Question

I have a functor base class and a functor derived class that looks like this:

class ReadSensor
{
public:
    ReadSensor();
    virtual ~ReadSensor(void){}
    virtual int operator()(void) = 0;
};

class ReadSensorDummy : public ReadSensor
{
public:
    ReadSensorDummy() : x(0) {}
    ReadSensorDummy(int x): x(x) {}
    ~ReadSensorDummy(void) {}
    int operator() (void) { return x;}
private:
    int x;
};

I am creating it as so:

ReadSensor *rs =  new ReadSensorDummy(5);

It compiles, but I get the following link error:

Error   2   error LNK2019: unresolved external symbol "public: __cdecl ReadSensor::ReadSensor(void)" (??0ReadSensor@@QEAA@XZ) referenced in function "public: __cdecl ReadSensorDummy::ReadSensorDummy(int)" (??0ReadSensorDummy@@QEAA@H@Z) W:\SafetySystemTest.obj

What is wrong? A dynamic_cast shouldn’t be needed, but I did try it and it didn’t help.

Answer 1

You have declared the intention to define a constructor for ReadSensor, but the compiler did not find one defined in any of your source files. You can try defining an empty one, or removing the declaration.