initial commit

third_party/eigen/doc/examples/class_FixedVectorBlock.cpp

@@ -0,0 +1,21 @@
+#include <Eigen/Core>
+#include <iostream>
+
+template <typename Derived>
+Eigen::VectorBlock<Derived, 2> firstTwo(Eigen::MatrixBase<Derived>& v) {
+  return Eigen::VectorBlock<Derived, 2>(v.derived(), 0);
+}
+
+template <typename Derived>
+const Eigen::VectorBlock<const Derived, 2> firstTwo(const Eigen::MatrixBase<Derived>& v) {
+  return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0);
+}
+
+int main(int, char**) {
+  Eigen::Matrix<int, 1, 6> v;
+  v << 1, 2, 3, 4, 5, 6;
+  std::cout << firstTwo(4 * v) << std::endl;  // calls the const version
+  firstTwo(v) *= 2;                           // calls the non-const version
+  std::cout << "Now the vector v is:" << std::endl << v << std::endl;
+  return 0;
+}