Applications of explicitly raising exceptions

Question

What are the applications and advantages of explicitly raising exceptions in a program. For example, if we consider Ada language specifically here provides an interface to raise exceptions in the program. Example:

raise <Exception>;

But what are the advantages and application areas where we would need to raise exceptions explicitly?

For example, in a procedure which accepts one of the parameters as string:

function Fixed_Str_To_Chr_Ptr (Source_String : String) return C.Strings.Chars_Ptr is
...
begin
...
  -- Check whether source string is of acceptable length
  if Source_String'Length <= 100 then
  ...
  else
    ...
    raise Constraint_Error;
  end if;

  return Ptr;

exception
     when Constraint_Error=>
        .. Do Something..
end Fixed_Str_To_Chr_Ptr;

Is there any advantage or good practice if I raise an exception in the above function and handle it when the passed string length bound exceeds the tolerable limits? Or a simple If-else handler logic should do the business?

Answer 1

I'll make my 2 cents an answer in order to bundle the various aspects. Let's start with the general question

But what are the advantages and application areas where we would need to raise exceptions explicitly?

There are a few typical reasons for raising exceptions. Most of them are not Ada-specific.

First of all there may be a general design decision to use or not use exceptions. Some general criteria:

• Exception handlers may incur a run time cost even if an exception is actually never thrown (cf. e.g. https://gcc.gnu.org/onlinedocs/gnat_ugn/Exception-Handling-Control.html). That may be unacceptable.
• Issues of inter-operability with other languages may preclude the use of exceptions, or at least require that none leave the part programmed in Ada.
• To a degree the decision is also a matter of taste. A programmer coming from a language without exceptions may feel more confident with a design which just relies on checking return values.
• Some programs will benefit from exceptions more than others. If traditional error handling obscures the actual program structure it may be time for exceptions. If, on the other hand, potential errors are few, easily detected and can be handled locally, exceptions may obscure potential execution paths more than handling errors traditionally would.

Once the general decision to use exceptions has been made the problem arises when and when not it is appropriate to raise them in your code. I mentioned one general criteria in my comment. What comes to mind:

• Exceptions should not be part of normal, expected program flow (they are called exceptions, not expectations ;-) ). This is partly because the control flow is harder to see and partly because of the potential run time cost.
• Errors which can be handled locally don't need exceptions. (It can still be useful to raise one in order to have a uniform error handling though. I'll discuss that below when I get to your code snippet.)
• On the other hand, exceptions are great if a function has no idea how to handle errors. This is particularly true for utility and library functions which can be called from a variety of contexts (GUI, console program, embedded, server ...). Exceptions allow the error to propagate up the call chain until somebody can handle it, without any error handling code in the intervening layers.
• Some people say that a library should only expose custom exceptions, at least for any anticipated errors. E.g. when an I/O exception occurs, wrap it in a custom exception and explicitly raise that custom exception instead.

Now to your specific code question:

Is there any advantage or good practice if I raise an exception in the above function and handle it when the passed string length bound exceeds the tolerable limits? Or a simple If-else handler logic should do the business?

I don't particularly like that (although I don't find it terrible) because my general argument above ("if you can handle it locally, don't raise") would indicate that a simple if/else is clearer.¹ For example, if the function is long the exception handler will be far away from the error location, so one may wonder where exactly the exception could occur (and finding one raise location is no guarantee that one has found them all, so the reviewer must scrutinize the whole function!).

It depends a bit on the specific circumstances though. Raising an exception may be elegant if an error can happen in several places. For example, if several strings can be too short it may be nice to have a centralized error handling through the exception handler, instead of scattering if/then/elses (nested??) across the function body. The situation is so common that a legitimate case can be made for using goto constructs in languages without exceptions. An exception is then clearly superior.

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¹But in all reality, how do you handle that error there? Do you have a guaranteed logging facility? What do you return? Does the caller know the result can be invalid? Maybe you should throw and not catch.

Answer 2

You raise an exception explicitly to control which exception is reported to the user of a subprogram. - Or in some cases just to control the message associated with the raised exception.

In very special cases you may also raise an exception as a program flow control.

Answer 3

There are two problems with the given example:

1. It's simple enough that control flow doesn't need the exception. That won't always be the case, however, and I'll come back to that in a moment.
2. Constraint_Error is a spectacularly bad exception to raise, to detect a string length error. The standard exceptions Program_Error, Constraint_Error, Storage_Error ought to be reserved for programming error conditions, and in most circumstances ought to bring down the executable before it can do any damage, with enough debugging information (a stack traceback at the very least) to let you find the mistake and guarantee it never happens again.

It's remarkably satisfying to get a Constraint_Error pointing spookily close to your mistake, instead of whatever undefined behaviour happens much later on... (It's useful to learn how to turn on stack tracebacks, which aren't generally on by default).

Instead, you probably want to define your own String_Size_Error exception, raise that and handle it. Then, anything else in your unshown code that raises Constraint_Error will be properly debugged instead of silently generating a faulty Chars_Ptr.

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For a valid use case for raising exceptions, consider a circuit simulator such as SPICE (or a CFD simulator for gas flow, etc). These tools, even when working properly, are prone to failures thanks to numerical problems that happen in matrix computations. (Two terms cancel, producing zero +/- rounding error, which causes infeasibly large numbers or divide-by-zero later on). It's often an iterative approximation, where the error should reduce in each step until it's an acceptably low value. But if a failure occurs, the error term will start growing...

Typically the simulation happens step by step, where each step is a sufficiently small time step, maybe 1 us or 1 ns. The main loop requests a step, and this request is passed to thousands of agents in the simulation representing components in a circuit, or triangles in a CFD mesh.

Any one of those agents may fail to compute a solution, and the cleanest way to handle a failure is to raise an exception, maybe Convergence_Error. There may be thousands of possible points where an exception can be raised.

Testing thousands of return codes would get ugly fast. But with exceptions, the main loop only needs one handler, which takes some corrective action such as reducing the simulation step size and running the step again.

Sanitizing user text input in a browser may be another good use case, closer to the example code.

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One word on the runtime cost of exceptions : the Gnat compiler and its RTS supports a "Zero Cost Exception" (ZCX) model - at least for some targets. There's a larger penalty when an exception is raised, as a tradeoff against eliminating the penalty in the normal case. If the penalty matters to you, refer to the documentation to see if it's worthwhile 9or even possible) in your case.

Answer 4

Exceptions should stay true to their name, which is to represent exceptional situations.