共享指针：
std :: shared_ptr是原始指针的容器。它是一个引用计数所有权模型，即它与std :: shared_ptr的所有副本协作维护其包含的指针的引用计数。因此，每当新的指针指向资源时，计数器就增加一次，而在调用对象的析构函数时，计数器就减少一次。
参考计数：
它是一种存储对资源(例如对象，内存块，磁盘空间或其他资源)的引用，指针或句柄数量的技术。
在引用计数大于零之前，即在删除std :: shared_ptr的所有副本之前，包含的原始指针引用的对象将不会被销毁。
何时使用：当我们想将一个原始指针分配给多个所有者时，应该使用shared_ptr。
有关共享和其他智能指针的更多信息和详细信息，请在此处阅读。
用户定义的共享指针实现：
程序：
CPP
#include
using namespace std;
// Class representing a reference counter class
class Counter
{
public:
// Constructor
Counter()
: m_counter(0){};
Counter(const Counter&) = delete;
Counter& operator=(const Counter&) = delete;
// Destructor
~Counter() {}
void reset()
{
m_counter = 0;
}
unsigned int get()
{
return m_counter;
}
// Overload post/pre increment
void operator++()
{
m_counter++;
}
void operator++(int)
{
m_counter++;
}
// Overload post/pre decrement
void operator--()
{
m_counter--;
}
void operator--(int)
{
m_counter--;
}
// Overloading << operator
friend ostream& operator<<(ostream& os,
const Counter& counter)
{
os << "Counter Value : " << counter.m_counter
<< endl;
return os;
}
private:
unsigned int m_counter{};
};
// Class representing a shared pointer
template
class Shared_ptr
{
public:
// Constructor
explicit Shared_ptr(T* ptr = nullptr)
{
m_ptr = ptr;
m_counter = new Counter();
if (ptr)
{
(*m_counter)++;
}
}
// Copy constructor
Shared_ptr(Shared_ptr& sp)
{
m_ptr = sp.m_ptr;
m_counter = sp.m_counter;
(*m_counter)++;
}
// Reference count
unsigned int use_count()
{
return m_counter->get();
}
// Get the pointer
T* get()
{
return m_ptr;
}
// Overload * operator
T& operator*()
{
return *m_ptr;
}
// Overload -> operator
T* operator->()
{
return m_ptr;
}
// Destructor
~Shared_ptr()
{
(*m_counter)--;
if (m_counter->get() == 0)
{
delete m_counter;
delete m_ptr;
}
}
friend ostream& operator<<(ostream& os,
Shared_ptr& sp)
{
os << "Address pointed : " << sp.get() << endl;
os << *(sp.m_counter) << endl;
return os;
}
private:
// Reference counter
Counter* m_counter;
// Shared pointer
T* m_ptr;
};
int main()
{
// ptr1 pointing to an integer.
Shared_ptr ptr1(new int(151));
cout << "--- Shared pointers ptr1 ---\n";
*ptr1 = 100;
cout << " ptr1's value now: " << *ptr1 << endl;
cout << ptr1;
{
// ptr2 pointing to same integer
// which ptr1 is pointing to
// Shared pointer reference counter
// should have increased now to 2.
Shared_ptr ptr2 = ptr1;
cout << "--- Shared pointers ptr1, ptr2 ---\n";
cout << ptr1;
cout << ptr2;
{
// ptr3 pointing to same integer
// which ptr1 and ptr2 are pointing to.
// Shared pointer reference counter
// should have increased now to 3.
Shared_ptr ptr3(ptr2);
cout << "--- Shared pointers ptr1, ptr2, ptr3 "
"---\n";
cout << ptr1;
cout << ptr2;
cout << ptr3;
}
// ptr3 is out of scope.
// It would have been destructed.
// So shared pointer reference counter
// should have decreased now to 2.
cout << "--- Shared pointers ptr1, ptr2 ---\n";
cout << ptr1;
cout << ptr2;
}
// ptr2 is out of scope.
// It would have been destructed.
// So shared pointer reference counter
// should have decreased now to 1.
cout << "--- Shared pointers ptr1 ---\n";
cout << ptr1;
return 0;
} 输出：
--- Shared pointers ptr1 ---
Address pointed : 0x1cbde70
Counter Value : 1
--- Shared pointers ptr1, ptr2 ---
Address pointed : 0x1cbde70
Counter Value : 2
Address pointed : 0x1cbde70
Counter Value : 2
--- Shared pointers ptr1, ptr2, ptr3 ---
Address pointed : 0x1cbde70
Counter Value : 3
Address pointed : 0x1cbde70
Counter Value : 3
Address pointed : 0x1cbde70
Counter Value : 3
--- Shared pointers ptr1, ptr2 ---
Address pointed : 0x1cbde70
Counter Value : 2
Address pointed : 0x1cbde70
Counter Value : 2
--- Shared pointers ptr1 ---
Address pointed : 0x1cbde70
Counter Value : 1
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