When is it appropriate to use C++ exceptions?

Question

I'm trying to design a class that needs to dynamically allocate some memory..

I had planned to allocate the memory it needs during construction, but how do I handle failed memory allocations? Should I throw an exception? I read somewhere that exceptions should only be used for "exceptional" cases, and running out of memory doesn't seem like an exceptional case to me..

Should I allocate memory in a separate initialization routine instead and check for failures and then destroy the class instance gracefully?

Or should I use exceptions instead? The class won't have anything useful to do if these memory allocations should fail..

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EDIT: The consensus seems to be that running out of memory IS an exceptional case.

Will see how to go about this.. Thanks.. :)

Answer 1

The usual behaviour in C++ when you are out of memory is to throw an exception. The built-in new operator does this by default.

Answer 2

I would argue that running out of memory (particularly heap memory) is an exceptional case, and if your class - and further, your application - cannot continue, I think exception throwing/handling is a very appropriate and graceful approach.

Answer 3

Assuming you are using new to allocate memory, and are not overriding the new operator, it will automatically throw the std::bad_alloc exception if it fails to allocate memory properly.

I read somewhere that exceptions should only be used for "exceptional" cases, and running out of memory doesn't seem like an exceptional case to me..

Running out of memory seems like a pretty exceptional case to me :)

It is very difficult to handle this sort of situation. You may want to return a meaningful error to the user of your application, but if it's a problem caused by lack of memory, you may not even be able to afford the memory to allocate the error message. It's a bit of a catch-22 situation really.

There is a defensive programming technique (sometimes called a memory parachute or rainy day fund) where you allocate a chunk of memory when your application starts. When you then handle the bad_alloc exception, you free this memory up, and use the available memory to close down the application gracefully, including displaying a meaningful error to the user. This is much better than crashing :)