"switch" on steroids

[kbw]

I bumped into this, a lot of it is pretty new to me. What do you think?

http://blog.robertelder.org/switch-statements-statement-expressions/

[mbozzi]

It's good to learn about the "statement expression" language extension.
It doesn't seem particularly well thought-through; all bets are obviously off when you start jumping into the middle of expressions. Seems a good enough reason to always use -std=C++11 -Wall -Wextra -pedantic.

I suppose, you can use switch to implement things like GOSUB.

In case you're unfamiliar, GOSUB is a primitive version of a function call. Jump somewhere, then return back to where you started later. The differences are relatively simple: no new lexical scope (edit: environment, technically; you can get a new scope by inserting braces), no explicit parameters, no explicit return values, no built-in ways to enforce those things. It's essentially a goto that you can return from if you want.

# include <stack>
# include <iostream>

int main() {
  std::stack <int> s;
# define GOSUB(proc) do {                               \
    s.push(__LINE__); goto proc; case __LINE__:         \
  } while (false)
# define RETURN() do {                                          \
    if (! s.empty()) { target = s.top(); s.pop(); goto _top_; } \
  } while (false)

  int target = 0;
 _top_:
  switch(target) { default:
  begin:
    std::cout << "Started!" << "\n";
    RETURN();
    GOSUB(elsewhere);
    std::cout << "Made it back!" << "\n";
    GOSUB(begin);

    std::cout << "Ending!" << "\n";
    break;

  elsewhere:
    std::cout << "Gosub'd!" << "\n";
    RETURN();
  }
}

Some notes:
With some more abuse it's maybe possible to move some macro anaphors into another namespace or something. But C++ is quite limited in this respect.

Certain toolchains provide a __COUNTER__ predefined macro, which would be a better choice then __LINE__, for obvious reasons, except it's not standard.

Of course, to make this "full-power", you need real macros. The C preprocessor is insufficient.

I can think of a way to allow multiple GOSUBs per line by (ab)using templates, but beyond that I'd need to have more thought.

[kbw]

It certainly gave me pause for thought, and it was new to me.

I was already familiar with Duff's Device. But this uses it to leapfrog into another place. It certainly cleans up some code in the example given, but you're always left with the question, "Is this right?"

[helios]

Whenever you ask yourself those kinds of questions, you should default to the "no" answer unless you have really good reasons.

[E0p9LyTq]

What do I think? The article's title sums it up:

"How to Get Fired Using Switch Statements & Statement Expressions"