added feature to pass void* pointers by popular demand

example/example14.cpp

 /*
-    example/example14.cpp -- opaque types
+    example/example14.cpp -- opaque types, passing void pointers
 
     Copyright (c) 2015 Wenzel Jakob <wenzel@inf.ethz.ch>
 
@@ -26,4 +26,7 @@ void init_ex14(py::module &m) {
         for (auto entry : l)
            std::cout << "  " << entry << 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; });
 }

example/example14.py

@@ -12,3 +12,7 @@ print_opaque_list(l)
 print("Back element is %s" % l.back())
 l.pop_back()
 print_opaque_list(l)
+
+from example import return_void_ptr, print_void_ptr
+
+print_void_ptr(return_void_ptr())

example/example14.ref

@@ -4,3 +4,4 @@ Opaque list:
 Back element is Element 2
 Opaque list: 
   Element 1
+Got void ptr : 1234

include/pybind11/cast.h

@@ -323,7 +323,26 @@ public:
     PYBIND11_TYPE_CASTER(void_type, _("NoneType"));
 };
 
-template <> class type_caster<void> : public type_caster<void_type> { };
+template <> class type_caster<void> : public type_caster<void_type> {
+public:
+    using type_caster<void_type>::cast;
+
+    bool load(handle h, bool) {
+        capsule c(h, true);
+        if (!c.check())
+            return false;
+        value = (void *) c;
+        return true;
+    }
+
+    static handle cast(void *ptr, return_value_policy /* policy */, handle /* parent */) {
+        return capsule(ptr).inc_ref();
+    }
+    operator void *() { return value; }
+private:
+    void *value;
+};
+
 template <> class type_caster<std::nullptr_t> : public type_caster<void_type> { };
 
 template <> class type_caster<bool> {