class_FixedVectorBlock.cpp
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	#include <Eigen/Core>
	#include <iostream>
	using namespace Eigen;
	using namespace std;

	template<typename Derived>
	Eigen::VectorBlock<Derived, 2>
	firstTwo(MatrixBase<Derived>& v)
	{
	return Eigen::VectorBlock<Derived, 2>(v.derived(), 0);
	}

	template<typename Derived>
	const Eigen::VectorBlock<const Derived, 2>
	firstTwo(const MatrixBase<Derived>& v)
	{
	return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0);
	}

	int main(int, char**)
	{
	Matrix<int,1,6> v; v << 1,2,3,4,5,6;
	cout << firstTwo(4*v) << endl; // calls the const version
	firstTwo(v) *= 2; // calls the non-const version
	cout << "Now the vector v is:" << endl << v << endl;
	return 0;
	}