C++ try catch does not work for array out of bound

Question

We have a QT based C++ application. In which we are using third party DLL files too. But, C++ try and catch does not work at all.

For example:

#include <QCoreApplication>
#include <QDebug>
#include <QException>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    int arr[10];
    try
    {
        arr[11] = 30;
    }
    catch (const std::out_of_range& e)
    {
        qDebug() << "Exception out of range occurred ..." << e.what();
    }
    catch (...)
    {
        qDebug() << "Unknown Exception occurred...";
    }

    return a.exec();
}

Above is the minimal example. In the above, it crashes the program. Is there a way to handle this?

Try `qDebug() << "Exception <"+e.what()+"> occured...";` in your first catch - do you ever see it? Both your error messages being the same might be confusing — Neil Gatenby, Feb 22 '19 at 12:14
This example doesn't quite work either, as going out of bounds with `operator[]` and an `int*` isn't supposed to throw an exception! — hegel5000, Feb 22 '19 at 13:19
Writing out of array bounds is undefined behavior. It's not required to crash, throw an exception, or do anything at all. Sorry. — Blastfurnace, Feb 22 '19 at 13:25
We have legacy application, and we have crashes like mentioned above also null pointer exception, Is there a way to prevent crashing of the application. — hetal agrawal, Feb 22 '19 at 13:28
It almost sounds like what you need is a container that acts like an array but throws if `operator[]` is accessed out of bounds. — Mooing Duck, May 11 '23 at 19:37

score 2 · Accepted Answer · answered Feb 22 '19 at 13:32

2

Reading or writing out of array bounds is undefined behavior. There's no guarantee that it will crash, throw an exception, or do anything at all. It's just malformed code.

If you want an array-like container with bounds checking there are std::array, std::vector, and perhaps std::deque in the standard library. They all have an at() member function that does bounds checking and will throw a std::out_of_range exception.

answered Feb 22 '19 at 13:32

Blastfurnace

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In this case, writing a std::set_terminate, will help? – hetal agrawal Feb 25 '19 at 11:41
1

@hetalagrawal Setting a terminate handler is useful to handle the case when exception handling fails (such as an uncaught `std::out_of_range`). Remember that out-of-bounds on a plain array is undefined behavior so `std::set_terminate` isn't the solution for the code in your question. – Blastfurnace Feb 25 '19 at 13:36
Why we can not handle null pointer access using try, catch block in c++. Is the reason is same as it leads to undefined behavior? – hetal agrawal Feb 28 '19 at 13:18
1

@hetalagrawal A segfault caused by a null pointer access isn't a C++ exception. See: [Is it possible to catch a segfault with try/catch?](https://stackoverflow.com/questions/40144571/is-it-possible-to-catch-a-segfault-with-try-catch) – Blastfurnace Feb 28 '19 at 16:52

score 1 · Answer 2 · answered Feb 22 '19 at 13:31

std::out_of_range

It may be thrown by the member functions of std::bitset and std::basic_string, by std::stoi and std::stod families of functions, and by the bounds-checked member access functions (e.g. std::vector::at and std::map::at)

Your try block has neither of these things. Out of bounds access to plain C style arrays is undefined behaviour. This is often manifested as crashes, as you have experienced.

score 0 · Answer 3 · edited May 11 '23 at 18:54

To answer your question:

"C++ try catch does not work for third party library"

No! C++ try catch does work with a third-party (Qt) library, as shown in the example below.

But the code that you are showing is not an MCVE. Therefore it is difficult to say, what exactly is causing the problem that you say you are having.

#include <QCoreApplication>
#include <QDebug>
#include <QException>

class testException : public QException
{
public:
    testException(QString const& message) :
        message(message)
    {}

    virtual ~testException()
    {}

    void raise() const { throw *this; }
    testException *clone() const { return new testException(*this); }

    QString getMessage() const {
        return message;
    }
private:
    QString message;
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    try
    {
        // throw std::out_of_range("blah");
        throw testException("blah");
    }
    catch (const std::out_of_range& e)
    {
        qDebug() << "Exception out of range occurred ...";
    }
    catch (...)
    {
        qDebug() << "Unknown Exception occured...";
    }

    return a.exec();
}

C++ try catch does not work for array out of bound

3 Answers3