C++ gives me an error: no match for call to

Question

I have a problem using static_cast. Here is my program:

#include <iostream>
using namespace std;

class Mtx { // base matrix
private:
    // refer to derived class
    Mtx& ReferToDerived() {
        return static_cast<Mtx&>(*this);
    }
    // entry() uses features of derived class
    virtual double& entry(int i, int j){
    return ReferToDerived() (i,j);    // error appears here
    }
protected:
    int dimn; // dimension of matrix
public:
    // define common functionality in base class that can
    // be called on derived classes
    double sum() { // sum all entries
        double d = 0;
        for (int i = 0; i < dimn; i++)
            for (int j = 0; j < dimn; j++) d += entry(i,j);
        return d;
    }
};

class FullMtx: public Mtx {
    double** mx;
public :
    FullMtx(int n) {
        dimn = n;
        mx = new double* [dimn] ;
        for (int i=0; i<dimn; i++) mx[i] = new double [dimn];
        for (int i=0; i<dimn; i++)
            for (int j=0; j<dimn; j++)
                mx[i][j] = 0; // initialization
    }
    double& operator() (int i, int j) { return mx[i] [j]; }
};

class SymmetricMtx : public Mtx {
    // store only lower triangular part to save memory
    double** mx ;
public :
    SymmetricMtx(int n) {
        dimn = n;
        mx = new double* [dimn];
        for (int i=0; i<dimn; i++) mx[i] = new double [i+1];
        for (int i=0; i<dimn; i++)
            for (int j=0; j <= i; j++)
                mx[i][j] = 0; // initialization
    }
    double& operator() (int i, int j) {
        if (i >= j ) return mx[i][j] ;
        else return mx[j][i]; // due to symmetry
    }
};

int main() 
{
    FullMtx A(1000);
    for (int i=0; i<1000; i++)
        for (int j=0; j<1000; j++)
        A(i,j)=1;
    cout << "sum of full matrix A = " << A.sum() << '\n';

    SymmetricMtx S(1000); // just assign lower triangular part
    for (int i=0; i<1000; i++)
        for (int j=0; j<1000; j++)
        S(i,j)=1;
    cout << "sum of symmetric matrix S = " << S.sum() << '\n';
}

When I run it, it says: no match for call to '(Mtx) (int&, int&)' And I don't understand what's wrong and how should I modify it? It should be written using virtual functions, but I don't know how can I write it correctly. Can someone help me? This program should count the sum of all the elements of the FullMatrix and SymmetricMatrix.

@Paula It is equivalent to `return *this;`. It's not clear what you're expecting it to accomplish. — molbdnilo, Apr 28 '16 at 16:05
You can just use `return *this;`. The `ReferToDerived()` function name implies something like it needs a template parameter. — πάντα ῥεῖ, Apr 28 '16 at 16:06
return *this doesn't work :( And I need the function without templates. — Caboom, Apr 28 '16 at 16:07
You should lookup for [CRTP](https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern) and consider how it could be used to solve your problem. — πάντα ῥεῖ, Apr 28 '16 at 16:10
@Paula _"And I need the function without templates."_ Why actually? — πάντα ῥεῖ, Apr 28 '16 at 16:11
because I am modifying the function that was with templates to one that uses virtual function. I need to see the difference between the times of execution in both — Caboom, Apr 28 '16 at 16:13
@πάντα ῥεῖ should I modify it somehow in a completely different way? without static_cast — Caboom, Apr 28 '16 at 16:39
@Paula _"should I modify it somehow in a completely different way?"_ You could use either dynamic (=> virtual functions) or static (=> CRTP) polymorphism. Depends on your use case what's the better choice. — πάντα ῥεῖ, Apr 28 '16 at 16:45
@πάντα ῥεῖ I would like to do it with dynamic polymorphism, bet I don't know how should I do it. — Caboom, Apr 28 '16 at 16:47
@Paula Introduce interfaces as abstract base classes (=> pure virtual function definitions). — πάντα ῥεῖ, Apr 28 '16 at 16:48
@πάνταῥεῖ so that ReferToDerived should become a pure virtual function? or do I have to declare something apart? I'm sorry, but I'm not too familiar with virtual functions. I am trying to get more familiar with it now... — Caboom, Apr 28 '16 at 17:04
You forgot to tell us what your program is supposed to do. We cannot magically divine that from code that, by your own admission, _does not do that thing_! — Lightness Races in Orbit, Apr 29 '16 at 12:20
@Lightness Races in Orbit sorry, I thought I did it. It's supposed to sum all elements of a matrix — Caboom, Apr 29 '16 at 12:36
@Lightness Races in Orbit I mean that the program is supposed to sum all the elements of a matrix FullMatrix and SymmetricMatrix and the program should be written using virtual functions. Do you understand what I mean? — Caboom, Apr 29 '16 at 12:58
@Lightness Races in Orbit, the question is how should I modify it and what should become virtual, so that so that it works well? — Caboom, Apr 29 '16 at 13:13
Sorry, I don't know how to make this any more clear. Instead of adding comments, edit your question to contain all the necessary detail. — Lightness Races in Orbit, Apr 29 '16 at 13:17
@Lightness Races in Orbit ups, sorry. I understood it only now and I edited it... — Caboom, Apr 29 '16 at 13:31

score 0 · Answer 1 · answered Apr 29 '16 at 12:17

remove virtual here

virtual double& entry(int i, int j){
    return (*this)() (i,j);    // error appears here
}

add virtual there

class Mtx {
...
    virtual double& operator() (int i, int j) = 0;
}

let the derived classes overload that operator and voala.

class FullMtx: public Mtx {
    ...
    virtual double& operator() (int i, int j) override { return mx[i] [j]; }
}

also this is just nonsense as already pointed out. you cast to same type, not the derived one. besides that you can't cast to derived type because in base you don't have the information about full inheritance - what if somebody derives from FullMtx without your knowledge ?

Mtx& ReferToDerived() {
    return static_cast<Mtx&>(*this);
}

I added and removed those virtual as you said. But in class Mtx { ... virtual double& operator() (int i, int j) = 0; } I get an errors that say: " 'virtual' outside class declaration " and " 'double& operator()(int, int)' must be a nonstatic member function ". — Caboom, Apr 29 '16 at 12:42

C++ gives me an error: no match for call to

1 Answers1