[fauigerzigerk]

>If you know the size of result in advance, there is no reason to use a stream for result.

Oh yes there is a reason. The reason is that streams are a convenient abstraction because there is very likely an operator<< for whatever transform(c) returns. And why is there an operator<<? Because streams abstract from the type of sink (string, file, network).

You are absolutely right, that this can be implemented a lot more efficiently. That's exactly my point. Abstractions lure us into using inefficient solutions without thinking.

You say "If you know the size of result in advance [...]". Well, exactly. Do you? That's something C programmers think about long and hard but you don't have to think about it if you use streams or even string abstractions naively.

This isn't just a case of "you can write bad code in any language". It's what abstractions do. Allow us to ignore stuff that is unnecessary if the goal is simply to arrive at a correct but less efficient solution.