Is checking by an exception safe for when array goes out bound?

Question

Here is my code:

template<class T>
void Matrix<T>::set(const int i, const int j, const T v) {
    try {
        m[i][j] = v;
    } catch (std::exception& e) {
        std::cout << "Standard exception: " << e.what() << std::endl;
    }
}

But, I am on the safe side here? I mean, it is possible that i and/or j are out of bound, but the program receives on segmentation fault, will then an exception been thrown?

If this is not a good approach, then should I use assert() maybe?

@juanchopanza For a guy with a gold tag of c++, *of course* it is. For an average guy like me, that knows **nothing** about exceptions it is not! Did you downvote for that reason?! — gsamaras, Oct 16 '15 at 23:18
No. It isn't that hard to search for this stuff. It has been answered a zillion times. — juanchopanza, Oct 16 '15 at 23:20
A zillion times @juanchopanza in places you have seen, but when I asked the question, I first search all the auto-filled question SO gave me... Why was I downvoted then? Damn it, it's not fair. — gsamaras, Oct 16 '15 at 23:23
I can re-open the question, no problem. The duplicate came from the "related" links, meaning it isn't hard to find. Don't worry too much about a single down-vote, there could be all kinds of reasons for that. — juanchopanza, Oct 16 '15 at 23:25
No @juanchopanza, you are a superuser, I trust you. Thank you for taking the time to answer my comments! — gsamaras, Oct 16 '15 at 23:26
@gsamaras Don't worry it happens sometimes. Arrays don't trow exceptions. Error handling in C++ is a complex opinionated topic (because C++ applications from embedded to mainframes are very different). I would recommend you to look at [famous book list](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) and specifically Chapter II.13 of [Bjarne's book](http://www.amazon.com/The-Programming-Language-4th-Edition/dp/0321563840), so you could find out your own truth. — Ivan Aksamentov - Drop, Oct 16 '15 at 23:26
Another good discussion of C++ error handling is in [Sutter, Alexandrescu -- Coding Standards](http://www.amazon.com/Coding-Standards-Rules-Guidelines-Practices/dp/0321113586). — Ivan Aksamentov - Drop, Oct 16 '15 at 23:33
This is one of the places where C and C++ line up well, excepting the exceptions of course. Correct use is not penalized with any sanity checking unless it is explicitly stated in the documentation. In the absence of documentation, assume unchecked. — user4581301, Oct 16 '15 at 23:35
You asked in the comments for where you can find the relevant information. Here is the site that I always use for this purpose: http://en.cppreference.com/w/cpp And more specifically to your question: http://en.cppreference.com/w/cpp/container/vector/operator_at As you see, it very clearly says that "no bounds checking is performed". As such, if `m` is an `std::vector<>`, you don't have the ghost of a chance to catch an out of bounds condition with your `try{} catch`. I have found the information at this site to be both very precise and very detailed. — cmaster - reinstate monica, Oct 16 '15 at 23:47
@cmaster thank you. It is interesting to leave this here too then: http://www.cplusplus.com/reference/stdexcept/out_of_range/ — gsamaras, Oct 16 '15 at 23:51

sehe · Answer 1 · 2015-10-16T22:58:10.810

Just add bounds checks.

C++ does not do bounds checking on arrays, let alone throw exceptions.

With vectors/std::array you can use the .at() method instead of [] operator to get checking.

Lastly, GSL introduces the array_view which lets you add "zero cost annotation" so that analysis tools can do the bounds checking. There is no de-facto standard implementation of GSL that has emerged yet. See also:

https://www.youtube.com/watch?v=1OEu9C51K2A

score 3 · Accepted Answer · answered Oct 16 '15 at 22:57

3

No.

Exception checking is maybe okay (but not at all elegant) when only reading from the array.

When writing, however, you risk overwriting "foreign" memory, that is a huge security risk, plus it might alter the execution flow by overwriting stack frames and who knows what - that is, there are certainly circumstances when the exception handler won't run even if it is actually expected to throw.

The best decision is to check indices on every iteration. Given that you mentioned asserting, I suppose you do know the array dimensions to start with.

answered Oct 16 '15 at 22:57

Victor - Reinstate Monica

1,264
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Yes I do, thanks! Do you think that the question was so bad? – gsamaras Oct 16 '15 at 23:13
No, actually. It is important to understand what's going on even for those who know `best practices`. – Victor - Reinstate Monica Oct 16 '15 at 23:15
Well the score is -1, so somebody things the question is bad and I would like to know why. – gsamaras Oct 16 '15 at 23:16
2

They probably wanted to refer that you could perhaps use the search in the first place, as it has been marked as a duplicate, and not to the question itself. – Victor - Reinstate Monica Oct 16 '15 at 23:24

score 3 · Answer 3 · answered Oct 16 '15 at 23:04

template<class T>
void Matrix<T>::set(const int i, const int j, const T v) {
    try {
        m[i][j] = v;
    } catch (std::exception& e) {
        std::cout << "Standard exception: " << e.what() << std::endl;
    }
}

You don't mention what type m is, but assuming it's a custom type that does bounds checking in operator[], then you should be okay. The bounds check will (or should) happen, and an exception should be thrown, before any attempt at an out-of-bounds write.

But note that C++ doesn't do bounds checking itself on raw (C) arrays, and neither do any of the standard library types in their operator[] implementations, so if m is one of these types then you'll never get an exception thrown. Instead, STL types provide an at() method which you should use instead if bounds checking is desired.

Is checking by an exception safe for when array goes out bound?

3 Answers3