virtual function and "no legal conversion for this pointer"

I have two classes as follow.

class NeuroShield
{
public:

    NeuroShield();
    uint16_t begin();

    void setNcr(uint16_t value);
    uint16_t getNcr();
    void setComp(uint8_t value);
    uint8_t getComp();
    void setLastComp(uint8_t value);
    void setIndexComp(uint16_t value);
    uint16_t getDist();
    void setCat(uint16_t value);
    uint16_t getCat();
    void setAif(uint16_t value);
    uint16_t getAif();
    void setMinif(uint16_t value);
    uint16_t getMinif();
    void setMaxif(uint16_t value);
    uint16_t getMaxif();
    uint16_t getNid();
    void setGcr(uint16_t value);
    uint16_t getGcr();
    void resetChain();
    void setNsr(uint16_t value);
    uint16_t getNsr();
    uint16_t getNcount();
    void setPowerSave();
    void forget();
    void forget(uint16_t maxif);

    void countTotalNeurons();
    void clearNeurons();

    void setContext(uint8_t context);
    void setContext(uint8_t context, uint16_t minif, uint16_t maxif);
    void getContext(uint8_t* context, uint16_t* minif, uint16_t* maxif);
    void setRbfClassifier();
    void setKnnClassifier();

    uint16_t broadcast(uint8_t vector[], uint16_t length);
    uint16_t learn(uint8_t vector[], uint16_t length, uint16_t category);
    uint16_t classify(uint8_t vector[], uint16_t length);
    uint16_t classify(uint8_t vector[], uint16_t length, uint16_t* distance, uint16_t* category, uint16_t* nid);
    uint16_t classify(uint8_t vector[], uint16_t length, uint16_t k, uint16_t distance[], uint16_t category[], uint16_t nid[]);

    void readNeuron(uint16_t nid, uint16_t model[], uint16_t* ncr, uint16_t* aif, uint16_t* cat);
    void readNeuron(uint16_t nid, uint16_t nuerons[]);
    uint16_t readNeurons(uint16_t neurons[]);
    void readCompVector(uint16_t* data, uint16_t size);
    void writeNeurons(uint16_t neurons[], uint16_t ncount);
    void writeCompVector(uint16_t* data, uint16_t size);

    uint16_t testCommand(uint8_t read_write, uint8_t reg, uint16_t data);

    uint16_t fpgaVersion();
    void nm500Reset();
    void ledSelect(uint8_t data);

    uint16_t total_neurons;

private:
    uint16_t support_burst_read = 0;
};

Another class is Parallel_process from opencv.

class Parallel_process : public cv::ParallelLoopBody
{

private:
    Mat gray_img;
    Mat orig_img;
    int size;
    int row;
    NeuroShield hnn;
    vector<uint16_t> dists;
public:
    uint16_t nm_cat, nm_nid;
    Parallel_process(Mat inputImgage, Mat orgImg, int row_, NeuroShield &hnn_) : gray_img(inputImgage), row(row_), hnn(hnn_){}

    virtual void operator()(const Range& range) const
    {
        for (int col = range.start; col < range.end; col = col +2)
        {
            uint8_t vector[NEURON_SIZE];
            Mat roi_img = gray_img(Rect(col, row, size, size));
            Mat res;
            resize(roi_img, res, Size(16, 16), 0, 0, INTER_LINEAR);
            uint8_t* data = (uint8_t*)res.data;
            for (int j = 0; j < VECTOR_SIZE; j++)
                vector[j] = *data++;
            uint16_t nm_dist;
            hnn.classify(vector, VECTOR_SIZE, &nm_dist, &nm_cat, &nm_nid);
            dists.push_back(nm_dist);

        }
    }
};

In main function, paraller process is called as

cv::parallel_for_(cv::Range(0, 8), Parallel_process(inputImgage, orgImg, row_, hnn, dists_))

But I have two compile errors at the following two lines.

hnn.classify(vector, VECTOR_SIZE, &nm_dist, &nm_cat, &nm_nid);
dists.push_back(nm_dist);

The errors are

Error   C2663   'NeuroShield::classify': 3 overloads have no legal conversion for 'this' pointer    
Error   C2663   'std::vector<uint16_t,std::allocator<_Ty>>::push_back': 2 overloads have no legal conversion for 'this' pointer 

What could be wrong?

2 answers

  • answered 2018-11-08 06:38 Swordfish

    You cannot modify an instance within a const-qualified function called upon it.*)

    Remove the const-qualifier from Parallel_process::operator()().

    *) short of members declared mutable.

  • answered 2018-11-08 07:15 rafix07

    Parallel_process class inherits from cv::ParallelLoopBody, so you have to override

    virtual void operator()(const Range& range) const
                                                ^^^^^
    

    const qualifier implies that any data members cannot be modified inside this method.

    NeuroShield hnn;
    

    is modified by the call

    hnn.classify(vector, VECTOR_SIZE, &nm_dist, &nm_cat, &nm_nid);
    

    because classify method of NeuroShield is non-const. You can make classify method as const then compiler won't complain. Second problem is with vector<uint16_t> dists;. This vector can be modified by operator()() only if you add mutable specifier to it.

    mutable vector<uint16_t> dists;
    

    All above explains why your code doesn't compile. The main issue in your code is the way you are using cv::ParallelLoopBody.

    The correct way is:

    1. prepare containers in which results will be stored

    2. pass this containers by reference/pointer to object derived from cv::ParallelLoopBody

    3. now in operator()() const you can modify data pointed by references/pointers [pointers are not changed, but pointed data can be - it is the key to resolve your problem]

    So

    int size;
    int row;
    NeuroShield& hnn; // make reference 
    vector<uint16_t>& dists; // make reference
    

    ctor:

    Parallel_process(Mat inputImgage, Mat orgImg, int row_, NeuroShield &hnn_, vector<uint16_t>& vec) : 
    gray_img(inputImgage), 
    row(row_), 
    hnn(hnn_),
    dists(vec) {}
    

    now these lines

       hnn.classify(vector, VECTOR_SIZE, &nm_dist, &nm_cat, &nm_nid);
       dists.push_back(nm_dist);
    

    should work. Probably you should some synchronization method while accessing dists vector, because this code is running concurrently.