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POSimplify config-file-driven factory
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copied!<p>Hej!</p> <p>Caution this is a long post -- if you are easily annoyed better skip it. ;-)</p> <p>The Project I am working on currently is about reading voltage measurements of different sensors and calculating the respective physical value. There are a number of channels each of which can have a different sensor attached. Basicly the kind of sensor is known but the details like sensitivity and bias can vary widely. While there is a parametrized class for each type of sensor, the parameters are pulled from config files.</p> <p>Suppose there is the following class hierachy:</p> <pre><code>class Sensor { public: virtual double Calculate(const double &arg)=0; }; class PTCResistor { PTCResistor(XMLNode &node); double Calculate(const double &arg); }; class Thermocouple { Thermocouple(XMLNode &node); double Calculate(const double &arg); }; </code></pre> <p>The main config file looks like this:</p> <pre><code><channels> <TurbineInletTemp sensor="Thermocouple.TypeK.xml" /> <CylinderHeadTemp sensor="PTCResistor.PT500.xml" /> ... </channels> </code></pre> <p>As you see each channel tag has a attribute sensor specifying some ohter XML-File which contians parameter information and the kind of sensor. It looks something like this:</p> <pre><code><sensor class="PTCResistor"> <param Rref="500" Tref="0"> ... </sensor> </code></pre> <p>The format of these reference XML-files can vary but is is understood by the constructor of each derived class.</p> <p>Upon program start the main config file is parsed and a factory class resolves the link to the XMLs containing the sensor specific information, examines the class tag and calls the respective constructors. Like this:</p> <pre><code>string chnTITFile = rootNode.GetNode("TurbineInletTemp").GetAttribute("sensor"); XMLNode chnTITNode = XMLNode.parseFile(RootPath + chnTITFile,"sensor") string className = chnTITNode .GetAttribute("class"); if(className == "Thermocouple") { Sensor* sensorTIT = new Thermocouple(chnTITNode); } else if(className = "PTCResistor") { Sensor* sensorTIT = new PTCResistor(chnTITNode); } </code></pre> <p>This is the leanest solution I came up with in the sense that there are only few places to alter when a new derived class is added. Basicly one writes a new derived class and the adds another if-branch in the factory. Unfortunately this involves a lot of string comparisons. Since the number of classes and channels can be rather large (and the target system is low-powered) I am concerned. </p> <p>Another possibility would be hash the class names. This would probably look like this:</p> <pre><code>string chnTITFile = rootNode.GetNode("TurbineInletTemp).GetAttribute("sensor"); XMLNode chnTITNode = XMLNode.parseFile(RootPath + chnTITFile,"sensor") string className = chnTITNode .GetAttribute("class"); switch(hash(className)) { case ThermocoupleHash: ... case PTCResistorHash: ... ... } </code></pre> <p>Problem here: need to maintain the enum containing the hash-values. </p> <p>As I am writing this code I feel that the process of adding a new sensor can be rather tedious. Any ideas how to reduce the mess? Or is this already the least of evil one can possibly get?</p> <p>Thanks for reading! Arne</p> <p>EDIT1</p> <p>As suggested by Neil Butterworth I consider to extend each class by a function like</p> <pre><code> static Sensor* Thermocouple::Create(XMLNode &node) { return new Thermocouple(node); } </code></pre> <p>and create a hash table that relates the class name string to a function pointer to this static function:</p> <pre><code>typedef Sensor* (*CreateFunct)(XMLNode &node); class SensorFactory { public: SensorFactory() { classNameMap["Thermocouple"] = Thermocouple::Create; classNameMap["PTCResistor"] = PTCResistor::Create; }; Sensor* ChannelByName(string chnName) { string chnFile= rootNode.GetNode(chnName).GetAttribute("sensor"); XMLNode chnSensorNode = XMLNode.parseFile(RootPath + chnFile,"sensor") string className = chnSensorNode.GetAttribute("class"); map<string, CreateFunct>::iterator iterat = classNameMap.find(className); if(iterat != classNameMap.end()) { CreateFunct f = iterat->second; return f(chnTITNode); } }; private: map<string, CreateFunct> classNameMap; } </code></pre> <p>This allows to create the matching sensor object for a channel identified by name. The lookup is accomplished by hash-comparision and one only needs to maintain map initialization in the constructor of the factory class.</p>

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