Problem throwing and catching custom exceptions in C++

Question

I've created the class Someting which throws an exception SomethingException (SomethingException inherits from std::exception) when it fails to instantiate. the problem is I can't catch SomethingException as such (I had to do a dirty trick to catch it).

There is somewhere in the program where it executes: This doesn't work, the exception is not caught and the program crashes.

try{ Something* s = new Something(); } catch (SomethingException* e){ std::cerr<<e.what(); }

In contrast this does work (exception is caught and the correct message shown) but I really have the feelin I shouldn't be doing this

try{ Something* s = new Something(); } catch (std::exception* e){ SomethingException* e2 = (SomethingException*) e; std::cerr<<e.what(); }

Because the pointer is casted I can only make this work if and only if one type of exception is thrown. The moment I need to catch various types this won't work.

Is there a way to caught a custom exception in a more correct way?

Edit:

The exception is thrown as follows

//... throw new SomethingException ("Errormessage"); //Custom exception constructor //...

The declaration of Something::Something() is

Something::Something() throw(...)

Using the declaration

Something::Something() throw(SomethingException) //or Something::Something() throw(SomethingException*)

Throws a lot of warnings (Warning C4290)

Your code feels like you are a Java programmer starting in C++. The way you use exceptions is not the usual C++ way. — Sjoerd
– Sjoerd, Commented Jun 12, 2011 at 21:51
Note: It is easier to derive exceptions from std::runtime_error than std::exception. This is because std::runtime_error will store the error message (passed in the constructor) for you that is returned by the what() method. Note: MSVC has a non standard std::exception that allows you to pass the error message in the constructor. — Loki Astari
– Loki Astari, Commented Jun 12, 2011 at 23:01

Jon Purdy · Accepted Answer · 2011-06-12 21:17:58Z

In general it's best to throw exceptions by value and catch them by reference:

try { throw SomethingException(); } catch (const SomethingException& error) { std::cerr << error.what() << '\n'; }

You would only be able to catch an exception with catch (SomethingException*) if you were to throw it with throw new SomethingException(). There isn't enough information in your question to tell, but the problem may be in how SomethingException derives from std::exception. Verify that or change it to inherit from, say, std::runtime_error or std::logic_error instead.

Also, don't use throw specifiers. Just don't. No compiler affords any benefit to using checked exceptions: in effect, checked exceptions aren't checked except to fail horribly (throwing std::bad_exception) in the event of an exception that doesn't conform to the specifier. That's probably what's happening in your code.

> throw new SomethingException() is the way is thrown. I'll try what you say anyway
This time it doesn't compile at all. 'std::runtime_exception doesn't have a default constructor' also the compiler throws a warning saying 'omitting exception specification except to assert the function is not __declspec(nothrow)' <-- rough translation
@NeonMan: The runtime_error and logic_error constructors take a string message, and what() must have the throw() specifier if you overload it.
@NeonMan: You should get rid of the throw(SomethingException) specifier on the function. There's really no reason catching a user-defined exception should fail to work, and there's not enough information in your post to tell where your problem is.
@NeonMan: I suggest no throw specifier whatsoever on the function declaration and, yes, throw SomethingException(); in the function body.

Richard Whitehead · Accepted Answer · 2014-03-22 12:48:13Z

For others who may have the problem where a custom exception, derived from std::exception, is being thrown but not caught, also check: - That the inheritance is public - If your exception is declared in another DLL, that the exception class is exported from the DLL. Strangely, if it is not, this does not produce a link error (in VS2012), it just fails to be caught.

This helped with GCC too (adding public after : in custom exception declaration).

Nick · Accepted Answer · 2011-06-12 20:09:02Z

Can you show the code where you throw the exception.

Another point is about throw-specifications - it's generally a bad idea. The problem is that C++, unlike Java, doesn't insist on throw-specs and so you get next to no benefit from them. All that they can do is potentially cause a core dump if your code (or some code that you call) throws an exception that isn't specified in the throw-spec

Added the throw line. I really like the way java uses the exceptions, unlike c++. Preventing an object for being constructed is essential in this case and, using a custom exxception is preferred.

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