· Do whatever you can in the current context and rethrow the same exception to a higher context.
· Do whatever you can in the current context and throw a different exception to a higher context.
· Terminate the program.
· Wrap functions (especially C library functions) that use ordinary error schemes so they produce exceptions instead.
· Simplify. If your exception scheme makes things more complicated, it is painful and annoying to use.
· Make your library and program safer. This is a short-term investment (for debugging) and a long-term investment (for application robustness).
When to use exception specifications
The exception specification is like a function prototype: it tells the user to write exception-handling code and what exceptions to handle. It tells the compiler the exceptions that might come out of this function so that it can detect violations at runtime.
Of course, you can’t always look at the code and anticipate which exceptions will arise from a particular function. Sometimes, the functions it calls produce an unexpected exception, and sometimes an old function that didn’t throw an exception is replaced with a new one that does, and you get a call to unexpected( ). Any time you use exception specifications or call functions that do, consider creating your own unexpected( ) function that logs a message and then either throws an exception or aborts the program.
As we explained earlier, you should avoid using exception specifications in template classes, since you can’t anticipate what types of exceptions the template parameter classes might throw.
Start with standard exceptions
Check out the Standard C++ library exceptions before creating your own. If a standard exception does what you need, chances are it’s a lot easier for your user to understand and handle.
If the exception type you want isn’t part of the standard library, try to derive one from an existing standard exception. It’s nice if your users can always write their code to expect the what( ) function defined in the exception( ) class interface.
Nest your own exceptions
If you create exceptions for your particular class, it’s a good idea to nest the exception classes either inside your class or inside a namespace containing your class, to provide a clear message to the reader that this exception is used only for your class. In addition, it prevents the pollution of the global namespace.
You can nest your exceptions even if you’re deriving them from C++ standard exceptions.
Use exception hierarchies
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.).