diff --git a/gtsam/inference/inferenceExceptions.cpp b/gtsam/inference/inferenceExceptions.cpp
new file mode 100644
index 000000000..8c2a95337
--- /dev/null
+++ b/gtsam/inference/inferenceExceptions.cpp
@@ -0,0 +1,60 @@
+/* ----------------------------------------------------------------------------
+
+ * GTSAM Copyright 2010, Georgia Tech Research Corporation,
+ * Atlanta, Georgia 30332-0415
+ * All Rights Reserved
+ * Authors: Frank Dellaert, et al. (see THANKS for the full author list)
+
+ * See LICENSE for the license information
+
+ * -------------------------------------------------------------------------- */
+
+/**
+ * @file    inferenceExceptions.cpp
+ * @brief   Exceptions that may be thrown by inference algorithms
+ * @author  Richard Roberts, Varun Agrawal
+ * @date    Apr 25, 2013
+ */
+
+#include <gtsam/inference/inferenceExceptions.h>
+
+#include <sstream>
+
+namespace gtsam {
+
+InconsistentEliminationRequested::InconsistentEliminationRequested(
+    const KeySet& keys, const KeyFormatter& key_formatter)
+    : keys_(keys.begin(), keys.end()), keyFormatter(key_formatter) {}
+
+const char* InconsistentEliminationRequested::what() const noexcept {
+  // Format keys for printing
+  std::stringstream sstr;
+  size_t nrKeysToDisplay = std::min(size_t(4), keys_.size());
+  for (size_t i = 0; i < nrKeysToDisplay; i++) {
+    sstr << keyFormatter(keys_.at(i));
+    if (i < nrKeysToDisplay - 1) {
+      sstr << ", ";
+    }
+  }
+  if (keys_.size() > nrKeysToDisplay) {
+    sstr << ", ... (total " << keys_.size() << " keys)";
+  }
+  sstr << ".";
+  std::string keys = sstr.str();
+
+  std::string msg =
+      "An inference algorithm was called with inconsistent "
+      "arguments.  "
+      "The\n"
+      "factor graph, ordering, or variable index were "
+      "inconsistent with "
+      "each\n"
+      "other, or a full elimination routine was called with "
+      "an ordering "
+      "that\n"
+      "does not include all of the variables.\n";
+  msg += ("Leftover keys after elimination: " + keys);
+  // `new` to allocate memory on heap instead of stack
+  return (new std::string(msg))->c_str();
+}
+}  // namespace gtsam
diff --git a/gtsam/inference/inferenceExceptions.h b/gtsam/inference/inferenceExceptions.h
index 339accad6..eb5d79e6f 100644
--- a/gtsam/inference/inferenceExceptions.h
+++ b/gtsam/inference/inferenceExceptions.h
@@ -18,9 +18,9 @@
 #pragma once
 
 #include <gtsam/global_includes.h>
+#include <gtsam/inference/Key.h>
 
 #include <exception>
-#include <sstream>
 
 namespace gtsam {
 
@@ -37,40 +37,10 @@ class InconsistentEliminationRequested : public std::exception {
 
   InconsistentEliminationRequested(
       const KeySet& keys,
-      const KeyFormatter& key_formatter = DefaultKeyFormatter)
-      : keys_(keys.begin(), keys.end()), keyFormatter(key_formatter) {}
+      const KeyFormatter& key_formatter = DefaultKeyFormatter);
 
   ~InconsistentEliminationRequested() noexcept override {}
-  const char* what() const noexcept override {
-    // Format keys for printing
-    std::stringstream sstr;
-    size_t nrKeysToDisplay = std::min(size_t(4), keys_.size());
-    for (size_t i = 0; i < nrKeysToDisplay; i++) {
-      sstr << keyFormatter(keys_.at(i));
-      if (i < nrKeysToDisplay - 1) {
-        sstr << ", ";
-      }
-    }
-    if (keys_.size() > nrKeysToDisplay) {
-      sstr << ", ... (total " << keys_.size() << " keys)";
-    }
-    sstr << ".";
-    std::string keys = sstr.str();
 
-    std::string msg =
-        "An inference algorithm was called with inconsistent "
-        "arguments.  "
-        "The\n"
-        "factor graph, ordering, or variable index were "
-        "inconsistent with "
-        "each\n"
-        "other, or a full elimination routine was called with "
-        "an ordering "
-        "that\n"
-        "does not include all of the variables.\n";
-    msg += ("Leftover keys after elimination: " + keys);
-    // `new` to allocate memory on heap instead of stack
-    return (new std::string(msg))->c_str();
-  }
+  const char* what() const noexcept override;
 };
 }  // namespace gtsam