[davidsiems]

You would put an assert inside of the divide function like so:

  int divide (int x, int y)
  {
    defend (y != 0);
    return x / y;
  }

Now divide is guaranteed to produce a correct result if it's called with correct data. It's up to the caller to make sure the data is correct, or an assert will happen.

Just to finish out the example to show how much cleaner asserting is compared to error handling:

  int foo(void)
  {
    volatile int x = 4, y = 28;
    return divide(x, y) + divide(y, x);
  }

  int main ()
  {
    return foo();
  }

That's not to say that error handling doesn't have its place, but it should only be used for data that you can't anticipate.

[wlievens]

That's a precondition check. While useful (essential), it doesn't cover all "forgot to check an error code" cases.

[davidsiems]

If you're really trying to write efficient code (which is what this article claims to care about) you don't 'forget' to do things like assert that the data is correct. You _guarantee_ that the data is correct and then you process it as fast as you can without having to check.

You only run into trouble with this method if the data verification step is the computationally expensive operation.