Allow nonconstructible holders

include/pybind11/pybind11.h

@@ -778,7 +778,12 @@ class cpp_function : public function {
         } else {
             if (overloads->is_constructor && !self_value_and_holder.holder_constructed()) {
                 auto *pi = reinterpret_cast<instance *>(parent.ptr());
-                self_value_and_holder.type->init_instance(pi, nullptr);
+                try {
+                    self_value_and_holder.type->init_instance(pi, nullptr);
+                } catch (const std::runtime_error &e) {
+                    PyErr_SetString(PyExc_RuntimeError, e.what());
+                    return nullptr;
+                }
             }
             return result.ptr();
         }
@@ -1298,6 +1303,34 @@ class class_ : public detail::generic_type {
     }
 
 private:
+    /// Initialize holder from pointer when the underlying object is destructible (public destructor)
+    template <typename T = type>
+    detail::enable_if_t<
+        !detail::is_instantiation<std::shared_ptr, holder_type>::value ||
+        std::is_destructible<T>::value
+    , void>
+    static init_shared_holder_from_pointer(detail::value_and_holder &v_h) {
+        new (std::addressof(v_h.holder<holder_type>())) holder_type(v_h.value_ptr<type>());
+        v_h.set_holder_constructed();
+    }
+
+    /// Do *not* initialize holder from pointer when the underlying object is *not* destructible
+    template <typename T = type>
+    detail::enable_if_t<
+        detail::is_instantiation<std::shared_ptr, holder_type>::value &&
+        !std::is_destructible<T>::value
+    , void>
+    static init_shared_holder_from_pointer(detail::value_and_holder &) {
+        pybind11_fail("Unable to construct C++ holder"
+#if defined(NDEBUG)
+            " (compile in debug mode for type information)"
+#else
+            ": cannot construct a `" + type_id<holder_type>() + "' holder around a non-destructible `" +
+            type_id<type>() + "' pointer"
+#endif
+        );
+    }
+
     /// Initialize holder object, variant 1: object derives from enable_shared_from_this
     template <typename T>
     static void init_holder(detail::instance *inst, detail::value_and_holder &v_h,
@@ -1312,8 +1345,7 @@ class class_ : public detail::generic_type {
         } catch (const std::bad_weak_ptr &) {}
 
         if (!v_h.holder_constructed() && inst->owned) {
-            new (std::addressof(v_h.holder<holder_type>())) holder_type(v_h.value_ptr<type>());
-            v_h.set_holder_constructed();
+            init_shared_holder_from_pointer(v_h);
         }
     }
 
@@ -1334,8 +1366,7 @@ class class_ : public detail::generic_type {
             init_holder_from_existing(v_h, holder_ptr, std::is_copy_constructible<holder_type>());
             v_h.set_holder_constructed();
         } else if (inst->owned || detail::always_construct_holder<holder_type>::value) {
-            new (std::addressof(v_h.holder<holder_type>())) holder_type(v_h.value_ptr<type>());
-            v_h.set_holder_constructed();
+            init_shared_holder_from_pointer(v_h);
         }
     }
 

tests/test_smart_ptr.cpp

@@ -212,6 +212,26 @@ TEST_SUBMODULE(smart_ptr, m) {
     py::class_<MyObject4b, MyObject4a>(m, "MyObject4b")
         .def(py::init<int>());
 
+    // Object with non-public destructor in a shared_ptr
+    class MyObject4c {
+    public:
+        MyObject4c() { print_created(this); }
+    protected:
+        ~MyObject4c() { print_destroyed(this); }
+    };
+    py::class_<MyObject4c, std::shared_ptr<MyObject4c>>(m, "MyObject4c")
+        .def(py::init());
+
+    // Object with non-public destructor in a shared_ptr with std::enable_shared_from_this<>
+    class MyObject4d : public std::enable_shared_from_this<MyObject4d> {
+    public:
+        MyObject4d() { print_created(this); }
+    protected:
+        ~MyObject4d() { print_destroyed(this); }
+    };
+    py::class_<MyObject4d, std::shared_ptr<MyObject4d>>(m, "MyObject4d")
+        .def(py::init());
+
     // test_large_holder
     class MyObject5 { // managed by huge_unique_ptr
     public:

tests/test_smart_ptr.py

@@ -136,6 +136,20 @@ def test_unique_deleter():
     assert cstats4b.alive() == 0  # Should be deleted
 
 
+def test_shared_ptr_non_public_destructor():
+    """#1178: Shared Pointers and Protected Destructors"""
+    with pytest.raises(RuntimeError) as excinfo:
+        m.MyObject4c()
+    assert "Unable to construct C++ holder" in str(excinfo.value)
+
+
+def test_shared_ptr_from_this_non_public_destructor():
+    """#1178: Shared Pointers and Protected Destructors"""
+    with pytest.raises(RuntimeError) as excinfo:
+        m.MyObject4d()
+    assert "Unable to construct C++ holder" in str(excinfo.value)
+
+
 def test_large_holder():
     o = m.MyObject5(5)
     assert o.value == 5