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#include <vector>
 
template <typename T>
class matrix
{
    std::vector< std::vector<T> > data;
    size_t N, M;    
public:
        T& at(size_t x, size_t y);
        matrix<T> operator+(const matrix<T>& m2) const;
};
 
template<class T>
T& matrix<T>::at(size_t x, size_t y)
{ return data[x][y]; }
 
template<class T>
matrix<T> matrix<T>::operator+(const matrix<T>& m2) const
{    
    matrix<T> res;
    for(unsigned int i=0; i<N; ++i)
        for(unsigned int j=0; j<M; ++j)
            res.at(i,j) = this->at(i, j) + m2.at(i, j);    
    return res;
}
 
int main()
{
    matrix<int> a, b;
    a = a + b;
    return 0;
}

prog.cpp: In member function 'matrix<T> matrix<T>::operator+(const matrix<T>&) const [with T = int]':
prog.cpp:30:13:   instantiated from here
prog.cpp:23:13: error: passing 'const matrix<int>' as 'this' argument of 'T& matrix<T>::at(size_t, size_t) [with T = int, size_t = unsigned int]' discards qualifiers
prog.cpp:30:13:   instantiated from here
prog.cpp:23:13: error: passing 'const matrix<int>' as 'this' argument of 'T& matrix<T>::at(size_t, size_t) [with T = int, size_t = unsigned int]' discards qualifiers

Standard output is empty