Added ability to print a tensor using an iostream.

unsupported/Eigen/CXX11/Tensor

diff --git a/unsupported/Eigen/CXX11/Tensor b/unsupported/Eigen/CXX11/Tensor
index 5a6246a..79510fd 100644
--- a/unsupported/Eigen/CXX11/Tensor
+++ b/unsupported/Eigen/CXX11/Tensor
@@ -64,6 +64,8 @@
 #include "unsupported/Eigen/CXX11/src/Tensor/TensorFixedSize.h"
 #include "unsupported/Eigen/CXX11/src/Tensor/TensorMap.h"
 
+#include "unsupported/Eigen/CXX11/src/Tensor/TensorIO.h"
+
 #include "Eigen/src/Core/util/ReenableStupidWarnings.h"
 
 #endif // EIGEN_CXX11_TENSOR_MODULE

unsupported/Eigen/CXX11/src/Tensor/TensorIO.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorIO.h b/unsupported/Eigen/CXX11/src/Tensor/TensorIO.h
new file mode 100644
index 0000000..959b5db
--- /dev/null
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorIO.h
@@ -0,0 +1,44 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2014 Benoit Steiner <benoit.steiner.goog@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_CXX11_TENSOR_TENSOR_IO_H
+#define EIGEN_CXX11_TENSOR_TENSOR_IO_H
+
+namespace Eigen {
+
+template <typename T>
+std::ostream& operator << (std::ostream& os, const TensorBase<T, ReadOnlyAccessors>& expr) {
+  // Evaluate the expression if needed
+  TensorForcedEvalOp<const T> eval = expr.eval();
+  TensorEvaluator<const TensorForcedEvalOp<const T>, DefaultDevice> tensor(eval, DefaultDevice());
+  tensor.evalSubExprsIfNeeded(NULL);
+
+  typedef typename T::Scalar Scalar;
+  typedef typename T::Index Index;
+  typedef typename TensorEvaluator<const TensorForcedEvalOp<const T>, DefaultDevice>::Dimensions Dimensions;
+  const Index total_size = internal::array_prod(tensor.dimensions());
+
+  // Print the tensor as a 1d vector or a 2d matrix.
+  if (internal::array_size<Dimensions>::value == 1) {
+    Map<Array<Scalar, Dynamic, 1> > array(tensor.data(), total_size);
+    os << array;
+  } else {
+    const Index first_dim = tensor.dimensions()[0];
+    Map<Array<Scalar, Dynamic, Dynamic> > matrix(tensor.data(), first_dim, total_size/first_dim);
+    os << matrix;
+  }
+
+  // Cleanup.
+  tensor.cleanup();
+  return os;
+}
+
+} // end namespace Eigen
+
+#endif // EIGEN_CXX11_TENSOR_TENSOR_IO_H

unsupported/test/CMakeLists.txt

diff --git a/unsupported/test/CMakeLists.txt b/unsupported/test/CMakeLists.txt
index 48435eb..99593b5 100644
--- a/unsupported/test/CMakeLists.txt
+++ b/unsupported/test/CMakeLists.txt
@@ -124,4 +124,5 @@
   ei_add_test(cxx11_tensor_striding "-std=c++0x")
 #  ei_add_test(cxx11_tensor_device  "-std=c++0x")
   ei_add_test(cxx11_tensor_thread_pool "-std=c++0x")
+  ei_add_test(cxx11_tensor_io "-std=c++0x")
 endif()

unsupported/test/cxx11_tensor_io.cpp

diff --git a/unsupported/test/cxx11_tensor_io.cpp b/unsupported/test/cxx11_tensor_io.cpp
new file mode 100644
index 0000000..b73c024
--- /dev/null
+++ b/unsupported/test/cxx11_tensor_io.cpp
@@ -0,0 +1,70 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2014 Benoit Steiner <benoit.steiner.goog@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include "main.h"
+#include <sstream>
+#include <string>
+#include <Eigen/CXX11/Tensor>
+
+
+static void test_output_1d()
+{
+  Tensor<int, 1> tensor(5);
+  for (int i = 0; i < 5; ++i) {
+    tensor(i) = i;
+  }
+
+  std::stringstream os;
+  os << tensor;
+
+  std::string expected("0\n1\n2\n3\n4");
+  VERIFY_IS_EQUAL(std::string(os.str()), expected);
+}
+
+
+static void test_output_2d()
+{
+  Tensor<int, 2> tensor(5, 3);
+  for (int i = 0; i < 5; ++i) {
+    for (int j = 0; j < 3; ++j) {
+      tensor(i, j) = i*j;
+    }
+  }
+
+  std::stringstream os;
+  os << tensor;
+
+  std::string expected("0  0  0\n0  1  2\n0  2  4\n0  3  6\n0  4  8");
+  VERIFY_IS_EQUAL(std::string(os.str()), expected);
+}
+
+
+static void test_output_expr()
+{
+  Tensor<int, 1> tensor1(5);
+  Tensor<int, 1> tensor2(5);
+  for (int i = 0; i < 5; ++i) {
+    tensor1(i) = i;
+    tensor2(i) = 7;
+  }
+
+  std::stringstream os;
+  os << tensor1 + tensor2;
+
+  std::string expected(" 7\n 8\n 9\n10\n11");
+  VERIFY_IS_EQUAL(std::string(os.str()), expected);
+}
+
+
+void test_cxx11_tensor_io()
+{
+  CALL_SUBTEST(test_output_1d());
+  CALL_SUBTEST(test_output_2d());
+  CALL_SUBTEST(test_output_expr());
+}