[fauigerzigerk]

I think the term "zero cost abstraction" is a bit misleading. Abstraction means that developers don't have to think about some lower level aspects that they would have given some thought otherwise. That tends to result in inefficiencies.

For instance, C programmers will think a lot more about the size of buffers, how big they need to be, can they be fixed size, can they be reused, etc, because expanding them is often a manual task. In C++ it's so easy to do things like this even though it could be done much more efficiently:

  string parse(const string& data) {
    stringstream result;
    for (char c : data) {
      if (someCondition(c)) {
        result << transform(c);
      }
    }
    return result.str();
  }

[petke]

I dont think I agree. Abstraction in my view is (mostly) about removing implementation details that dont change from one implementation to another. Its about removing boilerplate until just the fundamental features of the implementations remain. It would be a bad abstraction if details (low level or otherwise) that do change, are removed.

Im not sure I get your example. Are you implying that this C++ code is inefficient because the allocation for result is not done in advance? If you know the size of result in advance, there is no reason to use a stream for result. You can simply use std::string and reserve.

[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.