mirror of
https://github.com/pybind/pybind11.git
synced 2024-11-30 00:47:12 +00:00
443ab5946b
This commit also adds `doc()` to `object_api` as a shortcut for the
`attr("__doc__")` accessor.
The module macro changes from:
```c++
PYBIND11_PLUGIN(example) {
pybind11::module m("example", "pybind11 example plugin");
m.def("add", [](int a, int b) { return a + b; });
return m.ptr();
}
```
to:
```c++
PYBIND11_MODULE(example, m) {
m.doc() = "pybind11 example plugin";
m.def("add", [](int a, int b) { return a + b; });
}
```
Using the old macro results in a deprecation warning. The warning
actually points to the `pybind11_init` function (since attributes
don't bind to macros), but the message should be quite clear:
"PYBIND11_PLUGIN is deprecated, use PYBIND11_MODULE".
110 lines
3.8 KiB
ReStructuredText
110 lines
3.8 KiB
ReStructuredText
Functional
|
|
##########
|
|
|
|
The following features must be enabled by including :file:`pybind11/functional.h`.
|
|
|
|
|
|
Callbacks and passing anonymous functions
|
|
=========================================
|
|
|
|
The C++11 standard brought lambda functions and the generic polymorphic
|
|
function wrapper ``std::function<>`` to the C++ programming language, which
|
|
enable powerful new ways of working with functions. Lambda functions come in
|
|
two flavors: stateless lambda function resemble classic function pointers that
|
|
link to an anonymous piece of code, while stateful lambda functions
|
|
additionally depend on captured variables that are stored in an anonymous
|
|
*lambda closure object*.
|
|
|
|
Here is a simple example of a C++ function that takes an arbitrary function
|
|
(stateful or stateless) with signature ``int -> int`` as an argument and runs
|
|
it with the value 10.
|
|
|
|
.. code-block:: cpp
|
|
|
|
int func_arg(const std::function<int(int)> &f) {
|
|
return f(10);
|
|
}
|
|
|
|
The example below is more involved: it takes a function of signature ``int -> int``
|
|
and returns another function of the same kind. The return value is a stateful
|
|
lambda function, which stores the value ``f`` in the capture object and adds 1 to
|
|
its return value upon execution.
|
|
|
|
.. code-block:: cpp
|
|
|
|
std::function<int(int)> func_ret(const std::function<int(int)> &f) {
|
|
return [f](int i) {
|
|
return f(i) + 1;
|
|
};
|
|
}
|
|
|
|
This example demonstrates using python named parameters in C++ callbacks which
|
|
requires using ``py::cpp_function`` as a wrapper. Usage is similar to defining
|
|
methods of classes:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::cpp_function func_cpp() {
|
|
return py::cpp_function([](int i) { return i+1; },
|
|
py::arg("number"));
|
|
}
|
|
|
|
After including the extra header file :file:`pybind11/functional.h`, it is almost
|
|
trivial to generate binding code for all of these functions.
|
|
|
|
.. code-block:: cpp
|
|
|
|
#include <pybind11/functional.h>
|
|
|
|
PYBIND11_MODULE(example, m) {
|
|
m.def("func_arg", &func_arg);
|
|
m.def("func_ret", &func_ret);
|
|
m.def("func_cpp", &func_cpp);
|
|
}
|
|
|
|
The following interactive session shows how to call them from Python.
|
|
|
|
.. code-block:: pycon
|
|
|
|
$ python
|
|
>>> import example
|
|
>>> def square(i):
|
|
... return i * i
|
|
...
|
|
>>> example.func_arg(square)
|
|
100L
|
|
>>> square_plus_1 = example.func_ret(square)
|
|
>>> square_plus_1(4)
|
|
17L
|
|
>>> plus_1 = func_cpp()
|
|
>>> plus_1(number=43)
|
|
44L
|
|
|
|
.. warning::
|
|
|
|
Keep in mind that passing a function from C++ to Python (or vice versa)
|
|
will instantiate a piece of wrapper code that translates function
|
|
invocations between the two languages. Naturally, this translation
|
|
increases the computational cost of each function call somewhat. A
|
|
problematic situation can arise when a function is copied back and forth
|
|
between Python and C++ many times in a row, in which case the underlying
|
|
wrappers will accumulate correspondingly. The resulting long sequence of
|
|
C++ -> Python -> C++ -> ... roundtrips can significantly decrease
|
|
performance.
|
|
|
|
There is one exception: pybind11 detects case where a stateless function
|
|
(i.e. a function pointer or a lambda function without captured variables)
|
|
is passed as an argument to another C++ function exposed in Python. In this
|
|
case, there is no overhead. Pybind11 will extract the underlying C++
|
|
function pointer from the wrapped function to sidestep a potential C++ ->
|
|
Python -> C++ roundtrip. This is demonstrated in :file:`tests/test_callbacks.cpp`.
|
|
|
|
.. note::
|
|
|
|
This functionality is very useful when generating bindings for callbacks in
|
|
C++ libraries (e.g. GUI libraries, asynchronous networking libraries, etc.).
|
|
|
|
The file :file:`tests/test_callbacks.cpp` contains a complete example
|
|
that demonstrates how to work with callbacks and anonymous functions in
|
|
more detail.
|