[zhoulei_3c]

Hello, Foamers:
When I want to use the "tmp" class in openfoam to save memory, it through a compile error. And it is possibly to use tmp in PtrList just like tmp<volScarlarField>?

const Foam::tmp<Foam::PtrList<Foam::volScalarField>> Foam::macroscopicXS::lambda() const
{
    tmp<PtrList<volScalarField>> tlambda
    (
        new PtrList<volScalarField>
        (
            precGroups_
        )
    );

    PtrList<volScalarField>& lambda(tlambda.ref());
            
    forAll(lambda, energyI)
    {
        lambda.set
        (
            energyI,
            new volScalarField
            (

                "macroscopicXS::lambda",
                lambdaPtr_[energyI]
            )
        );
    }
    return tlambda;
}

Following are the error messages:

/home/zhoulei/OpenFOAM/OpenFOAM-v2006/src/OpenFOAM/lnInclude/tmpI.H: In instantiation of ‘void Foam::tmp<T>::clear() const [with T = Foam::PtrList<Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> >]’:
/home/zhoulei/OpenFOAM/OpenFOAM-v2006/src/OpenFOAM/lnInclude/tmpI.H:185:10:   required from ‘Foam::tmp<T>::~tmp() [with T = Foam::PtrList<Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> >]’
macroscopicXS/macroscopicXS.C:169:12:   required from here
/home/zhoulei/OpenFOAM/OpenFOAM-v2006/src/OpenFOAM/lnInclude/tmpI.H:329:19: error: ‘class Foam::PtrList<Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> >’ has no member named ‘unique’
         if (ptr_->unique())
             ~~~~~~^~~~~~
/home/zhoulei/OpenFOAM/OpenFOAM-v2006/src/OpenFOAM/lnInclude/tmpI.H:335:27: error: ‘class Foam::PtrList<Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> >’ has no member named ‘operator--’; did you mean ‘operator=’?
             ptr_->operator--();

[olesen]

There are a few reasons why what you are attempting doesn't make much sense. Here are some notes (in no particular order).
For anything to be a tmp it must have intrinsic ref counting. So you could wrap a labelField as a tmp, but not a labelList, since List does not have a refCount.
If you want the idea like a tmp, then you could consider refPtr instead. In many respects it behaves like a tmp (can hold a reference or its own storage) but without any ref counting. So it makes it somewhat like an autoPtr and a unique_ptr, but with being able to have a reference.

So your example could be changed to use a refPtr for the return type. However, you will still need to create the stored item itself as a raw pointer before stuffing that into the refPtr. Otherwise you will be trying to return a reference to the local variable, which then goes out of scope (= crash).
Take a git grep through the OpenFOAM codebase for refPtr uses to get an idea of how they work. A few examples come to mind: the PrecisionAdaptor is a fun one for handling float/double conversion for mixed precision compilations without adding overhead for the regular case. There is also some fun bits in the isosurface (iso-topo I think) where it is used to hold a reference to a field or subfield. Saves juggling pointers a bit.

Note however for your example with wrapping a PtrList. This would only be useful for a possibly cached value. If it's a regular PtrList, simply returning it will be fine (copy ellision etc).