of.cpp

const fvPatchList& patches = mesh.boundary();

    forAll(patches, patchi)
    {
        const fvPatch& curPatch = patches[patchi];
        if (isType<wallFvPatch>(curPatch))
        {
            forAll(curPatch, facei)
            {
                label faceCelli = curPatch.faceCells()[facei];
                //your stuff
            }
        }
    }