# Manipulating Object and Value Types

## inject-code

> The inject-code node inserts the given code into the generated code for the given type or function, and it is a child of the object-type, value-type, modify-function and add-function nodes.

> The code can be embedded into XML (be careful to use the correct XML entities for characters like ‘<’, ‘>’, ‘&’):

>     <value-type>
>         <inject-code class="native | target | target-declaration"
>             position="beginning | end" since="...">
>             // the code
>         </inject-code>
>     </value-type>

> or obtained from an external file:

>     <value-type>
>         <inject-code class="native | target | target-declaration"
>             position="beginning | end" since="..."
>             file="external_source.cpp"
>             snippet="label"/>
>     </value-type>

> The `class` attribute specifies which module of the generated code that will be affected by the code injection. The `class` attribute accepts the following values:

> > - native: The c++ code

> > - target: The binding code

> > - target-declaration: The code will be injected into the generated header file containing the c++ wrapper class definition.

> > - file: The file name

> > - snippet: The snippet label (optional)

> If the `position` attribute is set to beginning (the default), the code is inserted at the beginning of the function. If it is set to end, the code is inserted at the end of the function.

> The `since` attribute specify the API version where this code was injected.

> If a `snippet` label is given, the code between annotations of the form

>     // @snippet label
>     ...
>     // @snippet label

> will be extracted.

## modify-field

> The modify-field node allows you to alter the access privileges for a given C++ field when mapping it onto the target language, and it is a child of an object-type or a value-type node.

>     <object-type>
>         <modify-field name="..."
>             write="true | false"
>             read="true | false" />
>     </object-type>

> The `name` attribute is the name of the field, the optional `write` and `read` attributes specify the field’s access privileges in the target language API (both are set to true by default). The `remove` attribute is an optional attribute, which can mark the field to be discarded on generation; it has the same purpose of the deprecated tag remove.

## modify-function

> The modify-function node allows you to modify a given C++ function when mapping it onto the target language, and it is a child of an object-type or a value-type node. Use the modify-argument node to specify which argument the modification affects.

>     <object-type>
>         <modify-function signature="..."
>                          since="..."
>                          remove="all | c++"
>                          access="public | private | protected"
>                          allow-thread="true | auto | false"
>                          exception-handling="off | auto-off | auto-on | on"
>                          rename="..." />
>     </object-type>

> The `signature` attribute is a normalized C++ signature, excluding return values but including potential const declarations.

> The `since` attribute specify the API version when this function was modified.

> The `allow-thread` attribute specifies whether a function should be wrapped into `Py_BEGIN_ALLOW_THREADS` and `Py_END_ALLOW_THREADS`, that is, temporarily release the GIL (global interpreter lock). Doing so is required for any thread-related function (wait operations), functions that might call a virtual function (potentially reimplemented in Python), and recommended for lengthy I/O operations or similar. It has performance costs, though. The value `auto` means that it will be turned off for functions for which it is deemed to be safe, for example, simple getters.

> The `exception-handling` attribute specifies whether to generate exception handling code (nest the function call into try / catch statements). It accepts the following values:

> > - no, false: Do not generate exception handling code

> > - auto-off: Generate exception handling code for functions declaring a non-empty `throw` list

> > - auto-on: Generate exception handling code unless function declares `noexcept`

> > - yes, true: Always generate exception handling code

> The `remove`, `access` and `rename` attributes are optional attributes for added convenience; they serve the same purpose as the deprecated tags remove, access and rename.

## add-function

> The add-function node allows you to add a given function onto the target language, and it is a child of an object-type or value-type nodes if the function is supposed to be a method, or namespace-type and Including Snippets if the function is supposed to be a function inside a namespace or a global function.

> Typically when adding a function some code must be injected to provide the function logic. This can be done using the inject-code node.

>     <object-type>
>         <add-function signature="..." return-type="..." access="public | protected" static="yes | no" since="..."/>
>     </object-type>

> The `return-type` attribute defaults to void, the `access` to public and the `static` one to no.

> The `since` attribute specify the API version when this function was added.

## conversion-rule

> The conversion-rule node allows you to write customized code to convert the given argument between the target language and C++, and is a child of the value-type, object-type, primitive-type and container-type nodes.

> The code pointed by the file attribute is very tied to the generator using APIExtractor, so it don’t follow any rules, but the generator rules..

>     <value-type name="Foo">
>         <convertion-rule file="my_converter_implementation.h" since="..."/>
>     </value-type>

> The `since` attribute specify the API version when this conversion rule became valid.

> Note

> You can also use the conversion-rule node to specify how the conversion of a single function argument should be done in a function.

> The `file` and `snippet` attributes are also supported (see inject-code nodes).