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: Best::Best() is missing.
    // Best stackBest(Best::Private{}); // <- Will not compile: Best::Private 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();
}

Возможный вывод:

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