From ed14879a1942fdfa4e4ef053c842841ca9fd6fc4 Mon Sep 17 00:00:00 2001 From: Jason Rhinelander Date: Thu, 21 Jul 2016 21:31:05 -0400 Subject: [PATCH] Move support for return values of called Python functions Currently pybind11 always translates values returned by Python functions invoked from C++ code by copying, even when moving is feasible--and, more importantly, even when moving is required. The first, and relatively minor, concern is that moving may be considerably more efficient for some types. The second problem, however, is more serious: there's currently no way python code can return a non-copyable type to C++ code. I ran into this while trying to add a PYBIND11_OVERLOAD of a virtual method that returns just such a type: it simply fails to compile because this: overload = ... overload(args).template cast(); involves a copy: overload(args) returns an object instance, and the invoked object::cast() loads the returned value, then returns a copy of the loaded value. We can, however, safely move that returned value *if* the object has the only reference to it (i.e. if ref_count() == 1) and the object is itself temporary (i.e. if it's an rvalue). This commit does that by adding an rvalue-qualified object::cast() method that allows the returned value to be move-constructed out of the stored instance when feasible. This basically comes down to three cases: - For objects that are movable but not copyable, we always try the move, with a runtime exception raised if this would involve moving a value with multiple references. - When the type is both movable and non-trivially copyable, the move happens only if the invoked object has a ref_count of 1, otherwise the object is copied. (Trivially copyable types are excluded from this case because they are typically just collections of primitive types, which can be copied just as easily as they can be moved.) - Non-movable and trivially copy constructible objects are simply copied. This also adds examples to example-virtual-functions that shows both a non-copyable object and a movable/copyable object in action: the former raises an exception if returned while holding a reference, the latter invokes a move constructor if unreferenced, or a copy constructor if referenced. Basically this allows code such as: class MyClass(Pybind11Class): def somemethod(self, whatever): mt = MovableType(whatever) # ... return mt which allows the MovableType instance to be returned to the C++ code via its move constructor. Of course if you attempt to violate this by doing something like: self.value = MovableType(whatever) return self.value you get an exception--but right now, the pybind11-side of that code won't compile at all. --- example/example-virtual-functions.cpp | 61 ++++++++++++++++++++ example/example-virtual-functions.py | 35 ++++++++++++ example/example-virtual-functions.ref | 16 ++++++ include/pybind11/cast.h | 80 ++++++++++++++++++++++++++- include/pybind11/pytypes.h | 5 ++ 5 files changed, 196 insertions(+), 1 deletion(-) diff --git a/example/example-virtual-functions.cpp b/example/example-virtual-functions.cpp index d08c782af..55a087109 100644 --- a/example/example-virtual-functions.cpp +++ b/example/example-virtual-functions.cpp @@ -69,6 +69,53 @@ public: } }; +class NonCopyable { +public: + NonCopyable(int a, int b) : value{new int(a*b)} {} + NonCopyable(NonCopyable &&) = default; + NonCopyable(const NonCopyable &) = delete; + NonCopyable() = delete; + void operator=(const NonCopyable &) = delete; + void operator=(NonCopyable &&) = delete; + std::string get_value() const { + if (value) return std::to_string(*value); else return "(null)"; + } + ~NonCopyable() { std::cout << "NonCopyable destructor @ " << this << "; value = " << get_value() << std::endl; } + +private: + std::unique_ptr value; +}; + +// This is like the above, but is both copy and movable. In effect this means it should get moved +// when it is not referenced elsewhere, but copied if it is still referenced. +class Movable { +public: + Movable(int a, int b) : value{a+b} {} + Movable(const Movable &m) { value = m.value; std::cout << "Movable @ " << this << " copy constructor" << std::endl; } + Movable(Movable &&m) { value = std::move(m.value); std::cout << "Movable @ " << this << " move constructor" << std::endl; } + int get_value() const { return value; } + ~Movable() { std::cout << "Movable destructor @ " << this << "; value = " << get_value() << std::endl; } +private: + int value; +}; + +class NCVirt { +public: + virtual NonCopyable get_noncopyable(int a, int b) { return NonCopyable(a, b); } + virtual Movable get_movable(int a, int b) = 0; + + void print_nc(int a, int b) { std::cout << get_noncopyable(a, b).get_value() << std::endl; } + void print_movable(int a, int b) { std::cout << get_movable(a, b).get_value() << std::endl; } +}; +class NCVirtTrampoline : public NCVirt { + virtual NonCopyable get_noncopyable(int a, int b) { + PYBIND11_OVERLOAD(NonCopyable, NCVirt, get_noncopyable, a, b); + } + virtual Movable get_movable(int a, int b) { + PYBIND11_OVERLOAD_PURE(Movable, NCVirt, get_movable, a, b); + } +}; + int runExampleVirt(ExampleVirt *ex, int value) { return ex->run(value); } @@ -240,6 +287,20 @@ void init_ex_virtual_functions(py::module &m) { .def("run_bool", &ExampleVirt::run_bool) .def("pure_virtual", &ExampleVirt::pure_virtual); + py::class_(m, "NonCopyable") + .def(py::init()) + ; + py::class_(m, "Movable") + .def(py::init()) + ; + py::class_, NCVirtTrampoline>(m, "NCVirt") + .def(py::init<>()) + .def("get_noncopyable", &NCVirt::get_noncopyable) + .def("get_movable", &NCVirt::get_movable) + .def("print_nc", &NCVirt::print_nc) + .def("print_movable", &NCVirt::print_movable) + ; + m.def("runExampleVirt", &runExampleVirt); m.def("runExampleVirtBool", &runExampleVirtBool); m.def("runExampleVirtVirtual", &runExampleVirtVirtual); diff --git a/example/example-virtual-functions.py b/example/example-virtual-functions.py index 1f7196570..121f330c8 100644 --- a/example/example-virtual-functions.py +++ b/example/example-virtual-functions.py @@ -5,6 +5,8 @@ sys.path.append('.') from example import ExampleVirt, runExampleVirt, runExampleVirtVirtual, runExampleVirtBool from example import A_Repeat, B_Repeat, C_Repeat, D_Repeat, A_Tpl, B_Tpl, C_Tpl, D_Tpl +from example import NCVirt, NonCopyable, Movable + class ExtendedExampleVirt(ExampleVirt): def __init__(self, state): @@ -87,3 +89,36 @@ for cl in classes: if hasattr(obj, "lucky_number"): print("Lucky = %.2f" % obj.lucky_number()) +class NCVirtExt(NCVirt): + def get_noncopyable(self, a, b): + # Constructs and returns a new instance: + nc = NonCopyable(a*a, b*b) + return nc + def get_movable(self, a, b): + # Return a referenced copy + self.movable = Movable(a, b) + return self.movable + +class NCVirtExt2(NCVirt): + def get_noncopyable(self, a, b): + # Keep a reference: this is going to throw an exception + self.nc = NonCopyable(a, b) + return self.nc + def get_movable(self, a, b): + # Return a new instance without storing it + return Movable(a, b) + +ncv1 = NCVirtExt() +print("2^2 * 3^2 =") +ncv1.print_nc(2, 3) +print("4 + 5 =") +ncv1.print_movable(4, 5) +ncv2 = NCVirtExt2() +print("7 + 7 =") +ncv2.print_movable(7, 7) +try: + ncv2.print_nc(9, 9) + print("Something's wrong: exception not raised!") +except RuntimeError as e: + # Don't print the exception message here because it differs under debug/non-debug mode + print("Caught expected exception") diff --git a/example/example-virtual-functions.ref b/example/example-virtual-functions.ref index 795f7b03f..c5c9223de 100644 --- a/example/example-virtual-functions.ref +++ b/example/example-virtual-functions.ref @@ -77,5 +77,21 @@ VI_DT: VI_DT says: quack quack quack Unlucky = 1234 Lucky = -4.25 +2^2 * 3^2 = +NonCopyable destructor @ 0x1a6c3f0; value = (null) +36 +NonCopyable destructor @ 0x7ffc6d1fbaa8; value = 36 +4 + 5 = +Movable @ 0x7ffc6d1fbacc copy constructor +9 +Movable destructor @ 0x7ffc6d1fbacc; value = 9 +7 + 7 = +Movable @ 0x7ffc6d1fbacc move constructor +Movable destructor @ 0x1a6c4d0; value = 14 +14 +Movable destructor @ 0x7ffc6d1fbacc; value = 14 +Caught expected exception +NonCopyable destructor @ 0x29a64b0; value = 81 +Movable destructor @ 0x1a6c410; value = 9 Destructing ExampleVirt.. Destructing ExampleVirt.. diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h index 1127abcc8..34dbb28e1 100644 --- a/include/pybind11/cast.h +++ b/include/pybind11/cast.h @@ -809,9 +809,36 @@ public: