Don't create new sections during parsing

doc/news/changes/minor/20240229Munch

@@ -0,0 +1,6 @@
+Improved: The parse functions of ParameterHandler used to add subsections
+if subsections/parameters have not been defined. This led to an output
+of ParameterHandler::print_parameters() that also contained these subsections.
+This is not the case anymore.
+<br>
+(Peter Munch, Magdalena Schreter-Fleischhacker, 2024/02/12)

include/deal.II/base/parameter_handler.h

@@ -1252,10 +1252,11 @@ class ParameterHandler : public Subscriptor
                 const bool         alias_is_deprecated = false);
 
   /**
-   * Enter a subsection. If it does not yet exist, create it.
+   * Enter a subsection. If it does not yet exist, create it if requested.
    */
   void
-  enter_subsection(const std::string &subsection);
+  enter_subsection(const std::string &subsection,
+                   const bool         create_path_if_needed = true);
 
   /**
    * Leave present subsection.

source/base/parameter_handler.cc

@@ -664,17 +664,25 @@ namespace
           }
         else
           {
-            // it must be a subsection
-            prm.enter_subsection(demangle(p.first));
-            read_xml_recursively(p.second,
-                                 (current_path.empty() ?
-                                    p.first :
-                                    current_path + path_separator + p.first),
-                                 path_separator,
-                                 patterns,
-                                 skip_undefined,
-                                 prm);
-            prm.leave_subsection();
+            try
+              {
+                // it must be a subsection
+                prm.enter_subsection(demangle(p.first), !skip_undefined);
+                read_xml_recursively(p.second,
+                                     (current_path.empty() ?
+                                        p.first :
+                                        current_path + path_separator +
+                                          p.first),
+                                     path_separator,
+                                     patterns,
+                                     skip_undefined,
+                                     prm);
+                prm.leave_subsection();
+              }
+            catch (const ParameterHandler::ExcEntryUndeclared &)
+              {
+                // ignore undeclared entry assert
+              }
           }
       }
   }
@@ -969,10 +977,20 @@ ParameterHandler::declare_alias(const std::string &existing_entry_name,
 
 
 void
-ParameterHandler::enter_subsection(const std::string &subsection)
+ParameterHandler::enter_subsection(const std::string &subsection,
+                                   const bool         create_path_if_needed)
 {
   // if necessary create subsection
-  if (!entries->get_child_optional(get_current_full_path(subsection)))
+  const auto path_exists =
+    entries->get_child_optional(get_current_full_path(subsection));
+
+  if (!create_path_if_needed)
+    {
+      AssertThrow(path_exists,
+                  ExcEntryUndeclared(get_current_full_path(subsection)));
+    }
+
+  if (!path_exists)
     entries->add_child(get_current_full_path(subsection),
                        boost::property_tree::ptree());
 

tests/parameter_handler/parameter_handler_read_json_04.cc

@@ -0,0 +1,49 @@
+// ------------------------------------------------------------------------
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2020 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// Part of the source code is dual licensed under Apache-2.0 WITH
+// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
+// governing the source code and code contributions can be found in
+// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
+//
+// ------------------------------------------------------------------------
+
+
+
+// check ParameterHandler::parse_input() for json file
+
+#include <deal.II/base/parameter_handler.h>
+
+#include "../tests.h"
+
+
+int
+main()
+{
+  initlog();
+
+  ParameterHandler prm;
+
+  double test_0 = 0;
+  double test_1 = 0;
+
+  // test if underscore can be parsed
+  prm.add_parameter("test 0", test_0);
+  prm.add_parameter("test 1", test_1);
+
+  std::string source   = SOURCE_DIR;
+  std::string filename = source + "/prm/parameter_handler_read_json_04.json";
+
+  std::ifstream file;
+  file.open(filename);
+  prm.parse_input_from_json(file, true);
+
+  prm.print_parameters(deallog.get_file_stream(),
+                       ParameterHandler::OutputStyle::ShortJSON);
+
+  return 0;
+}

tests/parameter_handler/parameter_handler_read_json_04.output

@@ -0,0 +1,5 @@
+
+{
+    "test 0": "1",
+    "test 1": "1"
+}

tests/parameter_handler/prm/parameter_handler_read_json_04.json

@@ -0,0 +1,8 @@
+{
+    "test 0": "1",
+    "test 1": "1",
+    "test 2": "1",
+    "test 3": {
+        "test 4" : "1"
+    }
+}