diff --git a/include/ceres/iteration_callback.h b/include/ceres/iteration_callback.h
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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2015 Google Inc. All rights reserved.
+// http://ceres-solver.org/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+// * Neither the name of Google Inc. nor the names of its contributors may be
+//   used to endorse or promote products derived from this software without
+//   specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: sameeragarwal@google.com (Sameer Agarwal)
+//
+// When an iteration callback is specified, Ceres calls the callback
+// after each minimizer step (if the minimizer has not converged) and
+// passes it an IterationSummary object, defined below.
+
+#ifndef CERES_PUBLIC_ITERATION_CALLBACK_H_
+#define CERES_PUBLIC_ITERATION_CALLBACK_H_
+
+#include "ceres/types.h"
+#include "ceres/internal/disable_warnings.h"
+
+namespace ceres {
+
+// This struct describes the state of the optimizer after each
+// iteration of the minimization.
+struct CERES_EXPORT IterationSummary {
+  IterationSummary()
+      : iteration(0),
+        step_is_valid(false),
+        step_is_nonmonotonic(false),
+        step_is_successful(false),
+        cost(0.0),
+        cost_change(0.0),
+        gradient_max_norm(0.0),
+        gradient_norm(0.0),
+        step_norm(0.0),
+        eta(0.0),
+        step_size(0.0),
+        line_search_function_evaluations(0),
+        line_search_gradient_evaluations(0),
+        line_search_iterations(0),
+        linear_solver_iterations(0),
+        iteration_time_in_seconds(0.0),
+        step_solver_time_in_seconds(0.0),
+        cumulative_time_in_seconds(0.0) {}
+
+  // Current iteration number.
+  int iteration;
+
+  // Step was numerically valid, i.e., all values are finite and the
+  // step reduces the value of the linearized model.
+  //
+  // Note: step_is_valid is always true when iteration = 0.
+  bool step_is_valid;
+
+  // Step did not reduce the value of the objective function
+  // sufficiently, but it was accepted because of the relaxed
+  // acceptance criterion used by the non-monotonic trust region
+  // algorithm.
+  //
+  // Note: step_is_nonmonotonic is always false when iteration = 0;
+  bool step_is_nonmonotonic;
+
+  // Whether or not the minimizer accepted this step or not. If the
+  // ordinary trust region algorithm is used, this means that the
+  // relative reduction in the objective function value was greater
+  // than Solver::Options::min_relative_decrease. However, if the
+  // non-monotonic trust region algorithm is used
+  // (Solver::Options:use_nonmonotonic_steps = true), then even if the
+  // relative decrease is not sufficient, the algorithm may accept the
+  // step and the step is declared successful.
+  //
+  // Note: step_is_successful is always true when iteration = 0.
+  bool step_is_successful;
+
+  // Value of the objective function.
+  double cost;
+
+  // Change in the value of the objective function in this
+  // iteration. This can be positive or negative.
+  double cost_change;
+
+  // Infinity norm of the gradient vector.
+  double gradient_max_norm;
+
+  // 2-norm of the gradient vector.
+  double gradient_norm;
+
+  // 2-norm of the size of the step computed by the optimization
+  // algorithm.
+  double step_norm;
+
+  // For trust region algorithms, the ratio of the actual change in
+  // cost and the change in the cost of the linearized approximation.
+  double relative_decrease;
+
+  // Size of the trust region at the end of the current iteration. For
+  // the Levenberg-Marquardt algorithm, the regularization parameter
+  // mu = 1.0 / trust_region_radius.
+  double trust_region_radius;
+
+  // For the inexact step Levenberg-Marquardt algorithm, this is the
+  // relative accuracy with which the Newton(LM) step is solved. This
+  // number affects only the iterative solvers capable of solving
+  // linear systems inexactly. Factorization-based exact solvers
+  // ignore it.
+  double eta;
+
+  // Step sized computed by the line search algorithm.
+  double step_size;
+
+  // Number of function value evaluations used by the line search algorithm.
+  int line_search_function_evaluations;
+
+  // Number of function gradient evaluations used by the line search algorithm.
+  int line_search_gradient_evaluations;
+
+  // Number of iterations taken by the line search algorithm.
+  int line_search_iterations;
+
+  // Number of iterations taken by the linear solver to solve for the
+  // Newton step.
+  int linear_solver_iterations;
+
+  // All times reported below are wall times.
+
+  // Time (in seconds) spent inside the minimizer loop in the current
+  // iteration.
+  double iteration_time_in_seconds;
+
+  // Time (in seconds) spent inside the trust region step solver.
+  double step_solver_time_in_seconds;
+
+  // Time (in seconds) since the user called Solve().
+  double cumulative_time_in_seconds;
+};
+
+// Interface for specifying callbacks that are executed at the end of
+// each iteration of the Minimizer. The solver uses the return value
+// of operator() to decide whether to continue solving or to
+// terminate. The user can return three values.
+//
+// SOLVER_ABORT indicates that the callback detected an abnormal
+// situation. The solver returns without updating the parameter blocks
+// (unless Solver::Options::update_state_every_iteration is set
+// true). Solver returns with Solver::Summary::termination_type set to
+// USER_ABORT.
+//
+// SOLVER_TERMINATE_SUCCESSFULLY indicates that there is no need to
+// optimize anymore (some user specified termination criterion has
+// been met). Solver returns with Solver::Summary::termination_type
+// set to USER_SUCCESS.
+//
+// SOLVER_CONTINUE indicates that the solver should continue
+// optimizing.
+//
+// For example, the following Callback is used internally by Ceres to
+// log the progress of the optimization.
+//
+// Callback for logging the state of the minimizer to STDERR or STDOUT
+// depending on the user's preferences and logging level.
+//
+//   class LoggingCallback : public IterationCallback {
+//    public:
+//     explicit LoggingCallback(bool log_to_stdout)
+//         : log_to_stdout_(log_to_stdout) {}
+//
+//     ~LoggingCallback() {}
+//
+//     CallbackReturnType operator()(const IterationSummary& summary) {
+//       const char* kReportRowFormat =
+//           "% 4d: f:% 8e d:% 3.2e g:% 3.2e h:% 3.2e "
+//           "rho:% 3.2e mu:% 3.2e eta:% 3.2e li:% 3d";
+//       string output = StringPrintf(kReportRowFormat,
+//                                    summary.iteration,
+//                                    summary.cost,
+//                                    summary.cost_change,
+//                                    summary.gradient_max_norm,
+//                                    summary.step_norm,
+//                                    summary.relative_decrease,
+//                                    summary.trust_region_radius,
+//                                    summary.eta,
+//                                    summary.linear_solver_iterations);
+//       if (log_to_stdout_) {
+//         cout << output << endl;
+//       } else {
+//         VLOG(1) << output;
+//       }
+//       return SOLVER_CONTINUE;
+//     }
+//
+//    private:
+//     const bool log_to_stdout_;
+//   };
+//
+class CERES_EXPORT IterationCallback {
+ public:
+  virtual ~IterationCallback() {}
+  virtual CallbackReturnType operator()(const IterationSummary& summary) = 0;
+};
+
+}  // namespace ceres
+
+#include "ceres/internal/reenable_warnings.h"
+
+#endif  // CERES_PUBLIC_ITERATION_CALLBACK_H_