Support binary search for struct in cpp (#4245)

* Support binary search for struct in cpp

CreateVectorOfSortedStruct is provided for convenience.

* fix continuous-integration error

* add generated files

* compile Ability.cs in csharp test

* compile Ability.cs in csharp

* modify according to code review

src/idl_gen_cpp.cpp

@@ -1943,6 +1943,24 @@ class CppGenerator : public BaseGenerator {
           code_ += "  }";
         }
       }
+
+      // Generate a comparison function for this field if it is a key.
+      if (field.key) {
+        code_ += "  bool KeyCompareLessThan(const {{STRUCT_NAME}} *o) const {";
+        code_ += "    return {{FIELD_NAME}}() < o->{{FIELD_NAME}}();";
+        code_ += "  }";
+        auto type = GenTypeBasic(field.value.type, false);
+        if (parser_.opts.scoped_enums && field.value.type.enum_def &&
+            IsScalar(field.value.type.base_type)) {
+          type = GenTypeGet(field.value.type, " ", "const ", " *", true);
+        }
+
+        code_.SetValue("KEY_TYPE", type);
+        code_ += "  int KeyCompareWithValue({{KEY_TYPE}} val) const {";
+        code_ += "    const auto key = {{FIELD_NAME}}();";
+        code_ += "    return static_cast<int>(key > val) - static_cast<int>(key < val);";
+        code_ += "  }";
+      }
     }
     code_ += "};";
 

src/idl_gen_general.cpp

@@ -1291,7 +1291,9 @@ void GenStruct(StructDef &struct_def, std::string *code_ptr) {
       code += "); }\n";
     }
   }
-  if (struct_def.has_key) {
+  // Only generate key compare function for table,
+  // because `key_field` is not set for struct
+  if (struct_def.has_key && !struct_def.fixed) {
     if (lang_.language == IDLOptions::kJava) {
       code += "\n  @Override\n  protected int keysCompare(";
       code += "Integer o1, Integer o2, ByteBuffer _bb) {";