Can anyone explain C++ exception specifications to me?

Question

Can anyone explain Exception Specifications as used in C++ ?

• When are they used (I have rarely seen it used in code)
• What are the pros and cons (benefits/

Answer 1

They are generally regarded as a bad idea.

They say what a method will throw. The downside is that if that method throws anything else then your app terminates. So it's a guarantee but not in the same way that Java does it. And it adds the overhead of inspection.

Answer 2

The important thing to know is: exception specifications are deprecated in the next revision of C++, except for the no-throw specifier (throw()), which is basically officially saying "don't use them".

Putting throw() after a function means the function does not throw any exceptions. If it does anyway, the application will be terminated (probably - the unexpected handler is called), so the rest of your application can use that function knowing it will not throw an exception. This is can be handy for writing exception-safe code.

Example:

void MyFunction() throw() // does not throw any exceptions
{
    /* ... */
{

Answer 3

Basically, exception specifications allow the compiler to optimize the stack. The disadvantage is that it's a hell of a lot of extra spec. This means that you commonly see it in library work but not so much in working code.

I believe there's also some compile-time exception-safety thing going on, but since I've never used it, I can't be sure.

Answer 4

They indicate to the programmer which exceptions will be thrown from that function. They have the advantage that you have the guarantee that no other exceptions can be thrown from that function. However, there is no compile-time checking of whether the function actually throws any exceptions other than what is indicated in the throw specifier. Instead, it's checked at runtime. And if it fails, then unexpected() is called, which by default calls abort() which in turn by default terminates your program. So, unless you really want your program to die if you screw up and an unexpected exception gets thrown, it's probably a bad idea to use them.

The one place where I would actually recommend using them is on destructors. It's really bad if a destructor throws. They should never throw. So, if you use throw() on a destructor to indicate that it can't throw an exception, then you know that your program will die rather than continue in the bad state that it would be in after an exception was thrown from a destructor.

Regardless, if you use any kind of throw specifier, you have to be sure that you really do want your program to die if it's violated. So, generally-speaking, it's best not to use them.

Answer 5

When are they used (I have rarely seen it used in code)
Hopefully never as they are deprecated in the next version of C++ due for standardization next year.

What are the pros and cons (benefits/disadvantages) of using exception specifications?
They provide a way for readers of your code to know exactly what exceptions a function is allowed to throw. The problem is, that if an unexpected exception (one not in the specification) is thrown, then the program will be terminated (by default).