> For the complete documentation index, see [llms.txt](https://wiki.solids.group/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://wiki.solids.group/alamo/parmparse.md).

# ParmParse

## Selector

The `pp.select` automates the selection of polymorphic objects by instantiating the object (using either static dispatch or dynamic dispatch) and relaying the parser to the model's `Parse` function. For example:

```
    pp.select<Model::AllenCahn,Model::CahnHilliard>("phase_field", value.model);
```

sets `value.model` to either `Model::AllenCahn` or `Model::CahnHilliard` depending on what is set in `phase_field.type`. The string that is matched is set as the `name` field in the corresponding class, for example,

```
public:
    static constexpr const char* name = "allencahn";
```

indicating that an input value of `phase_field.type = allencahn` will relay to the Allen Cahn integrator, and any subsequent arguments will be given the `allencahn` prefix (e.g. `phase_field.allencahn.mobility=1` ).

### Argument forwarding

Sometimes the model that is selected requires additional arguments upon construction. The `IC::IC` family of classes requires an `amrex::Geometry` argument and optionally a unit. These can be passed in using the `pp.forward_args` function as:

```
pp.select<IC::Constant,IC::Expression>("ic",value.ic,pp.forward_args(value.geom));
```

{% hint style="warning" %}
This is a change introduced by [PR297](https://github.com/solidsgroup/alamo/pull/297). Previously, constructor arguments could be forwarded in simply as additional argument to select, like

```
pp.select<IC::Constant,IC::Expression>("ic",value.ic,value.geom);
```

i.e. without the `pp.forward_args` wrapper. This is no longer supported and will trigger compilation errors; all that you need to do to fix is to add the wrapper and it should compile as before.
{% endhint %}


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