PYBIND11_TYPE_CASTER(type, handle_type_name::name()); }; +// Our conditions for enabling moving are quite restrictive: +// At compile time: +// - T needs to be a non-const, non-pointer, non-reference type +// - type_caster::operator T&() must exist +// - the type must be move constructible (obviously) +// At run-time: +// - if the type is non-copy-constructible, the object must be the sole owner of the type (i.e. it +// must have ref_count() == 1)h +// If any of the above are not satisfied, we fall back to copying. +template struct move_is_plain_type : std::false_type {}; +template struct move_is_plain_type::value && !std::is_pointer::value && !std::is_reference::value && !std::is_const::value + >::type> : std::true_type {}; +template struct move_always : std::false_type {}; +template struct move_always::value && + !std::is_copy_constructible::value && std::is_move_constructible::value && + std::is_same>().operator T&()), T&>::value + >::type> : std::true_type {}; +template struct move_if_unreferenced : std::false_type {}; +template struct move_if_unreferenced::value && + !move_always::value && std::is_move_constructible::value && + std::is_same>().operator T&()), T&>::value + >::type> : std::true_type {}; +template using move_never = std::integral_constant::value && !move_if_unreferenced::value>; + NAMESPACE_END(detail) -template T cast(handle handle) { +template T cast(const handle &handle) { typedef detail::type_caster::type> type_caster; type_caster conv; if (!conv.load(handle, true)) { @@ -838,6 +865,57 @@ template object cast(const T &value, template T handle::cast() const { return pybind11::cast(*this); } template <> inline void handle::cast() const { return; } +template +typename std::enable_if::value || detail::move_if_unreferenced::value, T>::type move(object &&obj) { + if (obj.ref_count() > 1) +#if defined(NDEBUG) + throw cast_error("Unable to cast Python instance to C++ rvalue: instance has multiple references" + " (compile in debug mode for details)"); +#else + throw cast_error("Unable to move from Python " + (std::string) obj.get_type().str() + + " instance to C++ " + type_id() + " instance: instance has multiple references"); +#endif + + typedef detail::type_caster type_caster; + type_caster conv; + if (!conv.load(obj, true)) +#if defined(NDEBUG) + throw cast_error("Unable to cast Python instance to C++ type (compile in debug mode for details)"); +#else + throw cast_error("Unable to cast Python instance of type " + + (std::string) obj.get_type().str() + " to C++ type '" + type_id() + "''"); +#endif + + // Move into a temporary and return that, because the reference may be a local value of `conv` + T ret = std::move(conv.operator T&()); + return ret; +} + +// Calling cast() on an rvalue calls pybind::cast with the object rvalue, which does: +// - If we have to move (because T has no copy constructor), do it. This will fail if the moved +// object has multiple references, but trying to copy will fail to compile. +// - If both movable and copyable, check ref count: if 1, move; otherwise copy +// - Otherwise (not movable), copy. +template typename std::enable_if::value, T>::type cast(object &&object) { + return move(std::move(object)); +} +template typename std::enable_if::value, T>::type cast(object &&object) { + if (object.ref_count() > 1) + return cast(object); + else + return move(std::move(object)); +} +template typename std::enable_if::value, T>::type cast(object &&object) { + return cast(object); +} + +template T object::cast() const & { return pybind11::cast(*this); } +template T object::cast() && { return pybind11::cast(std::move(*this)); } +template <> inline void object::cast() const & { return; } +template <> inline void object::cast() && { return; } + + + template tuple make_tuple(Args&&... args_) { const size_t size = sizeof...(Args); diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h index c8a90f871..0a0f50878 100644 --- a/include/pybind11/pytypes.h +++ b/include/pybind11/pytypes.h @@ -89,6 +89,11 @@ public: } return *this; } + + // Calling cast() on an object lvalue just copies (via handle::cast) + template T cast() const &; + // Calling on an object rvalue does a move, if needed and/or possible + template T cast() &&; }; NAMESPACE_BEGIN(detail)