diff --git a/dlib/dnn/loss.h b/dlib/dnn/loss.h
index 7d4365ad6..ab2807b19 100644
--- a/dlib/dnn/loss.h
+++ b/dlib/dnn/loss.h
@@ -993,6 +993,36 @@ namespace dlib
         }
     }
 
+    std::ostream& operator<<(std::ostream& out, const std::vector<mmod_options::detector_window_details>& detector_windows)
+    {
+        // write detector windows grouped by label
+        // example output: aeroplane:74x30,131x30,70x45,54x70,198x30;bicycle:70x57,32x70,70x32,51x70,128x30,30x121;car:70x36,70x60,99x30,52x70,30x83,30x114,30x200
+
+        std::map<std::string, std::deque<mmod_options::detector_window_details>> detector_windows_by_label;
+        for (const auto& detector_window : detector_windows)
+            detector_windows_by_label[detector_window.label].push_back(detector_window);
+
+        size_t label_count = 0;
+        for (const auto& i : detector_windows_by_label)
+        {
+            const auto& label = i.first;
+            const auto& detector_windows = i.second;
+
+            if (label_count++ > 0)
+                out << ";";
+            out << label << ":";
+
+            for (size_t j = 0; j < detector_windows.size(); ++j)
+            {
+                out << detector_windows[j].width << "x" << detector_windows[j].height;
+                if (j + 1 < detector_windows.size())
+                    out << ",";
+            }
+        }
+
+        return out;
+    }
+
 // ----------------------------------------------------------------------------------------
 
     class loss_mmod_ 
@@ -1395,37 +1425,10 @@ namespace dlib
         {
             out << "loss_mmod\t (";
 
-            out << "detector_windows:(";
             auto& opts = item.options;
 
-            {
-                // write detector windows grouped by label
-                // example output: detector_windows:(aeroplane:74x30,131x30,70x45,54x70,198x30;bicycle:70x57,32x70,70x32,51x70,128x30,30x121;car:70x36,70x60,99x30,52x70,30x83,30x114,30x200)
-                typedef std::deque<const dlib::mmod_options::detector_window_details*> detector_windows;
-                std::map<std::string, detector_windows> detector_windows_by_label;
-                for (const auto& detector_window : opts.detector_windows)
-                    detector_windows_by_label[detector_window.label].push_back(&detector_window);
+            out << "detector_windows:(" << opts.detector_windows << ")";
 
-                size_t label_count = 0;
-                for (const auto& i : detector_windows_by_label)
-                {
-                    const auto& label = i.first;
-                    const auto& detector_windows = i.second;
-
-                    if (label_count++ > 0)
-                        out << ";";
-                    out << label << ":";
-
-                    for (size_t i = 0; i < detector_windows.size(); ++i)
-                    {
-                        out << detector_windows[i]->width << "x" << detector_windows[i]->height;
-                        if (i+1 < detector_windows.size())
-                            out << ",";
-                    }
-                }
-            }
-
-            out << ")";
             out << ", loss per FA:" << opts.loss_per_false_alarm;
             out << ", loss per miss:" << opts.loss_per_missed_target;
             out << ", truth match IOU thresh:" << opts.truth_match_iou_threshold;