Matrix Template Class passing parameter by const reference

Question

Trying to write a template class for a matrix object. Getting compiling errors:

c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(606): error C2558: class 'Matrix' : no copy constructor available or copy constructor is declared 'explicit' 1> with 1> [ 1> T=float 1> ] 1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(605) : while compiling class template member function 'void std::allocator<_Ty>::construct(_Ty *,const _Ty &)' 1>
with 1> [ 1> _Ty=Matrix 1> ] 1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(751) : see reference to function template instantiation 'void std::allocator<_Ty>::construct(_Ty *,const _Ty &)' being compiled 1> with 1> [ 1>
_Ty=Matrix 1> ] 1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\type_traits(743) : see reference to class template instantiation 'std::allocator<_Ty>' being compiled 1> with 1> [ 1>
_Ty=Matrix 1> ] 1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : see reference to class template instantiation 'std::is_empty<_Ty>' being compiled 1> with 1> [ 1>
_Ty=std::allocator> 1> ] 1> e:\projects\work\nns\fifteenstepstut\fifteensteps\fifteensteps\source.cpp(35) : see reference to class template instantiation 'std::vector<_Ty>' being compiled 1> with 1> [ 1>
_Ty=Matrix 1> ] 1> 1>Build FAILED. 1>

AND

ClCompile: 1> Source.cpp 1>e:\projects\work\nns\fifteenstepstut\fifteensteps\fifteensteps\matrix.h(80): error C2664: 'Matrix::Matrix(Matrix &)' : cannot convert parameter 1 from 'Matrix (__cdecl *)(void)' to 'Matrix &' 1>
with 1> [ 1> T=float 1> ] 1>
e:\projects\work\nns\fifteenstepstut\fifteensteps\fifteensteps\matrix.h(74) : while compiling class template member function 'Matrix Matrix::dot(const Matrix &)' 1> with 1> [ 1>
T=float 1> ] 1>
e:\projects\work\nns\fifteenstepstut\fifteensteps\fifteensteps\source.cpp(62) : see reference to function template instantiation 'Matrix Matrix::dot(const Matrix &)' being compiled 1> with 1> [ 1> T=float 1> ] 1>
e:\projects\work\nns\fifteenstepstut\fifteensteps\fifteensteps\source.cpp(38) : see reference to class template instantiation 'Matrix' being compiled 1> with 1> [ 1> T=float 1>
] 1>e:\projects\work\nns\fifteenstepstut\fifteensteps\fifteensteps\matrix.h(84): error C2664: 'Matrix::Matrix(Matrix &)' : cannot convert parameter 1 from 'Matrix (__cdecl *)(void)' to 'Matrix &' 1>
with 1> [ 1> T=float 1> ] 1> 1>Build FAILED.

"Matrix.h"

#include <vector>
#include <iostream>

template<typename T>
class Matrix {
private: 
    std::vector<T> data;
    int rows;
    int cols;

public:
    Matrix();
    Matrix(std::vector<T>, int rows, int cols);
    Matrix(Matrix<T>&); //change with this one
    //Matrix(const Matrix<T>&); //Will need to uncomment to test the 3rd error
    void print();
    Matrix<T> transpose();
    Matrix<T> dot(const Matrix<typename std::remove_reference<T>::type> &); //error 2
    //Matrix<T&> dot(const Matrix<T> &); //dumb idea?
    //Matrix<T> dot(const Matrix<T> &); //error 1
};

template <typename T>
Matrix<T>::Matrix() {
    data.clear();
    rows = 0;
    cols = 0;
}

template <typename T> 
Matrix<T>::Matrix(std::vector<T> elements, int numRows, int numCols) {
    rows = numRows;
    cols = numCols;

    data.clear();
    for(unsigned int i = 0; i < elements.size(); i++) {
        data.push_back(elements[i]);
    }
}

template <typename T> 
Matrix<T>::Matrix(Matrix<T>& matrix) {
    rows = matrix.rows;
    cols = matrix.cols;

