.D.learn - problem with alias this and Tuple

reply Johann Lermer <johann.lermer elvin.eu> writes:
Hi all,

I have a problem with Tuples and struct templates that contain an 
alias this:
```d
import std;

struct Element(T)
{
	T payload;
	alias payload this;

	this (T p)
	{
		payload = p;
	}
}

class Item {}

void main ()
{
	auto e = Element!Item (new Item());
	auto t = tuple(e);
}
```
dmd generates a warning: **Warning: struct Element has method 
toHash, however it cannot be called with const(Element!(Item)) 
this.
Element.toHash defined here: 
/usr/local/include/d/druntime/import/object.d(83)**

The culprit seems to be the 'alias payload this'. When I remove 
that line, the code works. However, in my code, I need that 
alias. Any ideas, what's wrong or what I could do? Thanks.
Oct 12 2021
parent reply Paul Backus <snarwin gmail.com> writes:
On Tuesday, 12 October 2021 at 09:30:57 UTC, Johann Lermer wrote:

 Hi all,

 I have a problem with Tuples and struct templates that contain 
 an alias this:
 ```d
 import std;

 struct Element(T)
 {
 	T payload;
 	alias payload this;

 	this (T p)
 	{
 		payload = p;
 	}
 }

 class Item {}

 void main ()
 {
 	auto e = Element!Item (new Item());
 	auto t = tuple(e);
 }
 ```
 dmd generates a warning: **Warning: struct Element has method 
 toHash, however it cannot be called with const(Element!(Item)) 
 this.
 Element.toHash defined here: 
 /usr/local/include/d/druntime/import/object.d(83)**

 The culprit seems to be the 'alias payload this'. When I remove 
 that line, the code works. However, in my code, I need that 
 alias. Any ideas, what's wrong or what I could do? Thanks.


All classes in D inherit from [`Object`][1], and `Object` has a 
[`toHash` method][2]. So what's happening here is (1) your `Item` 
class is inheriting `toHash` from `Object`, and (2) your 
`Element` struct is "inheriting" `toHash` from `Item` via `alias 
this`. This is why the compiler says, "struct `Element` has 
method `toHash`".

For historical reasons, `Object.toHash` is not a `const` method, 
which means it cannot be called on a `const(Object)`. So if you 
had a `const(Element!Item)` and you tried to call `.toHash` on 
it, the compiler would rewrite the call to `.payload.toHash` 
using `alias this`, and then it would fail to compile, because 
`payload` would be a `const(Item)` (a subclass of 
`const(Object)`). This is why the compiler says, "however, it 
cannot be called with `const(Element!(Item))`".

The simplest fix is to define a `toHash` method for `Element` 
using the built-in [`hashOf`][3] function:

```d
size_t toHash() const
{
     return hashOf(payload);
}
```

[1]: https://druntime.dpldocs.info/object.Object.html
[2]: https://druntime.dpldocs.info/object.Object.toHash.html
[3]: https://druntime.dpldocs.info/object.hashOf.2.html
Oct 12 2021
parent reply Johann Lermer <johann.lermer elvin.eu> writes:
Thanks, understood. But isn't this a deficiency in the library 
that should be fixed?
Oct 12 2021
next sibling parent Paul Backus <snarwin gmail.com> writes:
On Tuesday, 12 October 2021 at 15:55:40 UTC, Johann Lermer wrote:

 Thanks, understood. But isn't this a deficiency in the library 
 that should be fixed?


You mean the part about how `Object.toHash` doesn't work with 
`const`? Yes, it is a deficiency in the library. The problem is, 
it cannot be fixed without breaking backwards compatibility.
Oct 12 2021
prev sibling parent Mike Parker <aldacron gmail.com> writes:
On Tuesday, 12 October 2021 at 15:55:40 UTC, Johann Lermer wrote:

 Thanks, understood. But isn't this a deficiency in the library 
 that should be fixed?


The problem extends to more than just `toHash`. Take a look at 
this DConf 2019 presentation by Eduard Staniloiu on ProtoObject 
(as proposed by Andrei Alexandrescu):

https://youtu.be/EcG5mnOzZ0s

Eduard is now mentoring a Symmetry Autumn of Code participant 
(Robert Aron) whose project is implementing `ProtoObject`. You 
can get a summary of the project from Robert's first weekly 
update thread here:

https://forum.dlang.org/post/wezzcgwbnaguawzfhtpw forum.dlang.org
Oct 12 2021