OpenFOAM guide/Input and Output operations using dictionaries and the IOobject class

Many input/output operations are performed in OpenFOAM using the IOobject class, which is described in its header file as follows:

IOobject defines the attributes of an object for which implicit objectRegistry management is supported, and provides the infrastructure for performing stream I/O. An IOobject is constructed with an object name, a class name, an instance path, a reference to a objectRegistry, and parameters determining its storage status.

1 IOobject constructors

The constructor of an IOobject may have two forms:

• Construct from name, instance, registry and IO options

 
IOobject  	
(  
    const word &   	 	name,
    const word &  		instance,
    const objectRegistry &  	registry,
    readOption  		r = NO_READ,
    writeOption  		w = NO_WRITE,
    bool  			registerObject = true
)

• Construct from name, instance, local, registry and IO options

 
IOobject  	
(
    const word &   	 	name,
    const word &  		instance,
    const fileName &  		local,
    const objectRegistry &  	registry,
    readOption  		r = NO_READ,
    writeOption  		w = NO_WRITE,
    bool  			registerObject = true
)

While reading the two previous code snippets, remember that string inherits word, from which fileName is derived. Moreover, both Time and polyMesh inherit objectRegistry. As a consequence fvMesh, inheriting polyMesh, indirectly inherits objectRegistry too. For further information, see the IOobject class reference.

2 Read options

Read options, which define what is done on object construction and explicit reads, are:

• MUST_READ

 The object must be read from Istream on construction. An error message is produced if Istream does not exist or can't be read.

• READ_IF_PRESENT

 It reads the object from Istream if Istream exists, otherwise doesn't. An error message is produced only if Istream exists but can't be read.

• NO_READ

 Don't read the object.

3 Write options

Write options, which define what is done on object destruction and explicit writes, are:

• AUTO_WRITE

 The object is written automatically when requested to by the objectRegistry.

• NO_WRITE

 The object is not written automatically on destruction, but it can be written explicitly.

4 IOobject and dictionaries

Dictionaries can be read using the IOobject class when they are declared. Usually the readOption for a dictionary is MUST_READ, while the writeOption is NO_WRITE not to overwrite the settings contained in the dictionary. For example, for the usual transportProperties dictionary, the syntax is:

 
IOdictionary transportProperties
(
     IOobject
     (
          "transportProperties",
          runTime.constant(),
          mesh,
          IOobject::MUST_READ,
          IOobject::NO_WRITE
     )
);

where runTime.constant() gives the position of the dictionary, which is, in this case, contained in the constant directory of the considered case.

5 IOobect and fields

Input/output options for a field can be set similarly to