當進行拷貝或賦值操作時，每個shared_ptr都會紀錄有多少個其他shared_ptr指向相同的對象：
auto p = make_shared<int>(50);//p指向的對象只有p一個引用者
auto q = p; //p和q指向相同對象，此對象有兩個引用者；
智能指針類能記錄有多少個shared_ptr指向相同的對象，並能在恰當的時候自動釋放對象！
關於shared_ptr的引用計數測試demo：
#include <iostream> #include <memory> #include <vector>
using namespace std; int main() { vector<shared_ptr<int>> intPtr; vector<shared_ptr<int>> intPtr2; for (int i = 0; i < 10; ++i) { intPtr.emplace_back(make_shared<int>(i)); } for (int i = 0; i < 10; ++i) { cout << "intPtr " << i << " = " << *intPtr.at(i) << endl; } intPtr2 = intPtr; /* cout << "2: intPtr[0].use_count() = " << intPtr[0].use_count() << endl; cout << "2: intPtr2[0].use_count() = " << intPtr2[0].use_count() << endl; */ cout << "intPtr[0] = " << intPtr[0] << endl; cout << "intPtr2[0] = " << intPtr2[0] << endl; intPtr[0] = make_shared<int>(50); cout << "3: intPtr[0].use_count() = " << intPtr[0].use_count() << endl; cout << "3: intPtr2[0].use_count() = " << intPtr2[0].use_count() << endl; cout << "intPtr[0] = " << intPtr[0] << endl; cout << "intPtr2[0] = " << intPtr2[0] << endl; cout << "intPtr[0] = " << *intPtr[0] << endl; cout << "intPtr2[0] = " << *intPtr2[0] << endl; return 0; }