Archiving godbolt code.

Author: rwgk

godbolt/jorg_black_box.cpp

@@ -0,0 +1,100 @@
+// https://godbolt.org/z/eTEvG3
+
+#include <iostream>
+#include <memory>
+#include <typeindex>
+
+#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; }
+};
+
+struct BlackBox {
+  std::shared_ptr<void> magicptr;
+  std::type_index magictype;
+
+  BlackBox() : magictype(typeid(void)) {}
+  template <typename T>
+  BlackBox(std::shared_ptr<T> in)                      //
+      : magicptr(std::static_pointer_cast<void>(in)),  //
+        magictype(typeid(T)) {}
+
+  template <typename T>
+  std::shared_ptr<T> get() const {
+    if (magictype == std::type_index(typeid(T))) {
+      return std::static_pointer_cast<T>(magicptr);
+    } else
+      return {};
+  }
+};
+
+void drvd_void_base() {
+  std::cout << "hello drvd_void_base" << '\n';
+  BlackBox b;
+
+  // pybind11-wrapped C++ function returns shared_ptr<base> up-cast from drvd.
+  {
+    std::shared_ptr<base> shared_base_uc(new base);
+    std::cout << "In:  " << DUMP(shared_base_uc->id()) << '\n';
+    b = shared_base_uc;
+
+    std::shared_ptr<base> shared_base_dc = b.get<base>();
+    std::cout << "Out: " << DUMP(shared_base_dc->id()) << '\n';
+  }
+  {
+    std::shared_ptr<base> shared_base_uc(new drvd);
+    std::cout << "In:  " << DUMP(shared_base_uc->id()) << '\n';
+    b = shared_base_uc;
+
+    std::shared_ptr<base> shared_base_dc = b.get<base>();
+    std::cout << "Out: " << DUMP(shared_base_dc->id()) << '\n';
+  }
+  {
+    std::shared_ptr<drvd> shared_drvd_uc(new drvd);
+    std::cout << "In:  " << DUMP(shared_drvd_uc->id()) << '\n';
+    b = shared_drvd_uc;
+
+    std::shared_ptr<drvd> shared_drvd_dc = b.get<drvd>();
+    std::cout << "Out: " << DUMP(shared_drvd_dc->id()) << '\n';
+  }
+}
+
+void bad_cast() {
+  std::cout << "uh-oh bad_cast" << '\n';
+  BlackBox b;
+
+  // pybind11-wrapped C++ function returns shared_ptr<base> up-cast from drvd.
+
+  {
+    std::shared_ptr<base> shared_base_uc(new drvd);
+    std::cout << "In:  " << DUMP(shared_base_uc->id()) << '\n';
+    b = shared_base_uc;
+
+    std::cout << std::flush;
+
+    std::shared_ptr<drvd> shared_drvd_dc = b.get<drvd>();
+    if (shared_drvd_dc) {
+      std::cout << "Out: " << DUMP(shared_drvd_dc->id()) << '\n';
+    } else {
+      std::cout << "Out: nullptr, sorry!\n";
+    }
+  }
+}
+
+int main() {
+  std::cout << "hello main" << '\n';
+  drvd_void_base();
+  bad_cast();
+  return 0;
+}

godbolt/safe_vs_unsafe_cast_back_from_void_pointer.cpp

@@ -0,0 +1,37 @@
+// https://godbolt.org/z/8873Wd
+
+#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() {
+  // pybind11-wrapped C++ function returns shared_ptr<base> up-cast from drvd.
+  std::shared_ptr<base> shared_drvd_uc(new drvd);
+  std::shared_ptr<void> ivp = std::dynamic_pointer_cast<void>(shared_drvd_uc);
+  std::shared_ptr<drvd> shared_drvd_dc = std::static_pointer_cast<drvd>(ivp);
+  // SEGFAULT:
+  // std::shared_ptr<base> shared_drvd_dc_uc = std::static_pointer_cast<base>(ivp);
+  // Can upcast only from std::shared_ptr<drvd>:
+  std::shared_ptr<base> shared_drvd_dc_uc = shared_drvd_dc;
+  std::cout << DUMP(shared_drvd_dc_uc->id()) << '\n';
+}
+
+int main() {
+  drvd_void_base();
+  return 0;
+}