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I'm starting to write some code using C++ and I have this simple class method from a simple task manager program I'm writing as an experiment:

void Task::setText(string text) { if(text.length() > MIN_LENGTH) { this->text = text; } }

As you can see, this method sets the class text attribute to the one passed to the method if it's length is higher than the MIN_LENGHT variable that is defined above the code I've shown. So I have to do something if the condition does not evaluate to true for the string passed to the method.

In the C++ book I bought, error handlings are not explained, instead it just uses assert everywhere. As assert just aborts the program if the expression is false, and this is intended as input validation, I looked for a better approach.

This search led me to C++ exceptions. There it explains how to create exceptions by creating a class that inherits from exception.

Good OOP practice says that every class should be independent from the others in the program. So where should I put this exception class I create? In the same header I define my Task class in? Or should it be in task.cpp where I define every method of the class?

Maybe this is a silly question but just want to be secure and follow a good software architecture practices from the beginning.

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    "I didn't do what you asked for" is not the same as "you screwed up". Is passing too short a string something that should, potentially, terminate the program? You first have to decide what the caller should do in a case like this. Then you can think about how to transmit the necessary information. Commented Oct 5, 2012 at 22:30

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Recommendation #1: You need to read your book completely.

It is not true that in case of every error you should throw an exception. Exception should happen something less often that every 1000 calls to your function/method. 1000 is not a magic value here, in particular case other number might be appropriate.

In other words. The first question that you need to answer: how often this error may happen. If this may happen often, then the return value (bool or enum or int) is a better approach.

If you decided to use exception, it is better to derive it from std::exception. You should place exceptions of your project in a separate file. Think about a couple of classes, maybe 3-10. It is better to place a data field inside your exception class that should explain details of what happened instead of creating hundreds of different exception classes.

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While I agree with this answer, I would like to point out that assert serve the purpose of identifying PROGRAMMER errors (like, something that should NEVER happen unless the code is buggy) because they are disabled when compiled in release, when exceptions should be used to treat errors that come from the user. In your case, the question you should ask is "if the string is shorter than it should, is it a bug? or can it happen because the user typed the wrong thing, or because a file did not exist, etc."
+1 for recommending to use exceptions sparingly. And, if I could, another +1 for recommending that they be defined in a separate header. You should think carefully about error categories that are right for your program and create a very small number of exception classes that capture them. You should derive from ::std::exception or one of its sub-clasees, like ::std::invalid_argument.
@qdii: Excellent point about assert. I'm afraid that my edit obscured the fact that the original post was seemingly a bit ambiguous about why the condition might fail. The OP mentioned 'validation' in several places, so I assumed 'input validation'.
Agree. Assert should be used for checking consistency between parts of the code. If program works with a 256 elements table (and not anything else), an assert should be used to verify this. Errors is everything that comes into a program from the outside world: a file, database, network socket, user input, etc. No input from outside should cause a crash.
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the exception class can be in a separate .h .cpp file. That way you can reuse for something else.

As my own rule of thumb:

assertions are for things that should never happen but we check just to be sure 100% sure

Exceptions is for error handling things that can happen when your program is in production

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C++ standard library provides many exception class that you can use. For begining, you can use them. In your case, what applies seems to be range error.
If you still want to implement your own exception class, where you declare and implement it should depend on what they code for. If they code for a class specific error, you can put them in your class file. If you have to use them from multiple files, put them in they own .cpp/.hpp couple.

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