C++ matrix template class slice - c++

Question

In order to learn about C++ templates I am writing a simple Matrix class. So far it has been working well, but I want to add the ability to slice the Matrix to extract a sub-matrix. I am struggling to figure out how to define the size of the return matrix. I have tried the following:
#include <cstdint>
#include <array>
#include <initializer_list>
template<typename T, std::size_t M, std::size_t N>
class Matrix
{
public:
Matrix(void): m_data{0} {}
Matrix(const std::initializer_list<std::initializer_list<T>> m)
{
for(auto i = m.begin(); i != m.end(); i++)
{
for(auto j = i->begin(); j != i->end(); j++)
{
(*this)(i - m.begin(), j - i->begin()) = *j;
}
}
}
T& operator()(const std::size_t i, const std::size_t j)
{
return m_data.at(i + j * N);
}
const T& operator()(const std::size_t i, const std::size_t j) const
{
return m_data.at(i + j * N);
}
template<std::size_t X, std::size_t Y>
Matrix<T,X,Y> slice(const std::size_t iStart, const std::size_t iEnd, const std::size_t jStart, const std::size_t jEnd)
{
Matrix<T,iEnd-iStart+1,jEnd-jStart+1> result;
for(std::size_t i = iStart; i <= iEnd; i++)
{
for(std::size_t j = jStart; j <= jEnd; j++)
{
result(i - iStart, j - jStart) = (*this)(i,j);
}
}
return result;
}
};
int main(void)
{
Matrix<double,3,3> m1 = {{1,2,3},{4,5,6},{7,8,9}};
Matrix<double,2,2> m2 = m1.slice(0,1,0,1);
return 0;
}
But I just get an error saying that 'iEnd' is not a constant expression. What would be the correct way to go about this?

Answer 1

You cannot use function parameters to instantiate templates. You need to pass in the sizes as template arguments to slice:
template<std::size_t iStart, std::size_t iEnd, std::size_t jStart, std::size_t jEnd,
std::size_t I = iEnd-iStart+1, std::size_t J = jEnd-jStart+1>
Matrix<T,I,J> slice()
{
Matrix<T,I,J> result;
///...
return result;
}