enable passing C++ instances to void*-valued arguments

example/example14.cpp

@@ -49,10 +49,10 @@ void init_ex14(py::module &m) {
         std::cout << "]" << std::endl;
     });
 
-    m.def("return_void_ptr", []() { return (void *) 1234; });
-    m.def("print_void_ptr", [](void *ptr) { std::cout << "Got void ptr : " << (uint64_t) ptr << std::endl; });
+    m.def("return_void_ptr", []() { return (void *) 0x1234; });
+    m.def("print_void_ptr", [](void *ptr) { std::cout << "Got void ptr : 0x" << std::hex << (uint64_t) ptr << std::endl; });
     m.def("return_null_str", []() { return (char *) nullptr; });
-    m.def("print_null_str", [](char *ptr) { std::cout << "Got null str : " << (uint64_t) ptr << std::endl; });
+    m.def("print_null_str", [](char *ptr) { std::cout << "Got null str : 0x" << std::hex << (uint64_t) ptr << std::endl; });
 
     m.def("return_unique_ptr", []() -> std::unique_ptr<StringList> {
         StringList *result = new StringList();

example/example14.py

@@ -35,6 +35,11 @@ print_opaque_list(cvp.stringList)
 print_void_ptr(return_void_ptr())
 print_void_ptr(Example1())  # Should also work for other C++ types
 
+try:
+    print_void_ptr([1, 2, 3])  # This should not work
+except Exception as e:
+    print("Caught expected exception: " + str(e))
+
 print(return_null_str())
 print_null_str(return_null_str())
 

example/example14.ref

@@ -5,8 +5,14 @@ Back element is Element 2
 Opaque list: [Element 1]
 Opaque list: []
 Opaque list: [Element 1, Element 3]
-Got void ptr : 1234
+Got void ptr : 0x1234
+Called Example1 default constructor..
+Got void ptr : 0x7f9ba0f3c430
+Called Example1 destructor (0)
+Caught expected exception: Incompatible function arguments. The following argument types are supported:
+    1. (capsule) -> NoneType
+
 None
-Got null str : 0
-<example.StringList object at 0x104a47500>
+Got null str : 0x0
+<example.StringList object at 0x10d3277a0>
 Opaque list: [some value]

include/pybind11/cast.h

@@ -52,15 +52,18 @@ PYBIND11_NOINLINE inline internals &get_internals() {
     return *internals_ptr;
 }
 
-PYBIND11_NOINLINE inline detail::type_info* get_type_info(PyTypeObject *type) {
+PYBIND11_NOINLINE inline detail::type_info* get_type_info(PyTypeObject *type, bool throw_if_missing = true) {
     auto const &type_dict = get_internals().registered_types_py;
     do {
         auto it = type_dict.find(type);
         if (it != type_dict.end())
             return (detail::type_info *) it->second;
         type = type->tp_base;
-        if (!type)
+        if (!type) {
+            if (throw_if_missing)
                 pybind11_fail("pybind11::detail::get_type_info: unable to find type object!");
+            return nullptr;
+        }
     } while (true);
 }
 
@@ -382,13 +385,24 @@ public:
             value = nullptr;
             return true;
         }
+
+        /* Check if this is a capsule */
         capsule c(h, true);
-        if (!c.check())
-            return false;
+        if (c.check()) {
             value = (void *) c;
             return true;
         }
 
+        /* Check if this is a C++ type */
+        if (get_type_info((PyTypeObject *) h.get_type().ptr(), false)) {
+            value = ((instance<void> *) h.ptr())->value;
+            return true;
+        }
+
+        /* Fail */
+        return false;
+    }
+
     static handle cast(const void *ptr, return_value_policy /* policy */, handle /* parent */) {
         if (ptr)
             return capsule(ptr).release();

include/pybind11/pybind11.h

@@ -450,7 +450,7 @@ protected:
             return nullptr;
         } else {
             if (overloads->is_constructor) {
-                /* When a construtor ran successfully, the corresponding
+                /* When a constructor ran successfully, the corresponding
                    holder type (e.g. std::unique_ptr) must still be initialized. */
                 PyObject *inst = PyTuple_GetItem(args, 0);
                 auto tinfo = detail::get_type_info(Py_TYPE(inst));