# Code Injection Semantics

API Extractor provides the inject-code tag allowing the user to put custom written code to on specific locations of the generated code. Yet this is only part of what is needed to generate proper binding code, where the custom code should be written to depends upon the technology used on the generated binding code.

This is the `inject-code` tag options that matters to Shiboken.

>     <inject-code class="native | target" position="beginning | end">
>         // custom code
>     </inject-code>

## inject-code tag

The following table describes the semantics of `inject-code` tag as used on Shiboken. The `class` attribute specifies whether to code is injected into the C++ Wrapper or the Python Wrapper (see Code Generation Terminology). The `position` attribute specifies the location of the custom code in the function.

> Parent Tag | Class | Position | Meaning
> --- | --- | --- | ---
> value-type, object-type | native | beginning | Write to the beginning of a class wrapper `.cpp` file, right after the `#include` clauses. A common use would be to write prototypes for custom functions whose definitions are put on a `native/end` code injection.
> end | Write to the end of a class wrapper `.cpp` file. Could be used to write custom/helper functions definitions for prototypes declared on `native/beginning`.
> target | beginning | Put custom code on the beginning of the wrapper initializer function (`init_CLASS(PyObject *module)`). This could be used to manipulate the `PyCLASS_Type` structure before registering it on Python.
> end | Write the given custom code at the end of the class wrapper initializer function (`init_CLASS(PyObject *module)`). The code here will be executed after all the wrapped class components have been initialized.
> modify-function | native | beginning | Code here is put on the virtual method override of a C++ wrapper class (the one responsible for passing C++ calls to a Python override, if there is any), right after the C++ arguments have been converted but before the Python call.
> end | This code injection is put in a virtual method override on the C++ wrapper class, after the call to Python and before dereferencing the Python method and tuple of arguments.
> target | beginning | This code is injected on the Python method wrapper (`PyCLASS_METHOD(...)`), right after the decisor have found which signature to call and also after the conversion of the arguments to be used, but before the actual call.
> end | This code is injected on the Python method wrapper (`PyCLASS_METHOD(...)`), right after the C++ method call, but still inside the scope created by the overload for each signature.
> shell | declaration | Used only for virtual functions. This code is injected at the top.
> beginning | Used only for virtual functions. The code is injected when the function does not has a Python implementation, then the code is inserted before c++ call
> end | Same as above, but the code is inserted after c++ call
> typesystem | native | beginning | Write code to the beginning of the module `.cpp` file, right after the `#include` clauses. This position has a similar purpose as the `native/beginning` position on a wrapper class `.cpp` file, namely write function prototypes, but not restricted to this use.
> end | Write code to the end of the module `.cpp` file. Usually implementations for function prototypes inserted at the beginning of the file with a `native/beginning` code injection.
> target | beginning | Insert code at the start of the module initialization function (`initMODULENAME()`), before the calling `Py_InitModule`.
> end | Insert code at the end of the module initialization function (`initMODULENAME()`), but before the checking that emits a fatal error in case of problems importing the module.
> declaration | Insert code into module header.

## Anatomy of Code Injection

To make things clear let’s use a simplified example of generated wrapper code and the places where each kind of code injection goes.

Below is the example C++ class for whom wrapper code will be generated.

>     class InjectCode
>     {
>     public:
>         InjectCode();
>         double overloadedMethod(int arg);
>         double overloadedMethod(double arg);
>         virtual int virtualMethod(int arg);
>     };

From the C++ class, Shiboken will generate a `injectcode_wrapper.cpp` file with the binding code. The next section will use a simplified version of the generated wrapper code with the injection spots marked with comments.

There are a number of placeholders indicated by a percent sign `%`, which will be expanded when inserting the code. For a list, see Type System Variables.

### Noteworthy Cases

The type system description system gives the binding developer a lot of flexibility, which is power, which comes with responsibility. Some modifications to the wrapped API will not be complete without some code injection.

#### Removing arguments and setting a default values for them

A simple case is when a function have one argument removed, as when the C++ method `METHOD(ARG)` is modified to be used from Python as `METHOD()`; of course the binding developer must provide some guidelines to the generator on what to do to call it. The most common solution is to remove the argument and set a default value for it at the same time, so the original C++ method could be called without problems.

#### Removing arguments and calling the method with your own hands

If the argument is removed and no default value is provided, the generator will not write any call to the method and expect the `modify-function - target/beginning` code injection to call the original C++ method on its own terms. If even this custom code is not provided the generator will put an `#error` clause to prevent compilation of erroneous binding code.

#### Calling the method with your own hands always!

If your custom code to be injected contains a call to the wrapped C++ method, it surely means that you don’t want the generator to write another call to the same method. As expected Shiboken will detect the user written call on the code injection and will not write its own call, but for this to work properly the binding developer must use the template variable `%FUNCTION_NAME` instead of writing the actual name of the wrapped method/function.

In other words, use

>     <inject-code class="target" position="beginning | end">
>         %CPPSELF.originalMethodName();
>     </inject-code>

instead of

>     <inject-code class="target" position="beginning | end">
>        %CPPSELF.%FUNCTION_NAME();
>     </inject-code>

## Code Injection for Functions/Methods

### On The Native Side

Notice that this is only used when there is a C++ wrapper, i.e. the wrapped class is polymorphic.

