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Update OVERLOAD macros to support ref/ptr return type overloads
This adds a static local variable (in dead code unless actually needed) in the overload code that is used for storage if the overload is for some convert-by-value type (such as numeric values or std::string). This has limitations (as written up in the advanced doc), but is better than simply not being able to overload reference or pointer methods.
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@ -298,13 +298,11 @@ helper class that is defined as follows:
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The macro :func:`PYBIND11_OVERLOAD_PURE` should be used for pure virtual
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functions, and :func:`PYBIND11_OVERLOAD` should be used for functions which have
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a default implementation.
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There are also two alternate macros :func:`PYBIND11_OVERLOAD_PURE_NAME` and
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:func:`PYBIND11_OVERLOAD_NAME` which take a string-valued name argument between
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the *Parent class* and *Name of the function* slots. This is useful when the
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C++ and Python versions of the function have different names, e.g.
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``operator()`` vs ``__call__``.
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a default implementation. There are also two alternate macros
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:func:`PYBIND11_OVERLOAD_PURE_NAME` and :func:`PYBIND11_OVERLOAD_NAME` which
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take a string-valued name argument between the *Parent class* and *Name of the
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function* slots. This is useful when the C++ and Python versions of the
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function have different names, e.g. ``operator()`` vs ``__call__``.
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The binding code also needs a few minor adaptations (highlighted):
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@ -357,6 +355,25 @@ a virtual method call.
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Please take a look at the :ref:`macro_notes` before using this feature.
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.. note::
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When the overridden type returns a reference or pointer to a type that
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pybind11 converts from Python (for example, numeric values, std::string,
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and other built-in value-converting types), there are some limitations to
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be aware of:
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- because in these cases there is no C++ variable to reference (the value
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is stored in the referenced Python variable), pybind11 provides one in
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the PYBIND11_OVERLOAD macros (when needed) with static storage duration.
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Note that this means that invoking the overloaded method on *any*
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instance will change the referenced value stored in *all* instances of
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that type.
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- Attempts to modify a non-const reference will not have the desired
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effect: it will change only the static cache variable, but this change
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will not propagate to underlying Python instance, and the change will be
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replaced the next time the overload is invoked.
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.. seealso::
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The file :file:`tests/test_virtual_functions.cpp` contains a complete
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@ -867,14 +867,8 @@ template <typename type> using cast_is_temporary_value_reference = bool_constant
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!std::is_base_of<type_caster_generic, make_caster<type>>::value
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>;
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NAMESPACE_END(detail)
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template <typename T> T cast(const handle &handle) {
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using type_caster = detail::make_caster<T>;
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static_assert(!detail::cast_is_temporary_value_reference<T>::value,
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"Unable to cast type to reference: value is local to type caster");
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type_caster conv;
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template <typename T> make_caster<T> load_type(const handle &handle) {
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make_caster<T> conv;
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if (!conv.load(handle, true)) {
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#if defined(NDEBUG)
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throw cast_error("Unable to cast Python instance to C++ type (compile in debug mode for details)");
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@ -883,7 +877,16 @@ template <typename T> T cast(const handle &handle) {
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(std::string) handle.get_type().str() + " to C++ type '" + type_id<T>() + "''");
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#endif
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}
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return conv.operator typename type_caster::template cast_op_type<T>();
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return conv;
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}
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NAMESPACE_END(detail)
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template <typename T> T cast(const handle &handle) {
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static_assert(!detail::cast_is_temporary_value_reference<T>::value,
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"Unable to cast type to reference: value is local to type caster");
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using type_caster = detail::make_caster<T>;
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return detail::load_type<T>(handle).operator typename type_caster::template cast_op_type<T>();
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}
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template <typename T> object cast(const T &value,
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@ -900,7 +903,7 @@ template <typename T> T handle::cast() const { return pybind11::cast<T>(*this);
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template <> inline void handle::cast() const { return; }
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template <typename T>
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typename std::enable_if<detail::move_always<T>::value || detail::move_if_unreferenced<T>::value, T>::type move(object &&obj) {
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detail::enable_if_t<detail::move_always<T>::value || detail::move_if_unreferenced<T>::value, T> move(object &&obj) {
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if (obj.ref_count() > 1)
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#if defined(NDEBUG)
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throw cast_error("Unable to cast Python instance to C++ rvalue: instance has multiple references"
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@ -910,18 +913,8 @@ typename std::enable_if<detail::move_always<T>::value || detail::move_if_unrefer
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" instance to C++ " + type_id<T>() + " instance: instance has multiple references");
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#endif
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typedef detail::type_caster<T> type_caster;
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type_caster conv;
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if (!conv.load(obj, true))
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#if defined(NDEBUG)
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throw cast_error("Unable to cast Python instance to C++ type (compile in debug mode for details)");
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#else
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throw cast_error("Unable to cast Python instance of type " +
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(std::string) obj.get_type().str() + " to C++ type '" + type_id<T>() + "''");
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#endif
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// Move into a temporary and return that, because the reference may be a local value of `conv`
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T ret = std::move(conv.operator T&());
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T ret = std::move(detail::load_type<T>(obj).operator T&());
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return ret;
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}
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@ -930,24 +923,57 @@ typename std::enable_if<detail::move_always<T>::value || detail::move_if_unrefer
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// object has multiple references, but trying to copy will fail to compile.
