Accessing file system properties

Each file system entity provides accessor functions for the properties associated with it. All file system properties are readable, and each has a Get function associated with it. If a property can also be modified, the class includes a Set member function. For example, the TFileSystemEntity class contains declarations for GetModificationTime and SetModificationTime, so that you can read and modify the property that stores the entity's modification time.

In addition to the accessor functions, the file system entity classes provide pre-constructed identifiers for each file system property in the form of references to TPropertyID instances. You can use these identifiers to access specific properties. For example, TFileSystemEntity declares a reference to the modification time property using the identifier kModificationTime. You can access the modification time using this identifier instead of an accessor function.

Whether you access properties using accessor functions or identifiers is up to you. Accessor functions are often more convenient. When you retrieve multiple file system properties, however, you can optimize performance by using identifiers. If you retrieve three properties using accessor functions, the system makes three round trips to get each property. If you construct a property set with identifiers instead, and retrieve the values for the set, the system makes a single round trip to return all three property values.

Table 1 shows each of the properties defined by the file system. All of these properties are readable and none are polymorphic.

	Entity Kind	Property Identifier	Write
File System Properties	All	TPropertyIDFor<FileSystemEntityName> TFileSystemEntity::kName	Yes
	All	TPropertyIDFor<TTime> TFileSystemEntity::kCreationTime	Yes
	All	TPropertyIDFor<TTime> TFileSystemEntity::kModification Time	Yes
	TFile	TPropertyIDFor<FSEntitySize> TFile::kEndOfFile	No
	TVolume	TPropertyIDFor<FSEntitySize> TVolume::kTotalSpace	No
	TVolume	TPropertyIDFor<FSEntitySize> TVolume::kFreeSpace	No

NOTE These properties represent only a subset of the file system properties planned for later releases.

Behavior of file system properties

Every file system entity always has the file system properties associated with that particular type of entity. For example, any TFile instance has an End of File property, and any TDirectory instance has a Member Count. It follows, then, that file system entities do not have properties not associated with that type of entity. For example, a TFile never has a Free Space property, because that property does not apply to a file.

NOTE File system properties do not allow polymorphic values.

Accessing file system properties using functions

  TFileSystemEntityName fileName = myFile.GetName;
  myFile.SetName("NewFilename");

• Makes a property (named prop) using the kModificationTime identifier.
• Reads the property from a file system entity called anEntity, using the ReadProperty member function that the entity inherits from TFileSystemEntity.
• Extracts the modification time using TFileSystemEntity::GetValue and stores this in a TTime instance named modifyTime.

  TPropertyIDFor<TTime> prop(TFileSystemEntity::kModificationTime)
  anEntity->ReadProperty(prop);
  TTime           modifyTime = prop.GetValue();