== operator
 

Hi foamers,

I know the "==" operator as an equality operator in C, i.e. (a==b) suppose to return true or false.

But then I can't understand following lines of interFoam:
and
Can you please explain me the meaning of the == operator is these cases? What would be the difference if I would use instead of == for the first one?

Best regards,
Ilya

I think...

= only does stuff to the internalField()
== does stuff to the internalField() AND the boundaryField()

And how OF knows, if it has to use the C/C++ standard boolean or the OF-redefined "==" operator?

Hi, '=' is the usual operator used to replace the content of LHS with RHS. '==' is overloaded in fvMatrix class in order to write equations. It's only dividing sides.

Regards.

Even in this case: rho == alpha1*rho1 + (scalar(1) - alpha1)*rho2; ?

I thought in this line the volScalar rho is being computed, but if I understand you correctly an equation for rho is being defined? :eek:

regards,
Ilya

No Ilya, in your example it is an assignment just like Laurence already stated further up. Santiago was explaining the case when having an "==" within a solve().

Illya, from what source file did you take this line?:

Regards.

interFoam : alphaEqnSubCycle.H

Thanks, so for volFields it makes more sense to use == instead of =, if no boundary condition is prescribed for the LHS-field?

Aha, it's true I never paid attention in this particular '==' operator. Checking GeometricField.C, operator== and operator= copies both internal and boundary fields. The difference seems to be in another characteristic, operator= uses transfer method and operator== does not. But I don't realize the necessity of this operator in this case.

Regards.

Sry, but what does it mean?

Regards.

Nope: operator== will FORCE assignment, even if patch field type says it should not be so. For example for a fixedValue patch, operator== will change its value and operator= will do nothing.

Hrv

Aha, that explains the part I couldn't understand at all (GeometricField.C:01103):

i think it ends doing this (fvPatchField.C):

Thanks for the clarification!!

Regards.

P.S. fvPatchField class don't have a 'fixed value flag' to avoid overriding?

Greetings,

I am developing a timeVaryingMapped BC for tractionDisplacement based on the timeV..M..FixedValue and the tractionDisplacement BCs. In the first (timeV..M..FixedValues) I find that values are assigned by the "==" operator:
According to the discussion here, this should mean that the value (for given patch and variable) is imposed for each time it is calculated.
However, in the tractionDisplacement BC, the gradient() is calculated via a "=" operator.
I would like to know:

1. What is the meaning of "this->operator==(someField);" for a fixedGradient BC?
2. To assign gradient() for the custom BC I seek, should I use "gradient() == someVectorField;" or "gradient() = someVectorField;"

I appreciate your help/answer! Thanks
Hisham

Hi Illya,

operators are like other functions in C++. Although you don't see it, they look like, for example:

where "==" is the name of the function. Since they are overloaded (the final name of the function is defined by the arguments, return value, as well as by the "==" special character), the compiler recognizes by the types involved, which operator to call, in which class (type).

From fvMatrix.C:

"checkMethod" compares the "psi_" private data members (geometric fields) to see if they are compatible (mapped on the same mesh) and the dimensioned set to ensure compatible field dimensions.

Since it returns A - B (both coeff matrices), it seems to me that it just makes the entire algebraic system implicit (put B on the left side, or in your case, the field you get by fvc::reconstruct).

operator = ... from fvMatrix.C:
It has to call the lduMatrix::operator= because it inherits from it, this guy goes over lower, upper and diagonal part and executes the data copy. The other part copies the rest of the data that is stored in the fvMatrix.

There seems to be no difference between

rho = something

and

rho == something

If rho is a vol*Field, == will copy both the boundary and the internal field:

01154 template<class Type, template<class> class PatchField, class GeoMesh>
01155 void Foam::GeometricField<Type, PatchField, GeoMesh>::operator==
01156 (
01157 const tmp<GeometricField<Type, PatchField, GeoMesh> >& tgf
01158 )
01159 {
01160 const GeometricField<Type, PatchField, GeoMesh>& gf = tgf();
01161
01162 checkField(*this, gf, "==");
01163
01164 // only equate field contents not ID
01165
01166 dimensionedInternalField() = gf.dimensionedInternalField();
01167 boundaryField() == gf.boundaryField();
01168
01169 tgf.clear();
01170 }



and "=" seems to be doing the same thing:

01082 template<class Type, template<class> class PatchField, class GeoMesh>
01083 void Foam::GeometricField<Type, PatchField, GeoMesh>::operator=
01084 (
01085 const GeometricField<Type, PatchField, GeoMesh>& gf
01086 )
01087 {
01088 if (this == &gf)
01089 {
01090 FatalErrorIn
01091 (
01092 "GeometricField<Type, PatchField, GeoMesh>::operator="
01093 "(const GeometricField<Type, PatchField, GeoMesh>&)"
01094 ) << "attempted assignment to self"
01095 << abort(FatalError);
01096 }
01097
01098 checkField(*this, gf, "=");
01099
01100 // only equate field contents not ID
01101
01102 dimensionedInternalField() = gf.dimensionedInternalField();
01103 boundaryField() = gf.boundaryField();
01104 }

Both operators, work on GeometricField<Type, PatchField, GeoMesh> ( example: vol*Field) from both sides.

Hope this helps,
Tomislav

Ah, but == will force an overwrite of all the boundary conditions, whereas = will not.

I just checked it with an application, and you are completely right.... still,

I didn't notice that double "==" in the operator == of the GeometricField<blah ... and when I look in the BoundaryField, this operator is defined to do:

and the definition does exactly that:

:confused: It's all in the GeometricField.C and GeometricBoundaryField.C (GeometricBoundaryField is composited into GeometricField).

The only question is, what does this button do: "this->operator[](patchI) == bf[patchI]"