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109 lines
3.8 KiB
109 lines
3.8 KiB
Functional
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##########
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The following features must be enabled by including :file:`pybind11/functional.h`.
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Callbacks and passing anonymous functions
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=========================================
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The C++11 standard brought lambda functions and the generic polymorphic
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function wrapper ``std::function<>`` to the C++ programming language, which
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enable powerful new ways of working with functions. Lambda functions come in
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two flavors: stateless lambda function resemble classic function pointers that
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link to an anonymous piece of code, while stateful lambda functions
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additionally depend on captured variables that are stored in an anonymous
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*lambda closure object*.
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Here is a simple example of a C++ function that takes an arbitrary function
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(stateful or stateless) with signature ``int -> int`` as an argument and runs
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it with the value 10.
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.. code-block:: cpp
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int func_arg(const std::function<int(int)> &f) {
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return f(10);
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}
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The example below is more involved: it takes a function of signature ``int -> int``
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and returns another function of the same kind. The return value is a stateful
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lambda function, which stores the value ``f`` in the capture object and adds 1 to
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its return value upon execution.
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.. code-block:: cpp
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std::function<int(int)> func_ret(const std::function<int(int)> &f) {
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return [f](int i) {
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return f(i) + 1;
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};
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}
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This example demonstrates using python named parameters in C++ callbacks which
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requires using ``py::cpp_function`` as a wrapper. Usage is similar to defining
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methods of classes:
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.. code-block:: cpp
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py::cpp_function func_cpp() {
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return py::cpp_function([](int i) { return i+1; },
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py::arg("number"));
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}
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After including the extra header file :file:`pybind11/functional.h`, it is almost
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trivial to generate binding code for all of these functions.
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.. code-block:: cpp
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#include <pybind11/functional.h>
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PYBIND11_MODULE(example, m) {
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m.def("func_arg", &func_arg);
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m.def("func_ret", &func_ret);
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m.def("func_cpp", &func_cpp);
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}
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The following interactive session shows how to call them from Python.
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.. code-block:: pycon
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$ python
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>>> import example
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>>> def square(i):
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... return i * i
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...
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>>> example.func_arg(square)
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100L
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>>> square_plus_1 = example.func_ret(square)
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>>> square_plus_1(4)
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17L
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>>> plus_1 = func_cpp()
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>>> plus_1(number=43)
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44L
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.. warning::
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Keep in mind that passing a function from C++ to Python (or vice versa)
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will instantiate a piece of wrapper code that translates function
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invocations between the two languages. Naturally, this translation
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increases the computational cost of each function call somewhat. A
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problematic situation can arise when a function is copied back and forth
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between Python and C++ many times in a row, in which case the underlying
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wrappers will accumulate correspondingly. The resulting long sequence of
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C++ -> Python -> C++ -> ... roundtrips can significantly decrease
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performance.
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There is one exception: pybind11 detects case where a stateless function
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(i.e. a function pointer or a lambda function without captured variables)
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is passed as an argument to another C++ function exposed in Python. In this
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case, there is no overhead. Pybind11 will extract the underlying C++
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function pointer from the wrapped function to sidestep a potential C++ ->
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Python -> C++ roundtrip. This is demonstrated in :file:`tests/test_callbacks.cpp`.
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.. note::
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This functionality is very useful when generating bindings for callbacks in
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C++ libraries (e.g. GUI libraries, asynchronous networking libraries, etc.).
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The file :file:`tests/test_callbacks.cpp` contains a complete example
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that demonstrates how to work with callbacks and anonymous functions in
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more detail.
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