diff --git a/src/solvers/flattening/boolbv.cpp b/src/solvers/flattening/boolbv.cpp
index 6d597ec1c5e..f6438834896 100644
--- a/src/solvers/flattening/boolbv.cpp
+++ b/src/solvers/flattening/boolbv.cpp
@@ -660,17 +660,16 @@ void boolbvt::print_assignment(std::ostream &out) const
     out << pair.first << "=" << pair.second.get_value(prop) << '\n';
 }
 
-void boolbvt::build_offset_map(const struct_typet &src, offset_mapt &dest)
+boolbvt::offset_mapt boolbvt::build_offset_map(const struct_typet &src)
 {
-  const struct_typet::componentst &components=
-    src.components();
-
-  dest.resize(components.size());
-  std::size_t offset=0;
-  for(std::size_t i=0; i<components.size(); i++)
+  const struct_typet::componentst &components = src.components();
+  offset_mapt dest;
+  dest.reserve(components.size());
+  std::size_t offset = 0;
+  for(const auto &comp : components)
   {
-    std::size_t comp_width=boolbv_width(components[i].type());
-    dest[i]=offset;
-    offset+=comp_width;
+    dest.push_back(offset);
+    offset += boolbv_width(comp.type());
   }
+  return dest;
 }
diff --git a/src/solvers/flattening/boolbv.h b/src/solvers/flattening/boolbv.h
index f6e85baf83d..17c487594aa 100644
--- a/src/solvers/flattening/boolbv.h
+++ b/src/solvers/flattening/boolbv.h
@@ -250,7 +250,7 @@ class boolbvt:public arrayst
   void post_process_quantifiers();
 
   typedef std::vector<std::size_t> offset_mapt;
-  void build_offset_map(const struct_typet &src, offset_mapt &dest);
+  offset_mapt build_offset_map(const struct_typet &src);
 
   // strings
   numbering<irep_idt> string_numbering;
diff --git a/src/solvers/flattening/boolbv_typecast.cpp b/src/solvers/flattening/boolbv_typecast.cpp
index 838c8df84ac..724b59b6a78 100644
--- a/src/solvers/flattening/boolbv_typecast.cpp
+++ b/src/solvers/flattening/boolbv_typecast.cpp
@@ -530,10 +530,8 @@ bool boolbvt::type_conversion(
           op_struct.components();
 
         // build offset maps
-        offset_mapt op_offsets, dest_offsets;
-
-        build_offset_map(op_struct, op_offsets);
-        build_offset_map(dest_struct, dest_offsets);
+        const offset_mapt op_offsets = build_offset_map(op_struct);
+        const offset_mapt dest_offsets = build_offset_map(dest_struct);
 
         // build name map
         typedef std::map<irep_idt, std::size_t> op_mapt;