Use object class to hold partially converted python objects.

Using object class to hold converted object automatically deallocates
object if an exception is thrown or scope is left before constructing
complete Python object.

Additionally added method object::release() that allows to release
ownership of python object without decreasing its reference count.
diff --git a/docs/reference.rst b/docs/reference.rst
index bf7a8e2..2bc62b5 100644
--- a/docs/reference.rst
+++ b/docs/reference.rst
@@ -59,7 +59,7 @@
 
     Creates a :class:`handle` from the given raw Python object pointer.
 
-.. function:: PyObject * handle::ptr()
+.. function:: PyObject * handle::ptr() const
 
     Return the ``PyObject *`` underlying a :class:`handle`.
 
@@ -167,6 +167,12 @@
     Move constructor; steals the object from ``other`` and preserves its
     reference count.
 
+.. function:: PyObject* object::release()
+
+    Release ownership of underlying ``PyObject *``. Returns raw Python object
+    pointer without decreasing its reference count and resets handle to
+    ``nullptr``-valued pointer.
+
 .. function:: object::~object()
 
     Constructor, which automatically calls :func:`handle::dec_ref()`.
diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index 22f7ffb..a891034 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -398,16 +398,15 @@
     }
 
     static PyObject *cast(const type &src, return_value_policy policy, PyObject *parent) {
-        PyObject *o1 = type_caster<typename decay<T1>::type>::cast(src.first, policy, parent);
-        PyObject *o2 = type_caster<typename decay<T2>::type>::cast(src.second, policy, parent);
-        if (!o1 || !o2) {
-            Py_XDECREF(o1);
-            Py_XDECREF(o2);
+        object o1(type_caster<typename decay<T1>::type>::cast(src.first, policy, parent), false);
+        object o2(type_caster<typename decay<T2>::type>::cast(src.second, policy, parent), false);
+        if (!o1 || !o2)
             return nullptr;
-        }
         PyObject *tuple = PyTuple_New(2);
-        PyTuple_SetItem(tuple, 0, o1);
-        PyTuple_SetItem(tuple, 1, o2);
+        if (!tuple)
+            return nullptr;
+        PyTuple_SetItem(tuple, 0, o1.release());
+        PyTuple_SetItem(tuple, 1, o2.release());
         return tuple;
     }
 
@@ -502,25 +501,19 @@
 
     /* Implementation: Convert a C++ tuple into a Python tuple */
     template <size_t ... Indices> static PyObject *cast(const type &src, return_value_policy policy, PyObject *parent, index_sequence<Indices...>) {
-        std::array<PyObject *, size> results {{
-            type_caster<typename decay<Tuple>::type>::cast(std::get<Indices>(src), policy, parent)...
+        std::array<object, size> results {{
+            object(type_caster<typename decay<Tuple>::type>::cast(std::get<Indices>(src), policy, parent), false)...
         }};
-        bool success = true;
-        for (auto result : results)
-            if (result == nullptr)
-                success = false;
-        if (success) {
-            PyObject *tuple = PyTuple_New(size);
-            int counter = 0;
-            for (auto result : results)
-                PyTuple_SetItem(tuple, counter++, result);
-            return tuple;
-        } else {
-            for (auto result : results) {
-                Py_XDECREF(result);
-            }
+        for (const auto & result : results)
+            if (!result)
+                return nullptr;
+        PyObject *tuple = PyTuple_New(size);
+        if (!tuple)
             return nullptr;
-        }
+        int counter = 0;
+        for (auto & result : results)
+            PyTuple_SetItem(tuple, counter++, result.release());
+        return tuple;
     }
 
