c++11 - C++ Error Code C2784 could not deduce template argument for -

i new object oriented programming , c++. have been working on matrix class , squarematrix class , have been running problems can't seem figure out. error code have been getting is:

c2784: 'matrix<t,m,k> operator *(matrix<t,m,n> &,matrix<t,n,k> &)': not  deduce template argument 'matrix<t,m,n> &'  'std::vector<std::vector<double,std::allocator<_ty>>,std::allocator<std::vector<_ty,std::allocator<_ty>>>>' 

i unsure why, because have had other parts of code work. error reported in line "product.elements = product.elements * elements;"

//source.cpp #include"squarematrix.h" #include<iostream> #include<vector> using namespace std; int main() {     vector<double> = { 1, 2,4,5,6};     squarematrix<double,2> n;     n.assign(a);     cout << n << n.pow(2)<< endl;     return(0); }  //matrix.h #ifndef _matrix_ #define _matrix_ #include <iostream> #include <vector> #include <math.h> using namespace std; template<class t, int m, int n> class matrix { public:     vector<vector<t>> elements;     int nrow;     int ncol;     matrix();     matrix(matrix<t, m, n>&);  }; template<class t, int m, int n> matrix<t, m, n>::matrix() {     vector<t>temp(n, 0);     elements.assign(m, temp);     nrow = m;  //m=0     ncol = n;  //n=0 } template<class t, int m, int n> matrix<t, m, n>::matrix(matrix<t, m, n>& a) {     elements = a.elements;     nrow = m;     ncol = n; } template<class t, int m, int n, int k> matrix<t, m, k> operator*(const matrix<t, m, n>& a, const matrix<t, n, k>& b) { matrix<t, m, k> product; (int = 0; < m; i++) {     (int j = 0; j < k; j++) {         (int h = 0; h < n; h++)             product.elements[i][j] += a.elements[i][h] * b.elements[h][j];     } } return product; }  template<class t, int m, int n> ostream& operator<< (ostream& o, const matrix<t, m, n>& input) {     (int = 0; < m; ++i) {     (int j = 0; j < n; ++j)     o << input.elements[i][j] << " ";     o << endl;     }     return o; } #endif _matrix_  //squarematrix.h #ifndef _squarematrix_ #define _squarematrix_  #include "matrix.h" #include <iostream> #include <vector> using namespace std;  template<class t, int n> class squarematrix : public matrix<t, n, n> { public:     squarematrix();     squarematrix(squarematrix<t, n>&);      squarematrix<t, n> pow(int); //calculate a^k };  template<class t, int n> squarematrix<t, n>::squarematrix(){     vector<t>temp(n, 0);     elements.assign(n, temp);     nrow = n;  //n=0     ncol = n;  //n=0 } template<class t, int n> squarematrix<t, n>::squarematrix(squarematrix<t, n>& a){     elements = a.elements;     nrow = n;     ncol = n; } template<class t, int n> squarematrix<t, n> squarematrix<t, n>::pow(int k){     squarematrix<t, n> product;     product.elements = elements;     (int power = 2; power <= k; power++) {     product.elements = product.elements * elements;       }     return product; } #endif _squarematrix_ 

you don't need nrow , ncol - they're template parameters , known @ compile time.

but that's not problem - you're multiplying std::vector should multiplying squarematrix:

template<class t, int n> squarematrix<t, n> squarematrix<t, n>::pow(int k){     squarematrix<t, n> product = unit_matrix<t, n>();     (int power = 1; power <= k; power++) {         product = product * *this;     }     return product; } 

where i've used fictitious function creates unit matrix.
writing function left exercise.