c++ exception handling - c++

Question

Learning "try & catch". What is wrong with the following code?
Thanks for the advice.
Error in execution:
terminate called without an active exception
Aborted
The code:
#include <stdio.h>
int main()
{
int a = 3;
try
{
if (a < 5)
throw;
}
catch (...)
{
printf ("captured\n");
}
return 0;
}

Answer 1

Your throw; statement tries to rethrow a current exception but there probably isn't one. You need something like
throw some_exception_object();

Answer 2

Inside of a try block, you have to specify what to throw. The only place you can use throw by itself is inside of a catch block to re-throw the current exception. If you call throw by itself without a current exception being active, you will kill your app, as you have already discovered.
Try this:
#include <stdio.h>
int main()
{
int a = 3;
try
{
if (a < 5)
throw 1; // throws an int
}
catch (...)
{
printf ("captured\n");
}
return 0;
}
You can throw anything you want, as long as you throw something.

Answer 3

There are four things, two major and two minor. One thing at a time...
1. Rethrow usage w/o active exception
A throw; statement is used to re-throw an exception that is currently caught. For example:
try {
do_something();
} catch (const std::exception &) {
throw; // This statement re-throws an exception that was caught in this "catch" block.
}
In your case, you are using throw; without catching any exceptions (in order words — it does not appear inside catch block directly or indirectly), thus your program is terminated. When there is a need to throw and not to re-throw an exception, like in your case, you must specify an exception object to be thrown. For example:
throw std::runtime_error("Something bad happened");
2. catch-all clause which does not re-throw a caught exception
Your catch-all clause (catch (...)) is perfectly legal C++. However, it does not re-throw caught exception. Even though it is a legal C++ code, such a usage is a taboo. C and C++ runtime is usually using special types of exceptions to implement certain functionality. For example, NPTL is using exceptions to implement a thread cancellation. If you catch that exception using catch (...), a thread won't be cancelled and you are going to have a bad time. Generally, you have to catch exceptions by their types. In almost all cases, exceptions are inherited from std::exception, and so you have to write catch (const std::exception &) or, if you expect to catch an exact type, - catch(const TypeYouExpect &). If you must, however, use catch-all, remember to re-throw. For example:
try {
do_something();
} catch (...) {
throw; // DO NOT FORGET TO RE-THROW.
}
3. Header naming...
You are including C header whereas C++ provides its own headers for standard C features. So, header:
#include <stdio.h>
.. should be:
#include <cstdio>
C++ specific C functions get special treatment. For example, they become available in std namespace. So that you can use std::open() instead of just open() or ::open(). No big deal, but is highly recommended way to go.
4. Return from main.
Unlike C, C++'s main() function is very special. It allows you not to have return 0;. This is a default behavior. So, unless you really need to return some value, you may save yourself some time by not typing return 0;. Remember, however, that main is the only function like that, and that everywhere else you must explicitly return something unless a function is marked void.
Hope it helps. Good Luck!

Answer 4

You need to actually throw some object. Even something as simple as
throw "error";
will catch the error like you want it to.
see it in action here

Answer 5

The statement to throw an exception is:
throw <expression>;
This statement:
throw;
is also called the re-throw statement and is use to re-throw an existing exception that has been caught. It is typically used in a catch block, for example, you look at the exception and decide if you can continue, retry or abort. In case you decide to abort, you re-throw the exception so that somebody else down the call stack will catch it and handle this error.
For example:
// getResult() calls can fail with a deadlock exception
// This method will retry up to 3 times before failing
Result getResultWithRetry()
{
int nbTry = 3;
for(;;) {
try {
return getResult();
} catch (DeadLockException& e) {
if (nbTry == 0) {
throw; // re-throw the deadlock exception
}
}
--nbTry;
}
}