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new move value policy

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Wenzel Jakob 2016-04-25 23:04:27 +02:00
parent fbafdea672
commit f7b5874ca3
6 changed files with 130 additions and 60 deletions
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79
docs/advanced.rst
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@ -411,38 +411,63 @@ For this reason, pybind11 provides a several `return value policy` annotations
that can be passed to the :func:`module::def` and :func:`class_::def`
functions. The default policy is :enum:`return_value_policy::automatic`.
+--------------------------------------------------+---------------------------------------------------------------------------+
| Return value policy | Description |
+==================================================+===========================================================================+
| :enum:`return_value_policy::automatic` | Automatic: copy objects returned as values and take ownership of |
| | objects returned as pointers |
+--------------------------------------------------+---------------------------------------------------------------------------+
| :enum:`return_value_policy::automatic_reference` | Automatic variant 2 : copy objects returned as values and reference |
| | objects returned as pointers |
+--------------------------------------------------+---------------------------------------------------------------------------+
| :enum:`return_value_policy::copy` | Create a new copy of the returned object, which will be owned by Python |
+--------------------------------------------------+---------------------------------------------------------------------------+
| :enum:`return_value_policy::take_ownership` | Reference the existing object and take ownership. Python will call |
| | the destructor and delete operator when the reference count reaches zero |
+--------------------------------------------------+---------------------------------------------------------------------------+
| :enum:`return_value_policy::reference` | Reference the object, but do not take ownership and defer responsibility |
| | for deleting it to C++ (dangerous when C++ code at some point decides to |
| | delete it while Python still has a nonzero reference count) |
+--------------------------------------------------+---------------------------------------------------------------------------+
| :enum:`return_value_policy::reference_internal` | Reference the object, but do not take ownership. The object is considered |
| | be owned by the C++ instance whose method or property returned it. The |
| | Python object will increase the reference count of this 'parent' by 1 |
| | to ensure that it won't be deallocated while Python is using the 'child' |
+--------------------------------------------------+---------------------------------------------------------------------------+
+--------------------------------------------------+----------------------------------------------------------------------------+
| Return value policy | Description |
+==================================================+============================================================================+
| :enum:`return_value_policy::automatic` | This is the default return value policy, which falls back to the policy |
| | :enum:`return_value_policy::take_ownership` when the return value is a |
| | pointer. Otherwise, it uses :enum::`return_value::move` or |
| | :enum::`return_value::copy` for rvalue and lvalue references, respectively.|
| | See below for a description of what all of these different policies do. |
+--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::automatic_reference` | As above, but use policy :enum:`return_value_policy::reference` when the |
| | return value is a pointer. |
+--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::take_ownership` | Reference an existing object (i.e. do not create a new copy) and take |
| | ownership. Python will call the destructor and delete operator when the |
| | object's reference count reaches zero. Undefined behavior ensues when the |
| | C++ side does the same.. |
+--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::copy` | Create a new copy of the returned object, which will be owned by Python. |
| | This policy is comparably safe because the lifetimes of the two instances |
| | are decoupled. |
+--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::move` | Use ``std::move`` to move the return value contents into a new instance |
| | that will be owned by Python. This policy is comparably safe because the |
| | lifetimes of the two instances (move source and destination) are decoupled.|
+--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::reference` | Reference an existing object, but do not take ownership. The C++ side is |
| | responsible for managing the object's lifetime and deallocating it when |
| | it is no longer used. Warning: undefined behavior will ensue when the C++ |
| | side deletes an object that is still referenced by Python. |
+--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::reference_internal` | Reference the object, but do not take ownership. The object is considered |
| | be owned by the C++ instance whose method or property returned it. The |
| | Python object will increase the reference count of this 'parent' by 1 |
| | to ensure that it won't be deallocated while Python is using the 'child' |
+--------------------------------------------------+----------------------------------------------------------------------------+
.. warning::
Code with invalid call policies might access unitialized memory and free
Code with invalid call policies might access unitialized memory or free
data structures multiple times, which can lead to hard-to-debug
non-determinism and segmentation faults, hence it is worth spending the
time to understand all the different options above.
.. note::
The next section on :ref:`call_policies` discusses *call policies* that can be
specified *in addition* to a return value policy from the list above. Call
policies indicate reference relationships that can involve both return values
and parameters of functions.
.. note::
As an alternative to elaborate call policies and lifetime management logic,
consider using smart pointers (see :ref:`smart_pointers` for details) that
can be used to share reference count information between C++ and Python.
See below for an example that uses the
:enum:`return_value_policy::reference_internal` policy.
@ -466,6 +491,8 @@ See below for an example that uses the
}
.. _call_policies:
Additional call policies
========================
@ -557,6 +584,8 @@ The above signature would imply that Python needs to give up ownership of an
object that is passed to this function, which is generally not possible (for
instance, the object might be referenced elsewhere).
.. _smart_pointers:
Smart pointers
==============

