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This chapter is from the book

This chapter is from the book

11.5 Exceptions and Design Issues

There are some design issues associated with the use of exception handling in C++ programs. Although the support for exception handling is built into the language, not every C++ program should use exception handling. Exception handling should be used to communicate program anomalies between parts of the program that are developed independently, because throwing an exception is not as fast as a normal function call. For example, a library implementer may decide to communicate program anomalies to users of the library using exceptions. If a library function encounters an exceptional situation that it cannot handle locally, it may throw an exception to notify the program using the library.

In our example, our library defines the iStack class and its member functions. The function main() uses the library, and we should assume that the programmer writing main() is not the library implementer. The member functions of the class iStack are capable of detecting that a pop() operation is requested on an empty stack or that a push() operation is requested on a full stack, but the library implementer does not know the state of the program that caused the pop() or push() operations to be requested in the first place and cannot write pop() and push() to locally address this situation. Because these errors cannot be handled in the member functions, we decided to throw exceptions to notify the program using the library.

Even though C++ supports exception handling, C++ programs should use other error handling techniques (such as returning an error code) when appropriate. There is no clearcut answer to the question, "When should an error become an exception?" It is really up to the library implementer to decide what an "exceptional situation" is. Exceptions are part of a library's interface, and deciding which exceptions the library throws is an important phase of the library design. If the library is intended to be used within programs that cannot afford to crash, then the library must either handle the problem itself, or, if it can't, it must communicate program anomalies to the part of the program that uses the library and give the caller the choice as to which action should be taken when no meaningful action can be taken within the library code itself. Deciding what should be handled as an exception is a difficult part of the library design.

In our iStack example, it is debatable whether the push() member function should throw an exception if the stack is full. Another, some people would say better, implementation of push() is to handle this situation locally and grow the stack if it is full. After all, the only real limit is the memory available to our program. Our decision to throw an exception if the program attempts to push a value on a full stack may have been ill-considered. We can reimplement the member function push() to grow the stack if a request is made to push a value on a full stack:

void iStack::push( int value )
{
   // if full, grow the underlying vector
   if ( full() )
      _stack.resize( 2 * _stack.size() );

   _stack[ _top++ ] = value;
}

Similarly, should pop() throw an exception when a request is made to pop a value from an empty stack? One interesting observation is that the stack class of the C++ standard library (introduced in Chapter 6), does not throw an exception if a pop operation is requested and the stack is empty. Instead, the operation has undefined behavior: it is unknown what the program behavior is after such an operation has been requested. When the C++ standard library was designed, it was decided that an exception should not be thrown in this case. Allowing the program to continue running while an illegal state had been encountered was deemed appropriate in this situation. As we mentioned, different libraries will have different exceptions. There is no right answer to the question of what constitutes an exception.

Not all programs should worry about exceptions thrown by libraries. Although it is true that some systems cannot afford down time and should be built to handle exceptional situations, not every program has such requirements. Exception handling is primarily an aid to the implementation of fault-tolerant systems. Again, deciding whether our programs are to handle exceptions thrown from libraries or whether we should let the program terminate is a difficult part of the design process.

One last aspect of program design with exception handling is that the handling of exceptions in a program is usually layered. A program is usually built of components, and each component must decide which exceptions it will handle locally and which exceptions it will pass to higher levels of the program. What do we mean by component? For example, the text query system introduced in Chapter 6 can be broken into three components or layers. The first layer is the C++ standard library, which provides support for the basic operations on strings, maps, and so on. The second layer is the text query system itself, which defines functions, such as string_caps() and suffix_text(), that manipulate the text to be processed and uses the C++ standard library as a subcomponent. The third layer is the program that uses our text query system. Each component or layer is built independently, and each must decide which exceptional situation it will handle directly and which exceptions it will pass to higher levels of the program.

Not every function in a component or layer should be capable of dealing with exceptions. Usually, try blocks and associated catch clauses are used by functions that are the entry points into a program component. The catch clauses are designed to handle exceptions that the component does not want to let propagate to higher levels of the program. Exception specifications (discussed in Section 11.4) are also used with the functions that are the entry points into a component to guard against the escape of unwanted exceptions to higher levels of the program.

We look at other aspects of designing programs with exceptions in Chapter 19, after classes and class hierarchies have been introduced.

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