>     int InjectCodeWrapper::virtualMethod(int arg)
>     {
>         PyObject *method = BindingManager::instance().getOverride(this, "virtualMethod");
>         if (!py_override)
>             return this->InjectCode::virtualMethod(arg);

>         (... here C++ arguments are converted to Python ...)

>         // INJECT-CODE: <modify-function><inject-code class="native" position="beginning">
>         // Uses: pre method call custom code, modify the argument before the
>         // Python call.

>         (... Python method call goes in here ...)

>         // INJECT-CODE: <modify-function><inject-code class="native" position="end">
>         // Uses: post method call custom code, modify the result before delivering
>         // it to C++ caller.

>         (... Python method and argument tuple are dereferenced here ...)

>         return Shiboken::Converter<int>::toCpp(method_result);
>     }

### On The Target Side

All the overloads of a method from C++ are gathered together on a single Python method that uses an overload decisor to call the correct C++ method based on the arguments passed by the Python call. Each overloaded method signature has its own `beginning` and `end` code injections.

>     static PyObject *PyInjectCode_overloadedMethod(PyObject *self, PyObject *arg)
>     {
>         PyObject* py_result{};
>         if (PyFloat_Check(arg)) {
>             double cpp_arg0 = Shiboken::Converter<double >::toCpp(arg);

>             // INJECT-CODE: <modify-function><inject-code class="target" position="beginning">
>             // Uses: pre method call custom code.

>             py_result = Shiboken::Converter<double >::toPython(
>                 PyInjectCode_cptr(self)->InjectCode::overloadedMethod(cpp_arg0)
>             );

>             // INJECT-CODE: <modify-function><inject-code class="target" position="end">
>             // Uses: post method call custom code.

>         } else if (PyNumber_Check(arg)) {
>             (... other overload calling code ...)
>         } else goto PyInjectCode_overloadedMethod_TypeError;

>         if (PyErr_Occurred() || !py_result)
>             return {};

>         return py_result;

>         PyInjectCode_overloadedMethod_TypeError:
>             PyErr_SetString(PyExc_TypeError, "'overloadedMethod()' called with wrong parameters.");
>             return {};
>     }

## Code Injection for Wrapped Classes

### On The Native Side

Those injections go in the body of the `CLASSNAME_wrapper.cpp` file for the wrapped class.

>     // Start of ``CLASSNAME_wrapper.cpp``
>     #define protected public
>     // default includes
>     #include <shiboken.h>
>     (...)
>     #include "injectcode_wrapper.h"
>     using namespace Shiboken;

>     // INJECT-CODE: <value/object-type><inject-code class="native" position="beginning">
>     // Uses: prototype declarations

>     (... C++ wrapper virtual methods, if any ...)

>     (... Python wrapper code ...)

>     PyAPI_FUNC(void)
>     init_injectcode(PyObject *module)
>     {
>         (...)
>     }

>     (...)

>     // INJECT-CODE: <value/object-type><inject-code class="native" position="end">
>     // Uses: definition of functions prototyped at ``native/beginning``.

>     // End of ``CLASSNAME_wrapper.cpp``

### On The Target Side

Code injections to the class Python initialization function.

>     // Start of ``CLASSNAME_wrapper.cpp``

>     (...)

>     PyAPI_FUNC(void)
>     init_injectcode(PyObject *module)
>     {
>         // INJECT-CODE: <value/object-type><inject-code class="target" position="beginning">
>         // Uses: Alter something in the PyInjectCode_Type (tp_flags value for example)
>         // before registering it.

>         if (PyType_Ready(&PyInjectCode_Type) < 0)
>             return;

>         Py_INCREF(&PyInjectCode_Type);
>         PyModule_AddObject(module, "InjectCode",
>             ((PyObject*)&PyInjectCode_Type));

>         // INJECT-CODE: <value/object-type><inject-code class="target" position="end">
>         // Uses: do something right after the class is registered, like set some static
>         // variable injected on this same file elsewhere.
>     }

>     (...)

>     // End of ``CLASSNAME_wrapper.cpp``

## Code Injection for Modules

The C++ libraries are wrapped as Python modules, a collection of classes, functions, enums and namespaces. Shiboken creates wrapper files for all of them and also one extra `MODULENAME_module_wrapper.cpp` to register the whole module. Code injection xml tags who have the `typesystem` tag as parent will be put on this file.

### On The Native Side

This works exactly as the class wrapper code injections On The Native Side.

### On The Target Side

This is very similar to class wrapper code injections On The Target Side. Notice that the inject code at `target/end` is inserted before the check for errors to prevent bad custom code to pass unnoticed.

>     // Start of ``MODULENAME_module_wrapper.cpp``

>     (...)
>     initMODULENAME()
>     {
>         // INJECT-CODE: <typesystem><inject-code class="target" position="beginning">
>         // Uses: do something before the module is created.

>         PyObject *module = Py_InitModule("MODULENAME", MODULENAME_methods);

>         (... initialization of wrapped classes, namespaces, functions and enums ...)

>         // INJECT-CODE: <typesystem><inject-code class="target" position="end">
>         // Uses: do something after the module is registered and initialized.

>         if (PyErr_Occurred())
>             Py_FatalError("can't initialize module sample");
>     }

>     (...)

>     // Start of ``MODULENAME_module_wrapper.cpp``

In addition, code can be injected into the module header by specifying `target` and `declaration`. This is useful for type definitions.