## How to use `tuple_tree_generator`

`tuple_tree_generator` takes a description of a struct/enum and generates boilerplate required to make them YAML
serializable/deserializable, compatible with `KeyedObjectContainer`s, and so on.

This document explains how to add an enum or a class to rev.ng model.

**Note**: Never include the generated headers anywhere apart from the corresponding _hand-written_ header, which is the only file that should be included anywhere else (even in other headers generated from the same schema).

### Defining an enum

Enums can be defined by adding
```yaml
name: <Name>
type: enum
doc: <Documentation entry about the enum itself>

members:
  - name: <Member_1_Name>
    doc: <Documentation entry about the first enum member>
  - name: <Member_2_Name>
    doc: <Documentation entry about the second enum member>
```
to the corresponding schema (in this case `include/revng/Model/model-schema.yml`), and making a new header:
```cpp
#pragma once

// <copyright notice>

#include "revng/Model/Generated/Early/<Name>.h"

// <feel free to add other related definitions here>

#include "revng/Model/Generated/Late/<Name>.h"
```

**Note**: The header must be named `<Name>.h`, where `<Name>` is the same across the schema and the header internals (namely the `Early` and `Late` include statements).


The autogenerated header file will contain something similar to this:

```cpp
namespace <Name> {
  enum Values {
    Invalid = 0
    Member_1_Name = 1;
    Member_2_Name = 2;
    // . . .
  };
}
```

Note that an `Invalid` and `Count` members are **always** present, you don't need to add them manually.

You can refer to the enum with `EnumName::Values` and to its values with `EnumName::Values::MemberName` or
`EnumName::MemberName`.

### Defining a struct

Structs can be defined by adding
```yaml
name: <Name>
type: struct
doc: <Documentation entry about the struct itself>

fields:
  - name: <Field1_name>
    type: <Field1_type>
    doc: <Documentation entry about the first field>
  - name: <Field2_name>
    type: <Field2_type>
    doc: <Documentation entry about the second field>

key:
  # List the fields that uniquely identify an object inside a container here.
  - Index
  # These are also used for building the appropriate `TupleTreeReference` -- a "path" inside the tuple tree.
```
to the corresponding schema (in this case `include/revng/Model/model-schema.yml`), and making a new header:
```cpp
#pragma once

// <copyright notice>

// Include the dependencies of this new type here:
#include "revng/ADT/Something.h"
// Note that you can use *wrapper* headers generated from the same schema. Their order in the schema does *not* matter.
#include "revng/Model/<AnotherName>.h"

#include "revng/Model/Generated/Early/<Name>.h"

class model::<Name> : public model::generated::<Name> {
public:
  // Note: don't forget to inherit constructors from autogenerated class
  using model::generated::<ClassName>::<ClassName>;

  // You can also add any extra methods you want here,
  // but no fields are allowed - please use the scheme for those.
};

#include "revng/Model/Generated/Late/<Name>.h"
```

**Note**: The header must be named `<Name>.h`, where `<Name>` is the same across the schema and the header internals (namely the class name, as well as `Early` and `Late` include statements).

#### Sequence members

You can define a member composed of a sequence of items as follows

```yaml
name: MyStruct
type: struct
doc: Did you know `doc` is a required field everywhere it's available?

fields:
  - name: MyArray
    doc: |
      Please don't use non-KOC containers in the model and other tuple trees
      that take advantage of invalidation.
    sequence:
      type: SortedVector
      elementType: int
```

The sequence type must be instantiable with one type parameter (the element type)! Which means you cannot currently use `std::pair` and friends.

Note that, if the specified `elementType` has `abstract: true` set in its *class* definition (see [here](#upcastable-structs) for more details), like here:

```yaml
name: MyStruct
type: struct
doc: Did you know `doc` is a required field everywhere it's available?

fields:
  - name: MyArray
    doc: |
      Please don't use non-KOC containers in the model and other tuple trees
      that take advantage of invalidation.
    sequence:
      type: SortedVector
      elementType: MyAbstractType # <-- note the different type
```

the sequence will automatically be promoted to an upcastable pointer.

In this case:
```cpp
SortedVector<UpcastablePointer<MyAbstractType>>
```

#### Reference members

Members can also specify references like this:

```yaml
name: MyStruct
type: struct
doc: Did you know `doc` is a required field everywhere it's available?

fields:
  - name: MyRef
    doc: |
      These references are polymorphic,
      feel free to use abstract types, like here.
    referenceTo: model::TypeDefinition
```

This gets translated in C++ as a
```cpp
TupleTreeReference<model::TypeDefinition, model::Binary>
```
where `model::Binary` is the root type of the `TupleTree` this struct is a part of.

#### Upcastable structs

It is possible to define polymorphic classes. To do so, first define a common base class (like `Type`).
This class has to be marked with the attribute `abstract`.

**Note**: this will mean that the class cannot be instantiated on its own. If that's desirable, please add an inheritor that doesn't add any extra fields to use instead.

Example (`TypeDefinition.h`):

```YAML
name: TypeDefinition
type: struct

abstract: true # <-- notice this

key:
  - Kind
  # . . .

fields:
  - name: Kind
    type: model::TypeDefinitionKind::Values
  # . . .
```

Note that the `Kind` field is required! You will get a compilation time error if one is not present.

Also, the `<Name>Kind` enum is automatically added to the schema (so no need to do so manually).
**Note**: It's only added to the schema, you still have to make a header for it (see [how to define an enum](#defining-an-enum) for more details).

With the base out of the way, adding derived structs is as easy as making normal structs (see [here](#defining-a-struct)) with an extra `inherits: <ParentClass>` property.

For example:
```yaml
name: StructDefinition
type: struct

inherits: TypeDefinition # <-- notice this

fields:
  # . . .
```

Note, that derived objects will contain all the fields of their parent, so the same names must not be used again.