4
example/example1.ref
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@ -7,8 +7,8 @@ Called Example1 destructor (32)
Instance 1: Example1[value=320]
Instance 2: Example1[value=32]
Called Example1 copy constructor with value 320..
Called Example1 copy constructor with value 320..
Called Example1 destructor (320)
Called Example1 move constructor with value 320..
Called Example1 destructor (0)
Example1[value=320]
Called Example1 destructor (320)
Example1[value=320]

20
example/example3.ref
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@ -3,52 +3,52 @@ Value constructor
v1 = [1.000000, 2.000000]
v2 = [3.000000, -1.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
v1+v2 = [4.000000, 1.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
v1-v2 = [-2.000000, 3.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
v1-8 = [-7.000000, -6.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
v1+8 = [9.000000, 10.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
v1*8 = [8.000000, 16.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
v1/8 = [0.125000, 0.250000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
8-v1 = [7.000000, 6.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
8+v1 = [9.000000, 10.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
8*v1 = [8.000000, 16.000000]
Value constructor
Copy constructor
Move constructor
Destructor.
Destructor.
8/v1 = [8.000000, 4.000000]

6
example/example6.ref
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@ -1,13 +1,13 @@
Value constructor: Creating a sequence with 5 entries
s = <example.Sequence object at 0x1033bd8d0>
s = <example.Sequence object at 0x10c786c70>
len(s) = 5
s[0], s[3] = 0.000000 0.000000
12.34 in s: False
12.34 in s: True
s[0], s[3] = 12.340000 56.779999
Value constructor: Creating a sequence with 5 entries
Copy constructor: Creating a sequence with 5 entries
Freeing a sequence with 5 entries
Move constructor: Creating a sequence with 5 entries
Freeing a sequence with 0 entries
Value constructor: Creating a sequence with 5 entries
rev[0], rev[1], rev[2], rev[3], rev[4] = 0.000000 56.779999 0.000000 0.000000 12.340000
0.0 56.7799987793 0.0 0.0 12.3400001526

28
include/pybind11/cast.h
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@ -158,6 +158,7 @@ public:
const std::type_info *type_info,
const std::type_info *type_info_backup,
void *(*copy_constructor)(const void *),
void *(*move_constructor)(const void *),
const void *existing_holder = nullptr) {
void *src = const_cast<void *>(_src);
if (src == nullptr)
@ -204,6 +205,12 @@ public:
wrapper->value = copy_constructor(wrapper->value);
if (wrapper->value == nullptr)
throw cast_error("return_value_policy = copy, but the object is non-copyable!");
} else if (policy == return_value_policy::move) {
wrapper->value = move_constructor(wrapper->value);
if (wrapper->value == nullptr)
wrapper->value = copy_constructor(wrapper->value);
if (wrapper->value == nullptr)
throw cast_error("return_value_policy = move, but the object is neither movable nor copyable!");
} else if (policy == return_value_policy::reference) {
wrapper->owned = false;
} else if (policy == return_value_policy::reference_internal) {
@ -243,8 +250,16 @@ public:
return cast(&src, policy, parent);
}
static handle cast(type &&src, return_value_policy policy, handle parent) {
if (policy == return_value_policy::automatic || policy == return_value_policy::automatic_reference)
policy = return_value_policy::move;
return cast(&src, policy, parent);
}
static handle cast(const type *src, return_value_policy policy, handle parent) {
return type_caster_generic::cast(src, policy, parent, src ? &typeid(*src) : nullptr, &typeid(type), &copy_constructor);
return type_caster_generic::cast(
src, policy, parent, src ? &typeid(*src) : nullptr, &typeid(type),
&copy_constructor, &move_constructor);
}
template <typename T> using cast_op_type = pybind11::detail::cast_op_type<T>;
@ -253,11 +268,13 @@ public:
operator type&() { return *((type *) value); }
protected:
template <typename T = type, typename std::enable_if<detail::is_copy_constructible<T>::value, int>::type = 0>
static void *copy_constructor(const void *arg) {
return (void *) new type(*((const type *) arg));
}
static void *copy_constructor(const void *arg) { return (void *) new type(*((const type *) arg)); }
template <typename T = type, typename std::enable_if<!detail::is_copy_constructible<T>::value, int>::type = 0>
static void *copy_constructor(const void *) { return nullptr; }
template <typename T = type, typename std::enable_if<detail::is_move_constructible<T>::value, int>::type = 0>
static void *move_constructor(const void *arg) { return (void *) new type(std::move(*((type *) arg))); }
template <typename T = type, typename std::enable_if<!detail::is_move_constructible<T>::value, int>::type = 0>
static void *move_constructor(const void *) { return nullptr; }
};
template <typename type> class type_caster<std::reference_wrapper<type>> : public type_caster<type> {
@ -662,6 +679,7 @@ public:
using type_caster<type>::value;
using type_caster<type>::temp;
using type_caster<type>::copy_constructor;
using type_caster<type>::move_constructor;
bool load(handle src, bool convert) {
if (!src || !typeinfo) {
@ -702,7 +720,7 @@ public:
return type_caster_generic::cast(
src.get(), policy, parent,
src.get() ? &typeid(*src.get()) : nullptr, &typeid(type),
&copy_constructor, &src);
&copy_constructor, &move_constructor, &src);
}
protected:

