Add the buffer interface for wrapped STL vectors

Allows use of vectors as python buffers, so for example they can be adopted without a copy by numpy.asarray
Allows faster conversion of numeric buffers to vectors with memcpy instead of individually casting the elements

Author: patstew

include/pybind11/stl_bind.h

@@ -63,6 +63,7 @@ struct is_comparable<T, enable_if_t<container_traits<T>::is_pair>> {
 template <typename, typename, typename... Args> void vector_if_copy_constructible(const Args&...) { }
 template <typename, typename, typename... Args> void vector_if_equal_operator(const Args&...) { }
 template <typename, typename, typename... Args> void vector_if_insertion_operator(const Args&...) { }
+template <typename, typename, typename, typename... Args> void vector_buffer(const Args&...) { }
 
 template<typename Vector, typename Class_, enable_if_t<std::is_copy_constructible<typename Vector::value_type>::value, int> = 0>
 void vector_if_copy_constructible(Class_ &cl) {
@@ -126,6 +127,23 @@ template <typename Vector, typename Class_> auto vector_if_insertion_operator(Cl
     );
 }
 
+// Provide the buffer interface for vectors if we have data(), the contained type is numeric and we have a format for it
+template <typename Class_, typename Vector, typename T>
+enable_if_t<std::is_arithmetic<T>::value && !std::is_same<T, bool>::value, decltype(format_descriptor<T>::format(), std::declval<Vector>().data(), void())>
+vector_buffer(Class_& cl) {
+    cl.def_buffer([](Vector& v) -> py::buffer_info {
+        return py::buffer_info(v.data(), sizeof(T), py::format_descriptor<T>::format(), 1, {v.size()}, {sizeof(T)});
+    });
+
+    cl.def("__init__", [](Vector& vec, py::buffer buf) {
+        auto info = buf.request();
+        if (info.ndim != 1 || info.strides[0] != info.itemsize || info.itemsize != sizeof(T) || info.format != py::format_descriptor<T>::format())
+            throw pybind11::type_error();
+        new (&vec) Vector(info.shape[0]);
+        memcpy(vec.data(), info.ptr, info.shape[0] * sizeof(T));
+    });
+}
+
 NAMESPACE_END(detail)
 
 //
@@ -152,6 +170,9 @@ pybind11::class_<Vector, holder_type> bind_vector(pybind11::module &m, std::stri
     // Register stream insertion operator (if possible)
     detail::vector_if_insertion_operator<Vector, Class_>(cl, name);
 
+    // Register the buffer interface (if possible)
+    detail::vector_buffer<Class_, Vector, T>(cl);
+
     cl.def("__init__", [](Vector &v, iterable it) {
         new (&v) Vector();
         try {

tests/test_stl_binders.py

@@ -1,3 +1,8 @@
+import pytest
+
+with pytest.suppress(ImportError):
+    import numpy as np
+
 def test_vector_int():
     from pybind11_tests import VectorInt
 
@@ -25,6 +30,21 @@ def test_vector_int():
     del v_int2[0]
     assert v_int2 == VectorInt([0, 99, 2, 3])
 
+@pytest.requires_numpy
+def test_vector_buffer():
+    from pybind11_tests import VectorInt
+
+    a = np.array([1,2,3,4], dtype=np.int32)
+    with pytest.raises(TypeError) as e_info:
+        VectorInt(a)
+
+    a = np.array([1,2,3,4], dtype=np.uintc)
+    v = VectorInt(a)
+    assert v[2] == 3
+    m = np.asarray(v)
+    m[2] = 5
+    assert v[2] == 5
+
 
 def test_vector_custom():
     from pybind11_tests import El, VectorEl, VectorVectorEl