Thinking In C++. Volume 2: Practical Programming

Using exception hierarchies is a valuable way to classify the types of critical errors that might be encountered with your class or library. This gives helpful information to users, assists them in organizing their code, and gives them the option of ignoring all the specific types of exceptions and just catching the base-class type. Also, any exceptions added later by inheriting from the same base class will not force all existing code to be rewritten—the base-class handler will catch the new exception^.

Of course, the Standard C++ exceptions are a good example of an exception hierarchy and one on which you can build^.

Multiple inheritance (MI)

As you’ll read in Chapter 9, the only essential place for MI is if you need to upcast an object pointer to two different base classes—that is, if you need polymorphic behavior with both of those base classes. It turns out that exception hierarchies are useful places for multiple inheritance because a base-class handler from any of the roots of the multiply inherited exception class can handle the exception^.

Catch by reference, not by value

We explained in the section "Exception matching" earlier that you should catch exceptions by reference for two reasons:

·         To avoid making a needless copy of the exception object when it is passed to the handler,

·         To avoid object slicing when catching a derived exception as a base class object

Although you can also throw and catch pointers, by doing so you introduce more coupling—the thrower and the catcher must agree on how the exception object is allocated and cleaned up. This is a problem because the exception itself might have occurred from heap exhaustion. If you throw exception objects, the exception-handling system takes care of all storage^.

Throw exceptions in constructors

Because a constructor has no return value, you’ve previously had two ways to report an error during construction:^.

·         Set a nonlocal flag and hope the user checks it.

·         Return an incompletely created object and hope the user checks it.

This problem is serious because C programmers have come to rely on an implied guarantee that object creation is always successful, which is not unreasonable in C in which types are so primitive. But continuing execution after construction fails in a C++ program is a guaranteed disaster, so constructors are one of the most important places to throw exceptions—now you have a safe, effective way to handle constructor errors. However, you must also pay attention to pointers inside objects and the way cleanup occurs when an exception is thrown inside a constructor^.

Don’t cause exceptions in destructors

Because destructors are called in the process of throwing other exceptions, you’ll never want to throw an exception in a destructor or cause another exception to be thrown by some action you perform in the destructor. If this happens, a new exception can be thrown before the catch-clause for an existing exception is reached, which will cause a call to terminate( )^.

If you call any functions inside a destructor that can throw exceptions, those calls should be within a try block in the destructor, and the destructor must handle all exceptions itself. None must escape from the destructor^.

Avoid naked pointers

See Wrapped.cpp earlier in this chapter. A naked pointer usually means vulnerability in the constructor if resources are allocated for that pointer. A pointer doesn’t have a destructor, so those resources aren’t released if an exception is thrown in the constructor. Use auto_ptr for pointers that reference heap memory^.

Overhead

When an exception is thrown, there’s considerable runtime overhead (but it’s good overhead, since objects are cleaned up automatically!). For this reason, you never want to use exceptions as part of your normal flow-of-control, no matter how tempting and clever it may seem. Exceptions should occur only rarely, so the overhead is piled on the exception and not on the normally executing code. One of the important design goals for exception handling was that it could be implemented with no impact on execution speed when it wasn’t used; that is, as long as you don’t throw an exception, your code runs as fast as it would without exception handling. Whether this is actually true depends on the particular compiler implementation you’re using. (See the description of the "zero-cost model" later in this section.)^.