STL containers

Automatic conversion

When including the additional header file pybind11/stl.h, conversions between std::vector<>/std::list<>/std::array<>, std::set<>/std::unordered_set<>, and std::map<>/std::unordered_map<> and the Python list, set and dict data structures are automatically enabled. The types std::pair<> and std::tuple<> are already supported out of the box with just the core pybind11/pybind11.h header.

The major downside of these implicit conversions is that containers must be converted (i.e. copied) on every Python->C++ and C++->Python transition, which can have implications on the program semantics and performance. Please read the next sections for more details and alternative approaches that avoid this.

Note

Arbitrary nesting of any of these types is possible.

See also

The file tests/test_python_types.cpp contains a complete example that demonstrates how to pass STL data types in more detail.

Making opaque types

pybind11 heavily relies on a template matching mechanism to convert parameters and return values that are constructed from STL data types such as vectors, linked lists, hash tables, etc. This even works in a recursive manner, for instance to deal with lists of hash maps of pairs of elementary and custom types, etc.

However, a fundamental limitation of this approach is that internal conversions between Python and C++ types involve a copy operation that prevents pass-by-reference semantics. What does this mean?

Suppose we bind the following function

void append_1(std::vector<int> &v) {
   v.push_back(1);
}

and call it from Python, the following happens:

>>> v = [5, 6]
>>> append_1(v)
>>> print(v)
[5, 6]

As you can see, when passing STL data structures by reference, modifications are not propagated back the Python side. A similar situation arises when exposing STL data structures using the def_readwrite or def_readonly functions:

/* ... definition ... */

class MyClass {
    std::vector<int> contents;
};

/* ... binding code ... */

py::class_<MyClass>(m, "MyClass")
    .def(py::init<>())
    .def_readwrite("contents", &MyClass::contents);

In this case, properties can be read and written in their entirety. However, an append operation involving such a list type has no effect:

>>> m = MyClass()
>>> m.contents = [5, 6]
>>> print(m.contents)
[5, 6]
>>> m.contents.append(7)
>>> print(m.contents)
[5, 6]

Finally, the involved copy operations can be costly when dealing with very large lists. To deal with all of the above situations, pybind11 provides a macro named PYBIND11_MAKE_OPAQUE(T) that disables the template-based conversion machinery of types, thus rendering them opaque. The contents of opaque objects are never inspected or extracted, hence they can be passed by reference. For instance, to turn std::vector<int> into an opaque type, add the declaration

PYBIND11_MAKE_OPAQUE(std::vector<int>);

before any binding code (e.g. invocations to class_::def(), etc.). This macro must be specified at the top level (and outside of any namespaces), since it instantiates a partial template overload. If your binding code consists of multiple compilation units, it must be present in every file preceding any usage of std::vector<int>. Opaque types must also have a corresponding class_ declaration to associate them with a name in Python, and to define a set of available operations, e.g.:

py::class_<std::vector<int>>(m, "IntVector")
    .def(py::init<>())
    .def("clear", &std::vector<int>::clear)
    .def("pop_back", &std::vector<int>::pop_back)
    .def("__len__", [](const std::vector<int> &v) { return v.size(); })
    .def("__iter__", [](std::vector<int> &v) {
       return py::make_iterator(v.begin(), v.end());
    }, py::keep_alive<0, 1>()) /* Keep vector alive while iterator is used */
    // ....

The ability to expose STL containers as native Python objects is a fairly common request, hence pybind11 also provides an optional header file named pybind11/stl_bind.h that does exactly this. The mapped containers try to match the behavior of their native Python counterparts as much as possible.

The following example showcases usage of pybind11/stl_bind.h:

// Don't forget this
#include <pybind11/stl_bind.h>

PYBIND11_MAKE_OPAQUE(std::vector<int>);
PYBIND11_MAKE_OPAQUE(std::map<std::string, double>);

// ...

// later in binding code:
py::bind_vector<std::vector<int>>(m, "VectorInt");
py::bind_map<std::map<std::string, double>>(m, "MapStringDouble");

Please take a look at the General notes regarding convenience macros before using the PYBIND11_MAKE_OPAQUE macro.

See also

The file tests/test_opaque_types.cpp contains a complete example that demonstrates how to create and expose opaque types using pybind11 in more detail.

The file tests/test_stl_binders.cpp shows how to use the convenience STL container wrappers.