How to catch the null pointer exception ?

Question

  try {
        int* p = 0;
        *p = 1;
    } catch (...) {
        cout << "null pointer." << endl;
    }

I tried to catch the exception like this but it doesn't work,any help?

Answer 1

You cannot. De-referencing a null-pointer is a system thing.

On Linux, the OS raises signals in your application. Take a look at csignal to see how to handle signals. To "catch" one, you'd hook a function in that will be called in the case of SIGSEGV. Here you could try to print some information before you gracefully terminate the program.

Windows uses structured-exception-handling. You could use the instristics __try/__except, as outlined in the previous link. The way I did it in a certain debug utility I wrote was with the function _set_se_translator (because it closely matches hooks). In Visual Studio, make sure you have SEH enabled. With that function, you can hook in a function to call when the system raises an exception in your application; in your case it would call it with EXCEPTION_ACCESS_VIOLATION. You can then throw an exception and have it propagate back out as if an exception was thrown in the first place.

Answer 2

C++ doesn't do pointer checking (although I suppose some implementations could). If you try to write to a null pointer it is most likely going to crash hard. It will not throw an exception. If you want to catch this you need to check the value of the pointer yourself before you try to write to it.

Answer 3

Dereferencing a null (or pointer that's past-the-end of array, or a random invalid pointer) results in undefined behavior. There's no portable way to "catch" that.

Answer 4

As others have said, you can't do this in C++.

If I can make a broader point: even in a language that allows you to catch it, the better action is to not touch null pointers. Catching an error when it's already blown up in your face, then deciding to just move on like it didn't happen, is not a good coding strategy. Things like null pointer dereference, stack overflow, etc., should be seen as catastrophic events and defensively avoided, even if your language allows you to react to it differently.

Answer 5

There is no platform independent way to do this. Under Windows/MSVC++ you can use __try/__except

But I wouldn't recommend doing it anyway. You almost certainly cannot recover correctly from a segmentation fault.

Answer 6

There's no such thing as "null pointer exception" in C++. The only exceptions you can catch, is the exceptions explicitly thrown by throw expressions (plus, as Pavel noted, some standard C++ exceptions thrown intrinsically by standard operator new, dynamic_cast etc). There are no other exceptions in C++. Dereferencing null pointers, division by zero etc. does not generate exceptions in C++, it produces undefined behavior. If you want exceptions thrown in cases like that it is your own responsibility to manually detect these conditions and do throw explicitly. That's how it works in C++.

Whatever else you seem to be looking for has noting to do with C++ language, but rather a feature of particular implementation. In Visual C++, for example, system/hardware exceptions can be "converted" into C++ exceptions, but there's a price attached to this non-standard functionality, which is not normally worth paying.

Answer 7

Generally you can't. Even if you could it would be like trying to put a band aid on a submarine that has sprung a leak.

A crippled application can do far more damage than one that has crashed. My advise here would be to let it crash then fix why it crashed. Rinse. Repeat.