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

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 2

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 3

There is a very easy way to catch any kind of exception (division by zero, access violation, etc.) in Visual Studio using try -> catch (...) blocks.

A minor project tweaking is enough. Just enable the /EHa option in project settings. See Project Properties -> C/C++ -> Code Generation -> Modify the Enable C++ Exceptions to "Yes With SEH Exceptions". That's it!

See details here: http://msdn.microsoft.com/en-us/library/1deeycx5(v=vs.80).aspx

Answer 4

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 5

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 6

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 advice here would be to let it crash then fix why it crashed. Rinse. Repeat.

Answer 7

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 8

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 9

If you wanted to you could just do the pointer checking yourself and throw...

if (p == nullptr) throw std::exception("woot! a nullptr!")
p->foo();

so course this would only be to debug the problem, the nullptr should not occur in the first place :)

Answer 10

Short answer- you can't in a portable or standard way, because bugs like this are potentially corrupting the process itself.

Long answer- you can do more than you might think, and definitely more than the default of the program just crashing. However, you need to keep 3 things in mind:
1) These bugs are MORE severe than exceptions and often cannot present as exceptions to your logic.
2) Your detection and library handling of them WILL be platform-dependent on the back end, even though you can provide a clean abstract interface for public consumption.
3) There will always be some crashes that are so bad you cannot even detect them before the end.

Basically, faults like segfaults or heap corruption are not exceptions because they're corrupting the actual process running the program. Anything you coded into the program is part of the program, including exception handling, so anything beyond logging a nice error message before the process dies is inadvisable in the few cases it isn't impossible. In POSIX, the OS uses a signaling system to report faults like these, and you can register callback functions to log what the error was before you exit. In Windows, the OS can sometimes convert them into normal-looking exceptions which you can catch and recover from.

Ultimately, however, your best bet is to code defensively against such nightmares. On any given OS there will be some that are so bad that you cannot detect them, even in principle, before your process dies. For example, corrupting your own stack pointer is something that can crash you so badly that even your POSIX signal callbacks never see it.

Answer 11

In VC++ 2013 (and also earlier versions) you can put breakpoints on exceptions:

1. Press Ctrl + Alt + Delete (this will open the exception dialog).
2. Expand 'Win32 Exceptions'
3. Ensure that "0xC0000005 Access Violation" exception is checked.

Now debug again, a breakpoint will be hit exactly when the the null dereference happened.

Answer 12

There is no NULL pointer exception exist in c++ but still you want to catch the same then you need to provide your own class implementation for the same.

below is the example for the same.

class Exception {

public:
   Exception(const string& msg,int val) : msg_(msg),e(val) {}
  ~Exception( ) {}

   string getMessage( ) const {return(msg_);}
   int what(){ return e;}
private:
   string msg_;
   int e;
};

Now based on NULL pointer check it can be threw like , throw(Exception("NullPointerException",NULL)); and below is the code for catching the same.

 catch(Exception& e) {
      cout << "Not a valid object: " << e.getMessage( )<< ": ";

        cout<<"value="<<e.what()<< endl;
   }