diff --git a/src/ccstruct/points.h b/src/ccstruct/points.h
index c8c46ad161..418031f227 100644
--- a/src/ccstruct/points.h
+++ b/src/ccstruct/points.h
@@ -269,11 +269,11 @@ class TESS_API FCOORD {
   bool normalise();
 
   /// test equality
-  bool operator==(const FCOORD &other) {
+  bool operator==(const FCOORD &other) const {
     return xcoord == other.xcoord && ycoord == other.ycoord;
   }
   /// test inequality
-  bool operator!=(const FCOORD &other) {
+  bool operator!=(const FCOORD &other) const {
     return xcoord != other.xcoord || ycoord != other.ycoord;
   }
   /// rotate 90 deg anti
diff --git a/src/ccutil/params.h b/src/ccutil/params.h
index ba8043b56d..f514d8709c 100644
--- a/src/ccutil/params.h
+++ b/src/ccutil/params.h
@@ -253,7 +253,7 @@ class StringParam : public Param {
   bool empty() const {
     return value_.empty();
   }
-  bool operator==(const std::string &other) {
+  bool operator==(const std::string &other) const {
     return value_ == other;
   }
   void operator=(const std::string &value) {
diff --git a/src/dict/dawg.h b/src/dict/dawg.h
index 05e7c5c59b..0869f68df4 100644
--- a/src/dict/dawg.h
+++ b/src/dict/dawg.h
@@ -361,7 +361,7 @@ struct DawgPosition {
         dawg_index(dawg_idx),
         punc_index(punc_idx),
         back_to_punc(backtopunc) {}
-  bool operator==(const DawgPosition &other) {
+  bool operator==(const DawgPosition &other) const {
     return dawg_index == other.dawg_index && dawg_ref == other.dawg_ref &&
            punc_index == other.punc_index && punc_ref == other.punc_ref &&
            back_to_punc == other.back_to_punc;
diff --git a/src/lstm/recodebeam.cpp b/src/lstm/recodebeam.cpp
index 0bf28dab1e..4b00166fbf 100644
--- a/src/lstm/recodebeam.cpp
+++ b/src/lstm/recodebeam.cpp
@@ -322,7 +322,7 @@ void RecodeBeamSearch::ExtractBestPathAsWords(const TBOX &line_box,
 }
 
 struct greater_than {
-  inline bool operator()(const RecodeNode *&node1, const RecodeNode *&node2) {
+  inline bool operator()(const RecodeNode *&node1, const RecodeNode *&node2) const {
     return (node1->score > node2->score);
   }
 };