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cast: Add is_generic_type<T> #3857

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cast: Add is_generic_type<T>
EricCousineau-TRI Apr 11, 2022
58d4aae
Update include/pybind11/pytypes.h
EricCousineau-TRI Apr 11, 2022
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9 changes: 9 additions & 0 deletions include/pybind11/cast.h
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,15 @@ class type_caster : public type_caster_base<type> {};
template <typename type>
using make_caster = type_caster<intrinsic_t<type>>;

template <typename T>
struct is_generic_type<T, enable_if_t<std::is_base_of<type_caster_generic, make_caster<T>>::value>>
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From @rwgk in #2950

Two questions:

  1. Will this work if I add another specialization pair for is_base_of<smart_holder_type_caster_base_tag, ...>?

  2. What do you think about choosing a different name, to connect to the intended purpose instead of pegging the name on type_caster_generic, e.g. is_iterable_t_compatible? (Otherwise it will get really confusing that generic is connected to smart_holder_type_caster.)

(1) Yup! Can simply add the specialization and it should work out of the box.
(2) I know that was for prior PR; I believe this trait is more agnostic. See PR overview for rationale.

: public std::true_type {};

template <typename T>
struct is_generic_type<T,
enable_if_t<!std::is_base_of<type_caster_generic, make_caster<T>>::value>>
: public std::false_type {};

// Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
template <typename T>
typename make_caster<T>::template cast_op_type<T> cast_op(make_caster<T> &caster) {
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9 changes: 8 additions & 1 deletion include/pybind11/pytypes.h
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,11 @@ PYBIND11_NAMESPACE_BEGIN(detail)
class args_proxy;
bool isinstance_generic(handle obj, const std::type_info &tp);

// Indicates that type is generic and and does not have a specialized
// `type_caster<>` specialization. Defined in `cast.h`.
template <typename T, typename SFINAE = void>
struct is_generic_type;

// Accessor forward declarations
template <typename Policy>
class accessor;
Expand Down Expand Up @@ -476,7 +481,7 @@ inline void raise_from(error_already_set &err, PyObject *type, const char *messa
/** \ingroup python_builtins
\rst
Return true if ``obj`` is an instance of ``T``. Type ``T`` must be a subclass of
`object` or a class which was exposed to Python as ``py::class_<T>``.
`object` or a class which was exposed to Python as ``py::class_<T>`` (generic).
\endrst */
template <typename T, detail::enable_if_t<std::is_base_of<object, T>::value, int> = 0>
bool isinstance(handle obj) {
Expand All @@ -485,6 +490,8 @@ bool isinstance(handle obj) {

template <typename T, detail::enable_if_t<!std::is_base_of<object, T>::value, int> = 0>
bool isinstance(handle obj) {
static_assert(detail::is_generic_type<T>::value,
"isinstance<T>() requires specialization for this type");
return detail::isinstance_generic(obj, typeid(T));
}

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