OpenStudio provides parsing of and programmatic access to Input Data Dictionary (IDD) files, objects, and fields through the OpenStudio utilities library (in the openstudio namespace). IDD files of central importance to OpenStudio are wrapped into the IddFactory , and used to generate IddFileType, IddObjectType, and field-level enumeration classes (see IddEnums.hpp and IddFieldEnums.hxx).

Quick Links

Base classes and enums

IddFactory classes and enums

Background

OpenStudio consists of middleware and end-user tools for building energy analysis. As such, it needs to be able to model buildings at varying levels of detail, and to communicate with building energy simulation engines.

OpenStudio's primary simulation engine target is EnergyPlus, which is freely available and actively developed with support from the U.S. Department of Energy and the California Energy Commission.

EnergyPlus operates off of text input files written in its own Input Data File (IDF) format. IDF files are similar to XML files in that they are intended to conform to a data schema written using similar syntax. For XML, the schema format is XSD; for IDF, the schema format is IDD. Although idiosyncratic, IDD and IDF have the advantages of being native to EnergyPlus and more compact than XSD and XML.

For each release of EnergyPlus, valid IDF files are defined by the Energy+.idd file shipped with the release. Therefore, by adopting the IDD and IDF data formats directly, OpenStudio can more easily keep up with the EnergyPlus release cycle, support legacy building models written in IDF, and provide detailed modeling capability (by allowing the user lightly filtered access to EnergyPlus IDF objects). However, there is an upfront cost in that there are no third-party tools available for programmatic access to IDF and IDD files, as there are for XML and XSD.

This document, along with the documentation for files in openstudiocore/src/utilities/idd/ (accessible through links on this page) describes the classes and functions available through OpenStudio for parsing and accessing IddFiles, and IddObjects, including the two IddFiles of central importance to OpenStudio: the current versions of Energy+.idd and OpenStudio.idd, which are best accessed through the IddFactory . (Legacy versions of the OpenStudio.idd should also be loaded from disk using the IddFactory, to avoid duplicate parsing effort.)

File and Object Parsing

Objects of type IddObject and IddFile are typically constructed by parsing IDD text using the methods IddFile::load and IddObject::load. File, object, and field-level slash codes such as

\group
\memo
\min-fields
\autosizable

are captured and made available through methods

std::string groupName = IddObject::group();

and properties

std::string objectMemo = IddObject::properties().memo;
unsigned minNumFields = IddObject::properties().minFields; // returns 0 if no fields required
bool canAutosize = IddField::properties().autosizable;

Individual IddObjects are uniquely identified by the IddObject::name string, which is the initial token used to identify the object in the IDD file, and to identify IDF objects following that IddObject's schema. IddFields may or may not have a non-empty IddField::name. They are always uniquely identified by their index in IddObject::nonextensibleFields or IddObject::extensibleGroup.

In addition to containing objects, IddFiles typically start with a header, at the top of which a version string is specified, prefixed by '!IDD_Version '. This version string should be the default value for the Version Identifier field in the file's version object ('Version' in the EnergyPlus IDD, 'OS:Version' in the OpenStudio IDD). Version information is generally available through the IddFile, IddFactory , IddFileAndFactoryWrapper, IdfFile, and Workspace classes.

Programmatic Access

After parsing an IddFile, file-level information such as

std::string fileHeader = IddFile::header();

std::string fileVersion = IddFile::version();

can be accessed, as can its objects. Individual objects can be retrieved by name. Vectors of all objects, or of objects satisfying certain cardinality requirements can also be retrieved.

boost::optional<IddObject> optionalBuildingIdd = IddFile::getObject("Building");
if (optionalBuildingIdd) {
  std::cout << "Text of retrieved IddObject named '" << optionalBuildingIdd->name() << "':" << '\n';
  std::cout << optionalBuildingIdd.get();
}
std::vector<IddObject> allIddObjects = IddFile::objects();
std::vector<IddObject> onlyTheRequiredIddObjects = IddFile::requiredObjects();

IddFiles can be saved to file using the IddFile::save method. Note that the default behavior of this (and other OpenStudio save methods) is to not overwrite an existing file. If you want to overwrite the file in the case that it already exists, you will need to pass true in for the overwrite argument. Also, to maintain cross-platform compatibility, you should explicitly construct the openstudio::path argument either directly or with one of the toPath methods in utilities/core/Path.hpp. IDD text can be directed to other destinations using std::ostream, IddFile::print, IddObject::print, and operator<<.

IddObjects provide access to object level properties, and to fields.

IddObjectProperties objectProperties = IddObject::properties();
std::vector<IddField> fieldsBeforeExtensibleMarker = IddObject::nonextensibleFields();
std::vector<IddField> fieldsAtAndAfterExtensibleMarker = IddObject::extensibleGroup();

IddObjects, and the IdfObjects that instantiate them, start with a list of nonextensible fields, that is, with basic data fields that are not repeated. Optionally, the nonextensible fields may be followed by an extensible group, that is, a group of fields that may be repeated indefinitely. For instance, several EnergyPlus objects have an extensible group that defines a vertex. The corresponding IddObject only needs to define one vertex. The IdfObject instantiation can then define multiple vertices, each of which refers to the same IddObject::extensibleGroup.

IddObject provides direct support for indexing fields in extensible groups. Any IdfObject field that is in such a group can be accessed with either its overall field index, or with an ExtensibleIndex, which specifies the index of the ExtensibleGroup (ExtensibleIndex::group), and the index of the field within that group (ExtensibleIndex::field). IddObject converts between the two representations using IddObject::numFields and IddObjectProperties::numExtensible. All field and group indices use the C/C++ convention of labeling the first element with index 0.

Energy+.idd and OpenStudio.idd

Two particular IddFiles, the current Energy+.idd and OpenStudio.idd, are central to OpenStudio, and so the utilities/idd sub-project provides convenience functionality to support their consistent use throughout the rest of the project via the IddFactory .

The IddFactory is generated from ProposedEnergy+.idd (contains the same information as Energy+.idd, but has slightly modified slash codes, and only one copy of each extensible group.) and OpenStudio.idd.

All IDD objects in these two files are listed in the IddObjectType enumeration; and all fields of all of these objects are accessible through the enumerations indirectly listed in IddFieldEnums.hxx. (In most integrated developer environments (IDEs) you can right-click on the file names listed there to open and subsequently inspect the individual enumerations.) The field enumerations are split into Fields and ExtensibleFields, as in:

BuildingSurface_DetailedFields surfaceTypeField("surface type");
BuildingSurface_DetailedExtensibleFields yCoordinateField(
    BuildingSurface_DetailedExtensibleFields::VertexYcoordinate);

IddFieldEnums.hxx should never be included directly; please use it as a table of contents to find the specific files that should be #included in your source.

The IddFactory API and IddFile API are purposefully similar, as openstudio::IdfFile and openstudio::Workspace can be used with either an explicit IddFile, or with an implicit IDD that is specified by filtering the IddFactory with an IddFileType (typically IddFileType::EnergyPlus or IddFileType::OpenStudio). Instead of each of those classes handling the possibility of using a custom IDD or an IddFactory IDD on their own, this interchangeability is embodied in the IddFileAndFactoryWrapper, which explicitly provides a common API to these two in-memory instantiations of IDD.

The IddFactory also provides access to older versions of the OpenStudio.idd. In addition to the convenience of the IddFactory::instance().getIddFile syntax, accessing the legacy files in this way ensures that each file is only parsed once, which is necessary for some applications (for instance, the openstudio::osversion::VersionTranslator) to have acceptable performance.