diff --git a/src/goto-diff/change_impact.cpp b/src/goto-diff/change_impact.cpp
index 67bcc2535b0..32ce14e28fc 100644
--- a/src/goto-diff/change_impact.cpp
+++ b/src/goto-diff/change_impact.cpp
@@ -315,8 +315,7 @@ change_impactt::change_impactt(
 
 void change_impactt::change_impact(const irep_idt &function)
 {
-  unified_difft::goto_program_difft diff;
-  unified_diff.get_diff(function, diff);
+  unified_difft::goto_program_difft diff = unified_diff.get_diff(function);
 
   if(diff.empty())
     return;
diff --git a/src/goto-diff/unified_diff.cpp b/src/goto-diff/unified_diff.cpp
index 3974c8acef1..e5468a2b1f2 100644
--- a/src/goto-diff/unified_diff.cpp
+++ b/src/goto-diff/unified_diff.cpp
@@ -27,14 +27,12 @@ unified_difft::unified_difft(
 {
 }
 
-void unified_difft::get_diff(const irep_idt &function, goto_program_difft &dest)
-  const
+unified_difft::goto_program_difft
+unified_difft::get_diff(const irep_idt &function) const
 {
-  dest.clear();
-
   differences_mapt::const_iterator entry = differences_map_.find(function);
   if(entry == differences_map_.end())
-    return;
+    return {};
 
   goto_functionst::function_mapt::const_iterator old_fit =
     old_goto_functions.function_map.find(function);
@@ -50,20 +48,21 @@ void unified_difft::get_diff(const irep_idt &function, goto_program_difft &dest)
     new_fit == new_goto_functions.function_map.end() ? empty
                                                      : new_fit->second.body;
 
-  get_diff(old_goto_program, new_goto_program, entry->second, dest);
+  return get_diff(old_goto_program, new_goto_program, entry->second);
 }
 
-void unified_difft::get_diff(
+unified_difft::goto_program_difft unified_difft::get_diff(
   const goto_programt &old_goto_program,
   const goto_programt &new_goto_program,
-  const differencest &differences,
-  goto_program_difft &dest) const
+  const differencest &differences) const
 {
   goto_programt::instructionst::const_iterator old_it =
     old_goto_program.instructions.begin();
   goto_programt::instructionst::const_iterator new_it =
     new_goto_program.instructions.begin();
 
+  goto_program_difft dest;
+
   for(differencest::const_reverse_iterator rit = differences.rbegin();
       rit != differences.rend();
       ++rit)
@@ -89,6 +88,8 @@ void unified_difft::get_diff(
       break;
     }
   }
+
+  return dest;
 }
 
 void unified_difft::output_diff(
@@ -98,8 +99,8 @@ void unified_difft::output_diff(
   const differencest &differences,
   std::ostream &os) const
 {
-  goto_program_difft diff;
-  get_diff(old_goto_program, new_goto_program, differences, diff);
+  goto_program_difft diff =
+    get_diff(old_goto_program, new_goto_program, differences);
 
   bool has_diff = false;
   for(const auto &d : diff)
diff --git a/src/goto-diff/unified_diff.h b/src/goto-diff/unified_diff.h
index 428328591a0..e8a6e227e46 100644
--- a/src/goto-diff/unified_diff.h
+++ b/src/goto-diff/unified_diff.h
@@ -46,7 +46,7 @@ class unified_difft
   typedef std::list<std::pair<goto_programt::const_targett, differencet>>
     goto_program_difft;
 
-  void get_diff(const irep_idt &function, goto_program_difft &dest) const;
+  goto_program_difft get_diff(const irep_idt &function) const;
 
   const goto_functionst &old_goto_functions;
   const namespacet ns_old;
@@ -67,11 +67,10 @@ class unified_difft
     const goto_programt &new_goto_program,
     differencest &differences) const;
 
-  void get_diff(
+  goto_program_difft get_diff(
     const goto_programt &old_goto_program,
     const goto_programt &new_goto_program,
-    const differencest &differences,
-    goto_program_difft &dest) const;
+    const differencest &differences) const;
 
   void output_diff(
     const irep_idt &identifier,