Exceptions/Catch an exception thrown in a nested call: Difference between revisions

Show how to create a user-defined exception and show how to catch an exception raised from several nested calls away.

Sample output:

An unhandled exception leads to termination of the corresponding [[task]]. When the task is the main task of the program as in the example, the whole program is terminated. In the example the exception back tracing message is compiler-specific (in this case it is [[GNAT]] and further depends on the compiler options.

Output:

Output (the second message is output by the default error handler):

Output:

<lang>Foo entering bar.

</lang>

First exception will be caught and message will be displayed, second will be caught by the default exception handler, which as required by the C++ Standard, will call terminate(), aborting the task,

typically with an error message.

Output:

Sample output (the numbered lines are output from <code>trace</code>):

At this point, the debugger (if any) is invoked with the unhandled condition of type USER-CONDITION-2.

First exception will be caught and message will be displayed, second will be caught by default exception handler.

{{output}}

Output:

Sample output:

Output:

Output:

The output shows the first exception is caught and handled. The second exception is not handled, and results in the program finishing and printing a stack trace.

The panic/recover mechanism of Go is missing (by design) some elements of exception handling needed for this task. Specifically, a function that recovers a panic cannot resume execution of the remainder of the function. If foo recovers a panic in the first call to bar, there is no way for it to make the second call to bar. The solution here is to define a wrapper, or proxy function, called try. Function foo calls bar indirectly through try.

The output of this program is:

The following Unicon example makes use of support for exceptions found in the

[http://tapestry.tucson.az.us/unilib/ The Unicon Code Library]. <i>Since

exception support is not built into Unicon, but rather implemented as

Unicon code, there are limitations not found in languages that natively

support exceptions.</i>

When run, this example produces:

Note: it may be possible to implement exceptions in Icon; however, it would require a major rework and would likely be inelegant.

Methods that may throw an exception (or that call a method that may throw an exception that it does not catch) must explicitly declare that they can throw such an exception (or a superclass thereof), unless they are unchecked exceptions (subclasses of <code>RuntimeException</code> or <code>Error</code>):

Sample output:

The first line of the output is generated from catching the U0 exception in function foo.

Uncaught exceptions give information showing where the exception originated through the nested function calls together with the name of the uncaught exception, (U1) to stderr, then quit the running program.

[[Rhino]] output:

[[SpiderMonkey]] output:

Lasso currently does not currently have a try mechanic — but we can easily add one like so.

Output:

Output:

Sample output:

Sample output:

Sample output:

Output:

There is no extra syntax to add to functions and/or methods such as ''bar'', to say what exceptions they may raise or pass through them:

Sample output:

The first line of the output is generated from catching the U0 exception in function foo.

Uncaught exceptions give information showing where the exception originated through the nested function calls together with the name of the uncaught exception, (U1) to stderr, then quit the running program.

The counter for the number of calls to baz is kept in the global environment for simplicity, but you could hide it in your own environment. See ?new.env and ?get.

Output:

While the REXX language doesn't have a ''throw'' capability ''pe se'', it does have the ability to catch exceptions (by label).

<br>This type of exception handling (in REXX) has its limitation (the label is local to the program, not external subroutines).

'''output'''

The first call to foo causes the U0 exception. It gets rescued. The second call results in a U1 exception which is not rescued, so the program dumps a stacktrace and exits.

[[Category:Scala Implementations]]{{libheader|Scala}}<lang Scala>object ExceptionsTest extends App {

</lang>Exception U0 is caught, exception U1 is caught and re-thrown. Program execution is terminated as the U1 exception is not caught when thrown the second time.

When an [http://seed7.sourceforge.net/manual/errors.htm#Exceptions exception] is not [http://seed7.sourceforge.net/manual/errors.htm#Handlers handled]

{{works with|Smalltalk/X}}functional code, not class based, using blocks as functions, and anonymous exceptions (signals):

"traditional" implementation, using class based exceptions, and method invocations:

Running this program results in:

Run:

Foo calls bar, and bar calls baz. Normal termination of bar is bypassed if

baz raises an exception. The exception is caught or not by foo.

Here is an example bash session.

foo catches exception U0 and ignores it. It calls bar with the exception to throw. bar in turn calls baz with that exception, which it throws. A stack trace is printed when an uncaught exception bubbles up to the VM (which handles all catchable exceptions).

{{omit from|C}}

{{omit from|GUISS}}

{{omit from|M4}}

{{omit from|Retro}}