Move Constructor Syntax

[Arafel]

On 15/10/24 20:29, Max Samukha wrote:
> On Tuesday, 15 October 2024 at 17:52:43 UTC, Arafel wrote:
> 
>> a move constructor invalidates the source.
> 
> Only if the source is not used after the construction/assignment. 
> Otherwise, a copy is made. I'd recommend to skim through DIP1040 if only 
> for fun.
> 

I had a look at it before posting, and according to it [1] (my bold):

 > A Move Constructor for struct S is declared as:
 >
 >   ```d
 >      this(S s) { ... }
 >   ```
 >
 > [...]
 >
 > A _Move Constructor_ is a struct member constructor that moves, 
rather than copies, the argument corresponding to its first parameter 
into the object to be constructed. **The argument is invalid after this 
move**, and is not destructed.

Also, the examples I could see were about _implicit_ calls to the move 
constructor ([2]). I found no example of explicit calls to them, nor a 
description of what to expect in that case, but I might have missed it.

In any case, it would be then helpful to clarify it more prominently: 
what happens if a constructor with the signature of a move constructor 
is called explicitly like this?

```d
struct S {
	this (int i) { }
	this (S s) { }
}

void main() {
	S s1, s2;
	s1 = S(1);
	s2 = S(s1);
	// Is s1 valid here?
}
```

Because this is currently valid D code, even if Walter thinks it 
shouldn't (as per the bug referenced in DIP1040 itself [3]), and s1 is 
perfectly valid at the end of the program.

[1]: 
https://github.com/dlang/DIPs/blob/master/DIPs/DIP1040.md#move-constructor
[2]: 
https://github.com/dlang/DIPs/blob/master/DIPs/DIP1040.md#assignment-after-move
[3]: https://issues.dlang.org/show_bug.cgi?id=20424