std::enable_shared_from_this

#include <iostream>
#include <memory>
 
class Good : public std::enable_shared_from_this<Good>
{
public:
 std::shared_ptr <Good> getptr()
 {
 return shared_from_this();
 }
};
 
class Best : public std::enable_shared_from_this<Best>
{
 struct Private{ explicit Private() = default; };
 
public:
 // Constructor is only usable by this class
 Best(Private) {}
 
 // Everyone else has to use this factory function
 // Hence all Best objects will be contained in shared_ptr
 static std::shared_ptr <Best> create()
 {
 return std::make_shared <Best>(Private());
 }
 
 std::shared_ptr <Best> getptr()
 {
 return shared_from_this();
 }
};
 
struct Bad
{
 std::shared_ptr <Bad> getptr()
 {
 return std::shared_ptr <Bad>(this);
 }
 ~Bad() { std::cout << "Bad::~Bad() called\n"; }
};
 
void testGood()
{
 // Good: the two shared_ptr's share the same object
 std::shared_ptr <Good> good0 = std::make_shared <Good>();
 std::shared_ptr <Good> good1 = good0->getptr();
 std::cout << "good1.use_count() = " << good1.use_count() << '\n';
}
 
void misuseGood()
{
 // Bad: shared_from_this is called without having std::shared_ptr owning the caller
 try
 {
 Good not_so_good;
 std::shared_ptr <Good> gp1 = not_so_good.getptr();
 }
 catch (std::bad_weak_ptr & e)
 {
 // undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
 std::cout << e.what() << '\n';
 }
}
 
void testBest()
{
 // Best: Same but cannot stack-allocate it:
 std::shared_ptr <Best> best0 = Best::create();
 std::shared_ptr <Best> best1 = best0->getptr();
 std::cout << "best1.use_count() = " << best1.use_count() << '\n';
 
 // Best stackBest; // <- Will not compile because Best::Best() is private.
}
 
void testBad()
{
 // Bad, each shared_ptr thinks it is the only owner of the object
 std::shared_ptr <Bad> bad0 = std::make_shared <Bad>();
 std::shared_ptr <Bad> bad1 = bad0->getptr();
 std::cout << "bad1.use_count() = " << bad1.use_count() << '\n';
} // UB: double-delete of Bad
 
int main()
{
 testGood();
 misuseGood();
 
 testBest();
 
 testBad();
}

good1.use_count() = 2
bad_weak_ptr
best1.use_count() = 2
bad1.use_count() = 1
Bad::~Bad() called
Bad::~Bad() called
*** glibc detected *** ./test: double free or corruption