diff --git a/src/label_map.cpp b/src/label_map.cpp
index ff11d13567..8bb91d3c32 100644
--- a/src/label_map.cpp
+++ b/src/label_map.cpp
@@ -172,7 +172,7 @@ void LabelMap::create_lmap2lmap(LabelMap *lmap2, int mode)
    return numeric type
 ------------------------------------------------------------------------- */
 
-int LabelMap::find_or_create(std::string mylabel,
+int LabelMap::find_or_create(const std::string &mylabel,
                              std::vector<std::string> &labels, int ntypes)
 {
   for (int i = 0; i < ntypes; i++)
@@ -190,8 +190,10 @@ int LabelMap::find_or_create(std::string mylabel,
   }
 
   // if label cannot be found or created, need more space reserved
-
   error->all(FLERR,"Topology type exceeds system topology type");
+
+  // never reaches here, just to prevent compiler warning
+  return -1;
 }
 
 /* ----------------------------------------------------------------------
@@ -199,7 +201,7 @@ int LabelMap::find_or_create(std::string mylabel,
    return -1 if type not yet defined
 ------------------------------------------------------------------------- */
 
-int LabelMap::find(std::string mylabel, int mode)
+int LabelMap::find(const std::string &mylabel, int mode)
 {
   if (mode == Atom::ATOM)
     return search(mylabel,typelabel,natomtypes);
@@ -224,8 +226,8 @@ int LabelMap::find(std::string mylabel, int mode)
    return -1 if type not yet defined
 ------------------------------------------------------------------------- */
 
-int LabelMap::search(std::string mylabel,
-                     std::vector<std::string> labels, int ntypes)
+int LabelMap::search(const std::string &mylabel,
+                     const std::vector<std::string> &labels, int ntypes)
 {
   for (int i = 0; i < ntypes; i++)
     if (labels[i] == mylabel) return i+1;
@@ -381,7 +383,7 @@ char *LabelMap::read_string(FILE *fp)
    byte) into the restart file
 ------------------------------------------------------------------------- */
 
-void LabelMap::write_string(std::string str, FILE *fp)
+void LabelMap::write_string(const std::string &str, FILE *fp)
 {
   const char *cstr = str.c_str();
   int n = strlen(cstr) + 1;
diff --git a/src/label_map.h b/src/label_map.h
index 2f852faa69..5724b1e070 100644
--- a/src/label_map.h
+++ b/src/label_map.h
@@ -43,12 +43,12 @@ class LabelMap : protected Pointers {
 
    void allocate_type_labels();
    void modify_lmap(int, char **);
-   void merge_lmap(class LabelMap *, int);                           // copy another lmap into this one
-   void create_lmap2lmap(class LabelMap *, int);                     // index mapping between two lmaps
-   int find_or_create(std::string, std::vector<std::string> &, int); // look up type or create new type
-   int find(std::string, int);                                       // find numeric type of type label
-   int search(std::string, std::vector<std::string>, int);           // look up type index
-   int is_complete(int);                                             // check if all types are assigned
+   void merge_lmap(class LabelMap *, int);                                   // copy another lmap into this one
+   void create_lmap2lmap(class LabelMap *, int);                             // index mapping between two lmaps
+   int find_or_create(const std::string &, std::vector<std::string> &, int); // look up type or create new type
+   int find(const std::string &, int);                                       // find numeric type of type label
+   int search(const std::string &, const std::vector<std::string> &, int);   // look up type index
+   int is_complete(int);                                                     // check if all types are assigned
 
    // input/output for atom class label map
 
@@ -58,7 +58,7 @@ class LabelMap : protected Pointers {
 
  private:
    char *read_string(FILE *);
-   void write_string(std::string, FILE *);
+   void write_string(const std::string &, FILE *);
    int read_int(FILE *);
 };