53
include/pybind11/common.h
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@ -141,29 +141,46 @@ typedef Py_ssize_t ssize_t;
/// Approach used to cast a previously unknown C++ instance into a Python object
enum class return_value_policy : int {
/** Automatic: copy objects returned as values and take ownership of objects
returned as pointers */
/** This is the default return value policy, which falls back to the policy
return_value_policy::take_ownership when the return value is a pointer.
Otherwise, it uses return_value::move or return_value::copy for rvalue
and lvalue references, respectively. See below for a description of what
all of these different policies do. */
automatic = 0,
/** Automatic variant 2: copy objects returned as values and reference objects
returned as pointers */
/** As above, but use policy return_value_policy::reference when the return
value is a pointer. */
automatic_reference,
/** Reference the object and take ownership. Python will call the
destructor and delete operator when the reference count reaches zero */
/** Reference an existing object (i.e. do not create a new copy) and take
ownership. Python will call the destructor and delete operator when the
object’s reference count reaches zero. Undefined behavior ensues when
the C++ side does the same.. */
take_ownership,
/** Reference the object, but do not take ownership (dangerous when C++ code
deletes it and Python still has a nonzero reference count) */
/** Create a new copy of the returned object, which will be owned by
Python. This policy is comparably safe because the lifetimes of the two
instances are decoupled. */
copy,
/** Use std::move to move the return value contents into a new instance
that will be owned by Python. This policy is comparably safe because the
lifetimes of the two instances (move source and destination) are
decoupled. */
move,
/** Reference an existing object, but do not take ownership. The C++ side
is responsible for managing the object’s lifetime and deallocating it
when it is no longer used. Warning: undefined behavior will ensue when
the C++ side deletes an object that is still referenced by Python. */
reference,
/** Reference the object, but do not take ownership. The object is considered
be owned by the C++ instance whose method or property returned it. The
Python object will increase the reference count of this 'parent' by 1 */
reference_internal,
/// Create a new copy of the returned object, which will be owned by Python
copy
/** Reference the object, but do not take ownership. The object is
considered be owned by the C++ instance whose method or property
returned it. The Python object will increase the reference count of this
‘parent’ by 1 to ensure that it won’t be deallocated while Python is
using the ‘child’ */
reference_internal
};
/// Format strings for basic number types
@ -276,6 +293,12 @@ template <typename T> struct is_copy_constructible {
static const bool value = std::is_same<std::true_type, decltype(test<T>(nullptr))>::value;
};
template <typename T> struct is_move_constructible {
template <typename T2> static std::true_type test(decltype(new T2(std::declval<typename std::add_rvalue_reference<T2>::type>())) *);
template <typename T2> static std::false_type test(...);
static const bool value = std::is_same<std::true_type, decltype(test<T>(nullptr))>::value;
};
/// Helper type to replace 'void' in some expressions
struct void_type { };