    data.clear();
    for(unsigned int i = 0; i < matrix.data.size(); i++) {
        data.push_back(matrix.data[i]);
    }
}
/* To get compiler error, exchange with a above
template <typename T> 
Matrix<T>::Matrix(const Matrix<T>& matrix) {
    rows = matrix.rows;
    cols = matrix.cols;

    data.clear();
    for(unsigned int i = 0; i < matrix.data.size(); i++) {
        data.push_back(matrix.data[i]);
    }
}*/

template <typename T>
Matrix<T> Matrix<T>::dot(const Matrix<typename std::remove_reference<T>::type> & rhs) { //ERROR 2
//Matrix<&T> dot(const Matrix<T> &) {   
//Matrix<T> dot(const Matrix<T> &) { ERROR 1
    if(cols != rhs.rows) {
        std::cout << "Error! Can not resolve dot product on these matrices!" << std::endl;
        std::cout << "Requested: [" << rows << "x" << cols << "] <alt+7> [" << rhs.rows << "x" << rhs.cols << "]" << std::endl;
        Matrix<T> matrix();
        return matrix;
    }

    Matrix<T> matrix();
    return matrix;
}

template <typename T>
void Matrix<T>::print() {
    for(unsigned int i = 0; i < data.size(); i++) {
        std::cout << data[i] << ", ";
        if((i+1) % cols == 0)
            std::cout << std::endl;
    }
}

template <typename T>
Matrix<T> Matrix<T>::transpose() {
    std::vector<T> vec;
    for(unsigned int i = 0; i < data.size(); i++) {
        vec.push_back(data[(cols*(i%rows)+i/rows)]);
    }
    return Matrix<T>(vec, cols, rows);
}

I've read several different ideas on how to correct this, but not really sure what the issue is. A lot of places talk about passing just T as a const reference, but in this case, I'm passing the class as a const reference. Does not seem to like that.

I finally decided to see what happens if a implement a const reference copy constructor.

I then get this error:

unresolved external symbol "public: class Matrix __thiscall Matrix::dot(class Matrix const &)" (?dot@?$Matrix@M@@QAE?AV1@ABV1@@Z) referenced in function "void __cdecl testMatrixClass(void)" (?testMatrixClass@@YAXXZ)

How could I accomplish passing this class as a const reference if at all possible?

Thanks!

Test implementation

SOURCE.CPP

#include <iostream>
#include <vector>
#include "Matrix.h"
#include <string>
#include <fstream>
#include <sstream>


////TODO: Find alternatives to these...
//typedef std::vector<std::vector<float>> Matrix;
//typedef std::vector<float> Vector;
//using LMath::operator+;
//using LMath::operator==;



//void testMatrix(); //testing function.
//Matrix loadData(std::string); //Not implemented yet
//bool saveData(Matrix, std::string); //Not implemented yet


void testMatrixClass();

int main() {

    //testMatrix();
    testMatrixClass();

    return 0;
}


void testMatrixClass() {

    std::vector<Matrix<float>> testResults;
    std::vector<std::string> testInfo;

    Matrix<float> temp;
    testResults.push_back(temp);
    testInfo.push_back("Default Constructor");

    std::vector<float> tempVec;
    for(int i = 0; i < 9; i++) {
        tempVec.push_back((float)(i%3));
    }

    Matrix<float> temp2(tempVec, 3, 3);
    testResults.push_back(temp2);
    testInfo.push_back("Vector constructor");

    testResults.push_back(temp2.transpose());
    testInfo.push_back("Vector transpose");

    tempVec.push_back(10.0);
    Matrix<float> temp3(tempVec, 5, 2);
    testResults.push_back(temp3);
    testInfo.push_back("Vector constructor");

    testResults.push_back(temp3.transpose());
    testInfo.push_back("Vector transpose");

    testResults.push_back(temp2.dot(temp2));
    testInfo.push_back("Dot product");

    testResults.push_back(temp2.dot(temp3));
    testInfo.push_back("Error Dot Product");

    for(unsigned int i = 0; i < testResults.size(); i++) {
        std::cout << "Test: " << testInfo[i] << ": " << std::endl;;
        testResults[i].print();
        std::cout << std::endl;
    }

