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return value Consider the following interface, which provides a database lookup operation:
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interface DB { typedef sequence<Record> ResultSeq; typedef string QueryType; exception NotFound { // Bad approach QueryType failed_query; }; ResultSeq lookup(in QueryType query) raises(NotFound); };
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The lookup operation in this interface returns a sequence of results in response to a passed query If no matching records are found, it raises NotFound There are a number of things wrong with this interface When searching a database, it is expected that a search will occasionally not locate anything It is therefore inappropriate to raise an exception to indicate this Instead, you should use a parameter or return value to indicate the empty result In the preceding example, raising an exception is redundant because you can indicate the empty result by returning an empty sequence The NotFound exception complicates the interface unnecessarily The NotFound exception contains the failed_query member Because only one query is passed to the operation, there is only one possible query that can fail namely, the one that was passed to lookup The exception contains information that is already known to the caller, and that is pointless The DB interface does not allow the caller to find out why a query failed Was it because no records matched the query, or was it because the query contained a syntax error Compare the preceding version with this one:
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interface DB { typedef sequence<Record> ResultSeq; typedef string QueryType; exception SyntaxError { unsigned short position; }; ResultSeq lookup(in QueryType query) raises(SyntaxError); };
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This version is almost identical to the previous one However, the flaws are eliminated A search that returns no results is indicated by returning an empty sequence instead of raising an exception An exception is raised if the query itself is unacceptable This enables the caller to distinguish between a bad query and a query that merely did not return any results
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The exception contains useful information In this case, it contains the index of the character position in the query string at which a syntax error was found The DB example highlights some lessons that many designers still refuse to heed They can be summarized as follows Raise exceptions only for exceptional conditions Operations that raise exceptions for expected outcomes are ergonomically poor Consider the programmer who needs to call such an operation The C++ mapping maps IDL exceptions to C++ exceptions C++ exceptions are harder to deal with than normal return values or parameters because exceptions break the normal flow of control Forcing the programmer to catch an exception for expected behavior is simply bad style Make sure that exceptions carry useful information It is worse than useless to tell the caller something that is already known Make sure that exceptions convey precise information An exception should convey precisely one semantic error condition Do not lump several error conditions together so that the caller can no longer distinguish between them Make sure that exceptions carry complete information If exceptions carry incomplete information, the caller will probably need to make further calls to find out what exactly went wrong If the initial call did not work, there is a good chance that subsequent calls will also fail, and that can make precise error handling impossible for the caller Design interfaces so that they cater to the needs of the caller and not the needs of the implementer Computing abounds with difficult-to-use APIs that provide poor abstractions of functionality Typically, such APIs come into existence because they are written by the implementer of the functionality and not its user But good tools are built for the convenience of the tool user; the effort required by the tool maker to create the tool is usually considered irrelevant (within reason) APIs are tools, and you should build them to suit their users Do not use normal return values or parameters to indicate errors As you will see in the next section, operations can raise exceptions even if they do not have a raises expression If you use error codes instead of exceptions, callers end up with inconsistent and convoluted error handling because they must check for exceptions as well as an error return code
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