cpp-mathmatic-vector/include/math/vector/forward.h

#pragma once

#include <cstdlib>

namespace math::vector {
template <typename T, std::size_t N> class vector;

struct {
} fill;
using fill_t = decltype(fill);
}

namespace math::matrix {
template <typename T, std::size_t R, std::size_t C> class matrix;
}