}

Solution:

#include <iostream>
#include <vector>
template<typename T>
class Matrix {
private: 
    std::vector<T> data;
    int rows;
    int cols;

public:
    Matrix();
    Matrix(std::vector<T>, int rows, int cols);
    //Matrix(Matrix<T>&);
    Matrix(const Matrix<T>&);
    void print();
    Matrix<T> transpose();
    Matrix<T> dot(const Matrix<typename std::remove_reference<T>::type> &);
};

template <typename T>
Matrix<T>::Matrix() {
    data.clear();
    rows = 0;
    cols = 0;
}

template <typename T> 
Matrix<T>::Matrix(std::vector<T> elements, int numRows, int numCols) {
    rows = numRows;
    cols = numCols;

    data.clear();
    for(unsigned int i = 0; i < elements.size(); i++) {
        data.push_back(elements[i]);
    }
}

template <typename T> 
Matrix<T>::Matrix(const Matrix<T>& matrix) {
    rows = matrix.rows;
    cols = matrix.cols;

    data.clear();
    for(unsigned int i = 0; i < matrix.data.size(); i++) {
        data.push_back(matrix.data[i]);
    }
}


template <typename T>
void Matrix<T>::print() {
    for(unsigned int i = 0; i < data.size(); i++) {
        std::cout << data[i] << ", ";
        if((i+1) % cols == 0)
            std::cout << std::endl;
    }
}

template <typename T>
Matrix<T> Matrix<T>::transpose() {
    std::vector<T> vec;
    for(unsigned int i = 0; i < data.size(); i++) {
        vec.push_back(data[(cols*(i%rows)+i/rows)]);
    }
    return Matrix<T>(vec, cols, rows);
}

template <typename T>
Matrix<T> Matrix<T>::dot(const Matrix<typename std::remove_reference<T>::type> & rhs) {
    if(cols != rhs.rows) {
        std::cout << "Error! Can not resolve dot product on these matrices!" << std::endl;
        std::cout << "Requested: [" << rows << "x" << cols << "] <alt+7> [" << rhs.rows << "x" << rhs.cols << "]" << std::endl;
        Matrix<T> matrix;
        return matrix;
    }

    Matrix<T> matrix;
    return matrix;
}

The shown code fails to meet the requirements of a minimal reproducible example, as explained in stackoverflow.com's help center, and, as such, is likely to be completely unanswerable. — Sam Varshavchik
I have no compiler accessible right now, but what sticks out is that you return Matrix<T> in several places; that requires a copy method. Try returning a const Matrix<T> — Aganju
@Aganju Well I have implemented the copy constructor, however I tried this suggestion. No result was different for a few of the different ideas I've been trying. — Chemistpp
I'm not seeing a need for a copy constructor at all. The only complex Matrix member is a std::vector, and std::vector is Rule of Five complaint. Matrix should be able to observe the Rule of Zero. — user4581301

Sam Varshavchik Sam Varshavchik · Accepted Answer · 2018-10-27T00:37:26

There are two problems with the shown code.

1) The copy constructor is wrong:

Matrix(Matrix<T>&);

Copy constructors must take a const reference as a parameter:

Matrix(const Matrix<T>&);

The actual declaration, in the header file, needs to be changed as well.

2) The second problem is The Most Vexing Parse:

Matrix<T> matrix();
return matrix;

This should simply be changed to:

Matrix<T> matrix;
return matrix;

Or simply to:

return Matrix<T>();

This occurs in two places in the shown code.

Once the above two problems are fixed, the shown code compiles for me, with gcc 8.

Matrix Template Class passing parameter by const reference

1 Answers