Every closed question can be deleted, but you need to wait at least 2 days after the closure. From the help center:
It takes 3 votes, minimum, to delete a closed question. However, the number of delete votes required scales to the number of votes on the question and all its answers.
You must wait for a question to be closed for 2 days before you can vote for deletion. This restriction is removed for trusted users when a post scores -3 or lower.
The last sentence is probably what you were thinking about.
The article also warns about deleting duplicates:
Also, be cautious when deleting questions closed as duplicates; they can serve as a signpost, directing users to useful answers on another question.
Yes, this is clearly a duplicate and it should've been closed instead of answered. I'm not contesting that at all. But it shouldn't have been deleted (so I voted to undelete). For example, searching for "child class constructor" won't find the duplicate target but it will find the question you linked to. While "child class" is not the correct term for this, it's understandable that people will search for something like this. I don't care about "positive reception", all I care about is this (from the blog page about duplicates):
There's often benefit to having multiple subtle variants of a question around, as people tend to ask and search using completely different words, and the better our coverage, the better odds our fellow programmers can find the answer they're looking for.