Simple .shared_from_this() experiments.

Author: rwgk

shared_from_this_custom_deleters.cpp

@@ -0,0 +1,99 @@
+/* clang++ -std=c++11 shared_from_this_custom_deleters.cpp && a.out
+
+   Simple experiment, to show that `.shared_from_this()` ~only retrieves
+   the first `shared_ptr` created for a given pointee. (See obj1_reset() for
+   a more complete story.)
+ */
+
+#include <iostream>
+#include <memory>
+
+template <typename T>
+struct labeled_delete {
+  std::string label;
+  explicit labeled_delete(const std::string& label) : label{label} {}
+  void operator()(T* raw_ptr) {
+    std::cout << "labeled_delete::operator() " << label << std::endl;
+    if (label != "SkipDelete") {
+      delete raw_ptr;
+    }
+  }
+};
+
+struct Atype : std::enable_shared_from_this<Atype> {};
+
+#define SHOW_USE_COUNTS                                              \
+  std::cout << "obj1, obj2 use_counts: " << obj1.use_count() << ", " \
+            << obj2.use_count() << std::endl;
+
+void obj1_owns() {
+  std::cout << "\nobj1_owns()" << std::endl;
+  std::shared_ptr<Atype> obj1(new Atype, labeled_delete<Atype>("1st"));
+  std::shared_ptr<Atype> obj2(obj1.get(), labeled_delete<Atype>("SkipDelete"));
+  SHOW_USE_COUNTS
+  auto sft1 = obj1->shared_from_this();
+  SHOW_USE_COUNTS
+  auto sft2 = obj2->shared_from_this();
+  SHOW_USE_COUNTS
+}
+/* Expected output:
+obj1_owns()
+obj1, obj2 use_counts: 1, 1
+obj1, obj2 use_counts: 2, 1
+obj1, obj2 use_counts: 3, 1
+labeled_delete::operator() SkipDelete
+labeled_delete::operator() 1st
+*/
+
+void obj2_owns() {
+  std::cout << "\nobj2_owns()" << std::endl;
+  std::shared_ptr<Atype> obj1(new Atype, labeled_delete<Atype>("SkipDelete"));
+  std::shared_ptr<Atype> obj2(obj1.get(), labeled_delete<Atype>("2nd"));
+  SHOW_USE_COUNTS
+  auto sft1 = obj1->shared_from_this();
+  SHOW_USE_COUNTS
+  auto sft2 = obj2->shared_from_this();
+  SHOW_USE_COUNTS
+}
+/* Expected output:
+obj2_owns()
+obj1, obj2 use_counts: 1, 1
+obj1, obj2 use_counts: 2, 1
+obj1, obj2 use_counts: 3, 1
+labeled_delete::operator() 2nd
+labeled_delete::operator() SkipDelete
+*/
+
+void obj1_reset() {
+  std::cout << "\nobj1_reset()" << std::endl;
+  std::shared_ptr<Atype> obj1(new Atype, labeled_delete<Atype>("SkipDelete"));
+  std::shared_ptr<Atype> obj2(obj1.get(), labeled_delete<Atype>("ThisDeletes"));
+  obj1->shared_from_this();
+  obj2->shared_from_this();
+  obj1.reset();
+  bool got_bad_weak_ptr = false;
+  try {
+    obj2->shared_from_this();
+  } catch (const std::bad_weak_ptr&) {
+    got_bad_weak_ptr = true;
+  }
+  std::cout << "got_bad_weak_ptr: " << got_bad_weak_ptr << std::endl;
+  std::shared_ptr<Atype> obj3(obj2.get(), labeled_delete<Atype>("SkipDelete"));
+  // Working again based on the shared_ptr that was created after obj1 was
+  // reset:
+  obj2->shared_from_this();
+}
+/* Expected output:
+obj1_reset()
+labeled_delete::operator() SkipDelete
+got_bad_weak_ptr: 1
+labeled_delete::operator() SkipDelete
+labeled_delete::operator() ThisDeletes
+*/
+
+int main() {
+  obj1_owns();
+  obj2_owns();
+  obj1_reset();
+  return 0;
+}