When determining if a shared_ptr already exists, use a test on the we… (#2819)

* When determining if a shared_ptr already exists, use a test on the weak_ptr instead of a try/catch block.

* When determining if a shared_ptr already exists, use a test on the weak_ptr instead of a try/catch block.

* weak_from_this is only available in C++17 and later

* Switch to use feature flag instead of C++ version flag.

* Add Microsoft-specific check.

* Avoid undefined preprocessor macro warning treated as error.

* Simplify shared_from_this in init_holder

* Include <version> in detail/common.h (~stolen~ borrowed from @bstaletic's #2816)

* Move class_::get_shared_from_this to detail::try_get_shared_from_this

* Simplify try_get_shared_from_this by using weak_ptr::lock()

Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

Author: elkhrt

include/pybind11/detail/common.h

@@ -162,6 +162,11 @@
 #include <memory>
 #include <typeindex>
 #include <type_traits>
+#if defined(__has_include)
+#  if __has_include(<version>)
+#    include <version>
+#  endif
+#endif
 
 // #define PYBIND11_STR_LEGACY_PERMISSIVE
 // If DEFINED, pybind11::str can hold PyUnicodeObject or PyBytesObject
@@ -870,5 +875,23 @@ class any_container {
 // Forward-declaration; see detail/class.h
 std::string get_fully_qualified_tp_name(PyTypeObject*);
 
+template <typename T>
+inline static std::shared_ptr<T> try_get_shared_from_this(std::enable_shared_from_this<T> *holder_value_ptr) {
+// Pre C++17, this code path exploits undefined behavior, but is known to work on many platforms.
+// Use at your own risk!
+// See also https://en.cppreference.com/w/cpp/memory/enable_shared_from_this, and in particular
+// the `std::shared_ptr<Good> gp1 = not_so_good.getptr();` and `try`-`catch` parts of the example.
+#if defined(__cpp_lib_enable_shared_from_this) && (!defined(_MSC_VER) || _MSC_VER >= 1912)
+    return holder_value_ptr->weak_from_this().lock();
+#else
+    try {
+        return holder_value_ptr->shared_from_this();
+    }
+    catch (const std::bad_weak_ptr &) {
+        return nullptr;
+    }
+#endif
+}
+
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

include/pybind11/pybind11.h

@@ -1505,14 +1505,13 @@ class class_ : public detail::generic_type {
     template <typename T>
     static void init_holder(detail::instance *inst, detail::value_and_holder &v_h,
             const holder_type * /* unused */, const std::enable_shared_from_this<T> * /* dummy */) {
-        try {
-            auto sh = std::dynamic_pointer_cast<typename holder_type::element_type>(
-                    v_h.value_ptr<type>()->shared_from_this());
-            if (sh) {
-                new (std::addressof(v_h.holder<holder_type>())) holder_type(std::move(sh));
-                v_h.set_holder_constructed();
-            }
-        } catch (const std::bad_weak_ptr &) {}
+
+        auto sh = std::dynamic_pointer_cast<typename holder_type::element_type>(
+                detail::try_get_shared_from_this(v_h.value_ptr<type>()));
+        if (sh) {
+            new (std::addressof(v_h.holder<holder_type>())) holder_type(std::move(sh));
+            v_h.set_holder_constructed();
+        }
 
         if (!v_h.holder_constructed() && inst->owned) {
             new (std::addressof(v_h.holder<holder_type>())) holder_type(v_h.value_ptr<type>());