Programming in D p288. Destructor called too many times.

[Brother Bill]

On Tuesday, 10 February 2026 at 20:37:04 UTC, Nick Treleaven 
wrote:
> If you add a copy constructor to Duration, you can see when it 
> gets copied:
```
> 	this(ref Duration d)
> 	{
> 		this.tupleof = d.tupleof;
> 		writeln("copying Duration: ", toString);
> 	}
```

The ```ref``` is needed to avoid yet another copy.

Please explain what ```this.tupleof = d.tupleof;``` does in 
detail.