Undefined reference to `MySensitiveDetector::MySensitiveDetector(G4String)'

[weller]

I can recommend the references for introductory material on c++ in this post:

Beginner at geant4

Hi, A good first step is to copy one of the basic examples and just play with it. Change an input file according to what the README says you can do, run it and see how the output changes. When you understand (roughly) how it works, modify the code to do something different that is of interest to you, recompile it and run it. For example, change the geometry and/or materials and see how the outputs change. Modify the readout mechanism, be it in stepping action or a sensitive detector to output a…

seems you are missing a

public:
    MyDetectorConstruction(G4String);

entry (plus maybe implementation).