C++ language in step16

[yy.wayne]

In step16 function assemble_system() and assemble_multigrid(), there are 2 variables/functions defined as:

auto cell_worker =   

    [&](const typename DoFHandler<dim>::active_cell_iterator &cell, 

        ScratchData<dim> & scratch_data, 

        CopyData & copy_data) 

       { this->cell_worker(cell, scratch_data, copy_data); };
auto copier = 

    [&](const CopyData &cd) 

       { this->constraints.distribute_local_to_global(cd.cell_matrix,

                                                                                       cd.cell_rhs, 

                                                                                       cd.local_dof_indices, 

                                                                                       system_matrix, system_rhs); };

I'm not expert in C++ and these codes seem confusing to me. Are cell_worker and copier defined here variables or functions? And why we define a new cell_worker here (is it because we cannot directly pass this->cell_worker to MeshWorker::mesh_loop) ? Sorry it might be silly. Should I know the terminology for this "technique" I can google myself.

Thank you.

[Daniel Arndt]

cell_worker and copier are lambda functions. You can think of them as in-place structs with a call operator (operator()) with the specified signature. We define a new cell_worker object because the place we are passing it into expects the object to have a call operator that does what this->cell_worker implements.

Best,

Daniel

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[yy.wayne]

Thank you Daniel.