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// - If both movable and copyable, check ref count: if 1, move; otherwise copy
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// - Otherwise (not movable), copy.
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template <typename T> typename std::enable_if<detail::move_always<T>::value, T>::type cast(object &&object) {
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template <typename T> detail::enable_if_t<detail::move_always<T>::value, T> cast(object &&object) {
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return move<T>(std::move(object));
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}
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template <typename T> typename std::enable_if<detail::move_if_unreferenced<T>::value, T>::type cast(object &&object) {
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template <typename T> detail::enable_if_t<detail::move_if_unreferenced<T>::value, T> cast(object &&object) {
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if (object.ref_count() > 1)
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return cast<T>(object);
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else
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return move<T>(std::move(object));
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}
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template <typename T> typename std::enable_if<detail::move_never<T>::value, T>::type cast(object &&object) {
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template <typename T> detail::enable_if_t<detail::move_never<T>::value, T> cast(object &&object) {
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return cast<T>(object);
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}
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// Provide a ref_cast() with move support for objects (only participates for moveable types)
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template <typename T> detail::enable_if_t<detail::move_is_plain_type<T>::value, T>
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ref_cast(object &&object) { return cast<T>(std::move(object)); }
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template <typename T> T object::cast() const & { return pybind11::cast<T>(*this); }
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template <typename T> T object::cast() && { return pybind11::cast<T>(std::move(*this)); }
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template <> inline void object::cast() const & { return; }
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template <> inline void object::cast() && { return; }
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NAMESPACE_BEGIN(detail)
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struct overload_nothing {}; // Placeholder type for the unneeded (and dead code) static variable in the OVERLOAD_INT macro
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template <typename ret_type> using overload_local_t = conditional_t<
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cast_is_temporary_value_reference<ret_type>::value, intrinsic_t<ret_type>, overload_nothing>;
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template <typename T> enable_if_t<std::is_lvalue_reference<T>::value, T> storage_cast(intrinsic_t<T> &v) { return v; }
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template <typename T> enable_if_t<std::is_pointer<T>::value, T> storage_cast(intrinsic_t<T> &v) { return &v; }
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// Trampoline use: for reference/pointer types to value-converted values, we do a value cast, then
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// store the result in the given variable. For other types, this is a no-op.
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template <typename T> enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&o, intrinsic_t<T> &storage) {
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using type_caster = make_caster<T>;
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using itype = intrinsic_t<T>;
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storage = std::move(load_type<T>(o).operator typename type_caster::template cast_op_type<itype>());
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return storage_cast<T>(storage);
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}
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template <typename T> enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&, overload_nothing &) {
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pybind11_fail("Internal error: cast_ref fallback invoked"); }
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// Trampoline use: Having a pybind11::cast with an invalid reference type is going to static_assert, even
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// though if it's in dead code, so we provide a "trampoline" to pybind11::cast that only does anything in
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// cases where pybind11::cast is valid.
