Add helpers for logging mojom objects.

mojo/public/cpp/bindings/map.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_MAP_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_MAP_H_
 
+#include <iosfwd>
 #include <map>
 #include <type_traits>
 
 #include "mojo/public/cpp/bindings/lib/map_internal.h"
 #include "mojo/public/cpp/bindings/lib/template_util.h"
 
 namespace mojo {
 
 // A move-only map that can handle move-only values. Map has the following
 // characteristics:
@@ skipping 268 matching lines @@
       for (auto it = input.cbegin(); it != input.cend(); ++it) {
         result.insert(std::make_pair(
             TypeConverter<STLKey, MojoKey>::Convert(it.GetKey()),
             TypeConverter<STLValue, MojoValue>::Convert(it.GetValue())));
       }
     }
     return result;
   }
 };
 
+// Prints the contents of a map to an output stream for debugging purposes.
+template <typename Key, typename Value>
+std::ostream& operator<<(std::ostream& os, const mojo::Map<Key, Value>& map) {
+  if (map) {
+    os << "[";
+    bool first = true;
+    for (auto it = map.cbegin(); it != map.cend(); ++it) {
+      if (first)
+        first = false;
+      else
+        os << ", ";
+      os << "{" << it.GetKey() << ": " << it.GetValue() << "}";
+    }
+    os << "]";
+  } else {
+    os << "null";
+  }
+  return os;
+}
+
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_MAP_H_