[JBUNSW]

Hi there,

I have a general question regarding the concept of tmp<volScalarField>. To explain my question I use rho, as an example, from a combustion class in OF.

How can one calculate density?
The density can be accessed by invoking the rho() function which is a virtual function in basicPsiThermo class. So for example if thermo object has been setup correctly in a given class (such as ODEChemistryModel), then you can easily calculate density by equation of state of an ideal gas (there are other models available as well) by

this->thermo().rho()

which returns a tmp<volScalarField> accordingly:

virtual tmp<volScalarField> rho() const
{
	return p_*psi();
}

I was wondering what is the difference between the following four methods to access a tmp<volScalarField> quantity such as density (or any other tmp<volScalarField>).

Method 1:

const volScalarField& rho = this->db().objectRegistry::lookupObject<volScalarField>("rho");

Method 2:

tmp<volScalarField> trho = this->thermo().rho();
const volScalarField& rho = trho();

Method 3:

const volScalarField rho
(
	IOobject
	(
		"rho",
		this->time().timeName(),
		this->mesh(),
		IOobject::NO_READ,
		IOobject::NO_WRITE,
		false
	),
	this->thermo().rho()
);

Method 4:

const volScalarField rho = this->thermo().rho();

My question:
My understanding of the above is that method 1 is accessing the rho variable already stored in the objectRegistry and doesn't use the result of the virtual function rho(). The other methods all use the thermo().rho() function to calculate rho. In method 1 we do not create a "local" volScalarField variable and just use "referencing" to reference to some memory location. But in other 3 methods we are creating a "local" volScalarField variable and store the results in it.

1. Am I right?!
2. Why we use tmp<volScalarField> at all?!
3. Are all the above four methods correct?!
4. What is the difference between them?!

Thanks for your time,
Jalal