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template <typename T> enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&o) {
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return pybind11::cast<T>(std::move(o)); }
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template <typename T> enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&) {
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pybind11_fail("Internal error: cast_safe fallback invoked"); }
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template <> inline void cast_safe<void>(object &&) {}
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NAMESPACE_END(detail)
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template <return_value_policy policy = return_value_policy::automatic_reference,
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@ -1485,8 +1485,15 @@ template <class T> function get_overload(const T *this_ptr, const char *name) {
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#define PYBIND11_OVERLOAD_INT(ret_type, cname, name, ...) { \
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pybind11::gil_scoped_acquire gil; \
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pybind11::function overload = pybind11::get_overload(static_cast<const cname *>(this), name); \
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if (overload) \
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return overload(__VA_ARGS__).template cast<ret_type>(); }
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if (overload) { \
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pybind11::object o = overload(__VA_ARGS__); \
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if (pybind11::detail::cast_is_temporary_value_reference<ret_type>::value) { \
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static pybind11::detail::overload_local_t<ret_type> local_value; \
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return pybind11::detail::cast_ref<ret_type>(std::move(o), local_value); \
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} \
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else return pybind11::detail::cast_safe<ret_type>(std::move(o)); \
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} \
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}
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#define PYBIND11_OVERLOAD_NAME(ret_type, cname, name, fn, ...) \
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PYBIND11_OVERLOAD_INT(ret_type, cname, name, __VA_ARGS__) \
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@ -21,14 +21,22 @@ public:
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virtual int run(int value) {
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py::print("Original implementation of "
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"ExampleVirt::run(state={}, value={})"_s.format(state, value));
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"ExampleVirt::run(state={}, value={}, str1={}, str2={})"_s.format(state, value, get_string1(), *get_string2()));
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return state + value;
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}
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virtual bool run_bool() = 0;
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virtual void pure_virtual() = 0;
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// Returning a reference/pointer to a type converted from python (numbers, strings, etc.) is a
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// bit trickier, because the actual int& or std::string& or whatever only exists temporarily, so
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// we have to handle it specially in the trampoline class (see below).
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virtual const std::string &get_string1() { return str1; }
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virtual const std::string *get_string2() { return &str2; }
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private:
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int state;
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const std::string str1{"default1"}, str2{"default2"};
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};
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/* This is a wrapper class that must be generated */
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@ -65,6 +73,27 @@ public:
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in the previous line is needed for some compilers */
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);
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}
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// We can return reference types for compatibility with C++ virtual interfaces that do so, but
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// note they have some significant limitations (see the documentation).
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const std::string &get_string1() override {
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PYBIND11_OVERLOAD(
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const std::string &, /* Return type */
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ExampleVirt, /* Parent class */
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get_string1, /* Name of function */
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/* (no arguments) */
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);
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}
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const std::string *get_string2() override {
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PYBIND11_OVERLOAD(
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const std::string *, /* Return type */
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ExampleVirt, /* Parent class */
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get_string2, /* Name of function */
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/* (no arguments) */
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);
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}
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};
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class NonCopyable {
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@ -20,13 +20,23 @@ def test_override(capture, msg):
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print('ExtendedExampleVirt::run_bool()')
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return False
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def get_string1(self):
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return "override1"
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def pure_virtual(self):
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print('ExtendedExampleVirt::pure_virtual(): %s' % self.data)
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class ExtendedExampleVirt2(ExtendedExampleVirt):
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def __init__(self, state):
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super(ExtendedExampleVirt2, self).__init__(state + 1)
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def get_string2(self):
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return "override2"
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ex12 = ExampleVirt(10)
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with capture:
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assert runExampleVirt(ex12, 20) == 30
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assert capture == "Original implementation of ExampleVirt::run(state=10, value=20)"
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assert capture == "Original implementation of ExampleVirt::run(state=10, value=20, str1=default1, str2=default2)"
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with pytest.raises(RuntimeError) as excinfo:
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runExampleVirtVirtual(ex12)
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@ -37,7 +47,7 @@ def test_override(capture, msg):
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assert runExampleVirt(ex12p, 20) == 32
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assert capture == """
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ExtendedExampleVirt::run(20), calling parent..
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Original implementation of ExampleVirt::run(state=11, value=21)
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Original implementation of ExampleVirt::run(state=11, value=21, str1=override1, str2=default2)
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"""
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with capture:
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assert runExampleVirtBool(ex12p) is False
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@ -46,11 +56,19 @@ def test_override(capture, msg):
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runExampleVirtVirtual(ex12p)
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assert capture == "ExtendedExampleVirt::pure_virtual(): Hello world"
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ex12p2 = ExtendedExampleVirt2(15)
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with capture:
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assert runExampleVirt(ex12p2, 50) == 68
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assert capture == """
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ExtendedExampleVirt::run(50), calling parent..
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Original implementation of ExampleVirt::run(state=17, value=51, str1=override1, str2=override2)
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"""
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cstats = ConstructorStats.get(ExampleVirt)
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assert cstats.alive() == 2
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del ex12, ex12p
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assert cstats.alive() == 3
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del ex12, ex12p, ex12p2
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assert cstats.alive() == 0
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assert cstats.values() == ['10', '11']
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assert cstats.values() == ['10', '11', '17']
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assert cstats.copy_constructions == 0
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assert cstats.move_constructions >= 0
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