What is the benefit of terminating if … else if constructs with an else clause?

As mentioned in another answer, this is from the MISRA-C coding guidelines. The purpose is defensive programming, a concept which is often used in mission-critical programming.

That is, every if - else if must end with an else, and every switch must end with a default.

There are two reasons for this:

• Self-documenting code. If you write an else but leave it empty it means: “I have definitely considered the scenario when neither if nor else if are true”.

  Not writing an else there means: “either I considered the scenario where neither if nor else if are true, or I completely forgot to consider it and there’s potentially a fat bug right here in my code”.

• Stop runaway code. In mission-critical software, you need to write robust programs that account even for the highly unlikely. So you could see code like

  if (mybool == TRUE) 
  {
  } 
  else if (mybool == FALSE) 
  {
  }
  else
  {
    // handle error
  }
  

  This code will be completely alien to PC programmers and computer scientists, but it makes perfect sense in mission-critical software, because it catches the case where the “mybool” has gone corrupt, for whatever reason.

  Historically, you would fear corruption of the RAM memory because of EMI/noise. This is not much of an issue today. Far more likely, memory corruption occurs because of bugs elsewhere in the code: pointers to wrong locations, array-out-of-bounds bugs, stack overflow, runaway code etc.

  So most of the time, code like this comes back to slap yourself in the face when you have written bugs during the implementation stage. Meaning it could also be used as a debug technique: the program you are writing tells you when you have written bugs.

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EDIT

Regarding why else is not needed after every single if:

An if-else or if-else if-else completely covers all possible values that a variable can have. But a plain if statement is not necessarily there to cover all possible values, it has a much broader usage. Most often you just wish to check a certain condition and if it is not met, then do nothing. Then it is simply not meaningful to write defensive programming to cover the else case.

Plus it would clutter up the code completely if you wrote an empty else after each and every if.

MISRA-C:2012 15.7 gives no rationale why else is not needed, it just states:

Note: a final else statement is not required for a simple if
statement.