c++ - To throw or not to throw exceptions?

Question

I was talking to a friend of mine that through my new code I didn't treat exceptions, just because I didn't know how to do it in C++. His answer surprised me: "why in the hell would you want to throw excetions?". I asked him why, but he didn't have a satisfying answer, so I googled it. One of the first pages I found was a blog entry, where the guy who posted wasn't totally against exceptions, but a religious war started in the replies: http://weblogs.asp.net/alex_papadimoulis/archive/2005/03/29/396141.aspx

Now I begin to wonder: is it that bad to throw an exception? For a student like I am, is it wrong to learn programming using exceptions as a normal thing? (When I throw exceptions, I catch them in another level of the code to treat them, most of the times). I have a code example, and I want to know what should I do:

int x;
cout << "Type an integer: ";
cin >> x;

Anything that is typed there that is not an integer will trigger an exception already, right? This exception should be treated there. But when I have a possible exception inside a class that is being used elsewhere in the program, should I make this method throw an exception so I can treat it wherever I call it or should I make it return a standard value when it has any problems?

The exception is always good, always bad, or something to be used in "exceptional" situations? Why?

Answer 1

The C++ iostreams classes do not, by default, use exception handling. Generally one should use exceptions for situations where an error can occur, but such errors are "unusual" and "infrequent" (such as a disk failing, the network being down, etc.). For error conditions that you do expect (such as the user providing invalid input), you should probably not use exception handling, unless the logic that needs to deal with this situation is far removed from the logic in which the situation is detected, in which case using an exception is the way to go. There is nothing wrong with using exceptions, and using exceptions where they are merited is definitely good... just avoid using them where if...else works just fine.

As to the why:

1. Exceptions often provide a simple, elegant way of handling unexpected errors, but:
2. Exception propagation, depending on the compiler and platform, may be slower than ordinary control flow, so if you can handle the situation locally, then that will be at least as fast... and you especially don't want to slow down the common, expected cases (such as invalid input). Also:
3. Exceptions require more type information to be present and also require code that is unrelated to throwing or catching exceptions to be "exception-safe" which is why some object to exceptions.