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POC++ error handling pattern
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copied!<p>We have a large code base with a lot of code which does this kind of thing:</p> <pre><code>bool DoSomething(CString Value) { if(Value == "bad") { AfxMessageBox("description of error"); return false; } return true; } </code></pre> <p>or even just this:</p> <pre><code>bool DoSomething(CString Value) { if(Value == "bad") { return false; } return true; } </code></pre> <p>We have considered various alternatives:</p> <ul> <li>Exceptions - Basically we’re sold on the cons as described here: <a href="http://www.codeproject.com/Articles/38449/C-Exceptions-Pros-and-Cons" rel="nofollow noreferrer">http://www.codeproject.com/Articles/38449/C-Exceptions-Pros-and-Cons</a></li> <li>Passing an additional string ref parm which is populated with error text - but it requires a pre-instantiation of the error string prior to the call, and adds bulk to the parm lists</li> <li>Populating a ‘last_error’ member variable - this seems to suffer (imo) the drawbacks expressed here: <a href="https://stackoverflow.com/questions/9099981/is-getlasterror-kind-of-design-pattern-is-it-good-mechanism">Is GetLastError() kind of design pattern? Is it good mechanism?</a></li> <li>Passing back an enum (or the like) which can be mapped to a description of the error with some other function - but this feels ‘heavy’ for small functions and objects, and also creates space between the place the error occurs and the place where the messages are maintained (though I suppose in multi-ligual environment they’d appreciate the centralization of the text).</li> </ul> <p>So I wondered if we could create a set of classes like the following:</p> <pre><code>class CResult { protected: CResult() { // Don't have to initialize because the derived class will do it } public: operator bool() { return m_Result; }; bool m_Result; CString m_Message; }; class CSuccess : public CResult { public: CSuccess() { m_Result = true; } }; class CFailure : public CResult { public: CFailure(const CString & Message) { m_Result = false; m_Message = Message; } }; </code></pre> <p>Then the above code could look like this:</p> <pre><code>CResult DoSomething(CString Value) { if(Value == "bad") { return CFailure("description of error"); } return CSuccess(); } </code></pre> <p>What I like about it:</p> <ul> <li>The code is still readable and the error message is maintained near the error condition</li> <li>The programmer will be slightly more compelled to actually provide an error string on an error condition (yes, they could provide blanks but that would seem a more egregious mistake, imo)</li> <li>The caller doesn’t have to create any specialized variables prior to the function call, and can still treat the function result like a bool - or in a case where the error is not ignored, easily retrieve an explanation</li> <li>The next programmer can what error model is in use just by looking at the function definition</li> </ul> <p>The main drawback I see is that there is higher overhead on success, in that an object and a string and a bool will all be instantiated - but a lot of the times in our app the code in question is not performance sensitive, such as validating user input, etc.</p> <p>Am I missing some other big drawback? Is there an even better solution?</p>

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