Archiving godbolt code.

Author: rwgk

godbolt/model_of_pybind11_behavior_and_dynamic_pointer_cast_alternative.cpp

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+// https://godbolt.org/z/sdhTqY
+
+#include <iostream>
+#include <memory>
+
+#define DUMP(expr) #expr " = " << (expr)
+
+struct base {
+  base() : base_id{100} {}
+  virtual ~base() = default;
+  virtual int const id() { return base_id; }
+  int base_id;
+};
+struct monkeywrench {
+  virtual int const monkey_id() { return 150; }
+  virtual ~monkeywrench() = default;
+};
+struct drvd : monkeywrench, base {
+  int const id() override { return 2 * base_id; }
+};
+
+void drvd_void_base() {
+  std::cout << "hello drvd_void_base" << std::endl;
+
+  // pybind11-wrapped C++ function returns shared_ptr<base> up-cast from drvd.
+  std::shared_ptr<base> shared_drvd_uc(new drvd);
+  std::cout << "have shared_drvd_uc" << std::endl;
+#if 0
+  std::cout << "dynamic_pointer_cast<drvd>" << std::endl;
+  std::shared_ptr<drvd> shared_drvd_dc = std::dynamic_pointer_cast<drvd>(shared_drvd_uc);
+#elif 1
+  std::cout << "dynamic_pointer_cast<void> + static_pointer_cast<drvd>" << std::endl;
+  std::shared_ptr<void> ivp = std::dynamic_pointer_cast<void>(shared_drvd_uc);
+  std::cout << "have ivp" << std::endl;
+  std::shared_ptr<drvd> shared_drvd_dc = std::static_pointer_cast<drvd>(ivp);  // Is this well-defined?
+#else
+  std::cout << "pybind11 <=2.6 behavior" << std::endl;
+  // intermediate void pointer.
+  const void *ivp = &shared_drvd_uc;
+  std::cout << "have ivp" << std::endl;
+  // intermediate shared_ptr<drvd> pointer.
+  const std::shared_ptr<drvd> *isdp = (const std::shared_ptr<drvd>*) ivp;  // ROOT CAUSE FOR SEGFAULT BELOW
+  std::cout << "have isdp" << std::endl;
+  std::shared_ptr<drvd> shared_drvd_dc = *isdp;
+#endif
+  std::cout << "have shared_drvd_dc" << std::endl;
+  std::cout << DUMP(shared_drvd_dc.use_count()) << std::endl;
+  std::cout << DUMP(shared_drvd_dc->id()) << std::endl;  // SEGFAULT HERE
+}
+
+int main() {
+  std::cout << "hello main" << std::endl;
+  drvd_void_base();
+  return 0;
+}