 protected:
@@ -600,26 +593,20 @@
 
 template <typename... Args> inline object handle::call(Args&&... args_) {
     const size_t size = sizeof...(Args);
-    std::array<PyObject *, size> args{
-        { detail::type_caster<typename detail::decay<Args>::type>::cast(
-            std::forward<Args>(args_), return_value_policy::reference, nullptr)... }
+    std::array<object, size> args{
+        { object(detail::type_caster<typename detail::decay<Args>::type>::cast(
+            std::forward<Args>(args_), return_value_policy::reference, nullptr), false)... }
     };
-    bool fail = false;
-    for (auto result : args)
-        if (result == nullptr)
-            fail = true;
-    if (fail) {
-        for (auto result : args) {
-            Py_XDECREF(result);
-        }
+    for (const auto & result : args)
+        if (!result)
+            throw cast_error("handle::call(): unable to convert input arguments to Python objects");
+    object tuple(PyTuple_New(size), false);
+    if (!tuple)
         throw cast_error("handle::call(): unable to convert input arguments to Python objects");
-    }
-    PyObject *tuple = PyTuple_New(size);
     int counter = 0;
-    for (auto result : args)
-        PyTuple_SetItem(tuple, counter++, result);
-    PyObject *result = PyObject_CallObject(m_ptr, tuple);
-    Py_DECREF(tuple);
+    for (auto & result : args)
+        PyTuple_SetItem(tuple.ptr(), counter++, result.release());
+    PyObject *result = PyObject_CallObject(m_ptr, tuple.ptr());
     if (result == nullptr && PyErr_Occurred())
         throw error_already_set();
     return object(result, false);
diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index c0bb47b..eedb73d 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -29,8 +29,7 @@
     handle() : m_ptr(nullptr) { }
     handle(const handle &other) : m_ptr(other.m_ptr) { }
     handle(PyObject *ptr) : m_ptr(ptr) { }
-    PyObject *ptr() { return m_ptr; }
-    const PyObject *ptr() const { return m_ptr; }
+    PyObject *ptr() const { return m_ptr; }
     void inc_ref() const { Py_XINCREF(m_ptr); }
     void dec_ref() const { Py_XDECREF(m_ptr); }
     int ref_count() const { return (int) Py_REFCNT(m_ptr); }
@@ -60,6 +59,12 @@
     object(object &&other) { m_ptr = other.m_ptr; other.m_ptr = nullptr; }
     ~object() { dec_ref(); }
 
+    PyObject * release() {
+      PyObject *tmp = m_ptr;
+      m_ptr = nullptr;
+      return tmp;
+    }
+
     object& operator=(object &other) {
         Py_XINCREF(other.m_ptr);
         Py_XDECREF(m_ptr);
diff --git a/include/pybind11/stl.h b/include/pybind11/stl.h
index cee5be7..c4f4b19 100644
--- a/include/pybind11/stl.h
+++ b/include/pybind11/stl.h
@@ -43,17 +43,17 @@
     }
 
     static PyObject *cast(const type &src, return_value_policy policy, PyObject *parent) {
-        PyObject *list = PyList_New(src.size());
+        object list(PyList_New(src.size()), false);
+        if (!list)
+          return nullptr;
         size_t index = 0;
         for (auto const &value: src) {
-            PyObject *value_ = value_conv::cast(value, policy, parent);
-            if (!value_) {
-                Py_DECREF(list);
+            object value_ (value_conv::cast(value, policy, parent), false);
+            if (!value_)
                 return nullptr;
-            }
-            PyList_SET_ITEM(list, index++, value_); // steals a reference
+            PyList_SET_ITEM(list.ptr(), index++, value_.release()); // steals a reference
         }
-        return list;
+        return list.release();
     }
     PYBIND11_TYPE_CASTER(type, detail::descr("list<") + value_conv::name() + detail::descr(">"));
 };
@@ -77,17 +77,15 @@
     }
 
     static PyObject *cast(const type &src, return_value_policy policy, PyObject *parent) {
-        PyObject *set = PySet_New(nullptr);
+        object set(PySet_New(nullptr), false);
+        if (!set)
+          return nullptr;
         for (auto const &value: src) {
-            PyObject *value_ = value_conv::cast(value, policy, parent);
-            if (!value_ || PySet_Add(set, value_) != 0) {
-                Py_XDECREF(value_);
-                Py_DECREF(set);
+            object value_(value_conv::cast(value, policy, parent), false);
+            if (!value_ || PySet_Add(set.ptr(), value_.ptr()) != 0)
                 return nullptr;
-            }
-            Py_DECREF(value_);
         }
-        return set;
+        return set.release();
     }
     PYBIND11_TYPE_CASTER(type, detail::descr("set<") + value_conv::name() + detail::descr(">"));
 };
@@ -116,20 +114,16 @@
     }
 
     static PyObject *cast(const type &src, return_value_policy policy, PyObject *parent) {
-        PyObject *dict = PyDict_New();
+        object dict(PyDict_New(), false);
+        if (!dict)
+          return nullptr;
         for (auto const &kv: src) {
-            PyObject *key   = key_conv::cast(kv.first, policy, parent);
-            PyObject *value = value_conv::cast(kv.second, policy, parent);
-            if (!key || !value || PyDict_SetItem(dict, key, value) != 0) {
-                Py_XDECREF(key);
-                Py_XDECREF(value);
-                Py_DECREF(dict);
+            object key(key_conv::cast(kv.first, policy, parent), false);
+            object value(value_conv::cast(kv.second, policy, parent), false);
+            if (!key || !value || PyDict_SetItem(dict.ptr(), key.ptr(), value.ptr()) != 0)
                 return nullptr;
-            }
-            Py_DECREF(key);
-            Py_DECREF(value);
         }
-        return dict;
+        return dict.release();
     }
 
     PYBIND11_TYPE_CASTER(type, detail::descr("dict<") + key_conv::name() + detail::descr(", ") + value_conv::name() + detail::descr(">"));