C++ generated code is missing toXML const method

[danny-smit]

The IVEF sdk generates classes with a toXML method, which is declared as follows:

const QString& toXML(bool outputNamespace = true);

However there is no 'const' variant of these methods, which prevent implementors of this method to properly design their code with the use of const methods in the application implementing this code.

Although this method has the nice feature of caching previously generated XML, in interface processes, this is often only done once for each received piece of data. Therefore adding a method with the signature as follows would be very helpfull:

QString toXML(bool outputNamespace = true) const;