added destructor-only version of capsule & tests

Author: wjakob

include/pybind11/pytypes.h

@@ -1032,6 +1032,16 @@ class capsule : public object {
             pybind11_fail("Could not set capsule context!");
     }
 
+    capsule(void (*destructor)()) {
+        m_ptr = PyCapsule_New(reinterpret_cast<void *>(destructor), nullptr, [](PyObject *o) {
+            auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, nullptr));
+            destructor();
+        });
+
+        if (!m_ptr)
+            pybind11_fail("Could not allocate capsule object!");
+    }
+
     template <typename T> operator T *() const {
         T * result = static_cast<T *>(PyCapsule_GetPointer(m_ptr, nullptr));
         if (!result) pybind11_fail("Unable to extract capsule contents!");

tests/test_python_types.cpp

@@ -470,6 +470,24 @@ test_initializer python_types([](py::module &m) {
     m.def("return_none_bool",   []() -> bool *        { return nullptr; });
     m.def("return_none_int",    []() -> int *         { return nullptr; });
     m.def("return_none_float",  []() -> float *       { return nullptr; });
+
+    m.def("return_capsule_with_destructor",
+        []() {
+            py::print("creating capsule");
+            return py::capsule([]() {
+                py::print("destructing capsule");
+            });
+        }
+    );
+
+    m.def("return_capsule_with_destructor_2",
+        []() {
+            py::print("creating capsule");
+            return py::capsule((void *) 1234, [](void *ptr) {
+                py::print("destructing capsule: {}"_s.format((size_t) ptr));
+            });
+        }
+    );
 });
 
 #if defined(_MSC_VER)

tests/test_python_types.py

@@ -512,3 +512,22 @@ def test_builtins_cast_return_none():
     assert m.return_none_bool() is None
     assert m.return_none_int() is None
     assert m.return_none_float() is None
+
+def test_capsule_with_destructor(capture):
+    import pybind11_tests as m
+    with capture:
+        a = m.return_capsule_with_destructor()
+        del a
+    assert capture.unordered == """
+        creating capsule
+        destructing capsule
+    """
+
+    with capture:
+        a = m.return_capsule_with_destructor_2()
+        del a
+    print(capture)
+    assert capture.unordered == """
+        creating capsule
+        destructing capsule: 1234
+    """