C++ 中 shared_ptr weak_ptr

shared_ptr

std::shared_ptr<int> sp1 = new int();  // shared count = 1, weak count = 0
std::shared_ptr<int> sp2(sp1);         // shared count = 2, weak count = 0
std::shared_ptr<int> sp3 = sp2;        // shared count = 3, weak count = 0

std::weak_ptr<int> wp1(sp1);           // shared count = 3, weak count = 1
std::weak_ptr<int> wp2(wp1);           // shared count = 3, weak count = 2
std::weak_ptr<int> wp3 = wp2;          // shared count = 3, weak count = 3

shared_ptr weak_ptr 使用 reset 或者指向另一个 managed object导致 shared count或weak count相应的减一。

1. 类继承中使用shared_ptr

class Base {};
class Derived : public Base {};
......
shared_ptr<Derived> dp1(new Derived);
shared_ptr<Base> bp1 = dp1;
shared_ptr<Base> bp2(dp1);
shared_ptr<Base> bp3(new Derived); 

2. casting shared_ptr

shared_ptr<Base> base_ptr (new Base);
shared_ptr<Derived> derived_ptr;
// if static_cast<Derived *>(base_ptr.get()) is valid, then the following is valid:
derived_ptr = static_pointer_cast<Derived>(base_ptr);

3. make_shared

使用shared_ptr = new int()，会导致两次内存分配：int对象的内存分配跟shared_ptr内部的 manager object 一次内存分配。make_shared 对此进行了优化，一次性分配 int + manager object 内存空间大小。
make_shared 用法：

shared_ptr<Thing> p (make_shared<Thing>(42, "I'm a Thing!"));
shared_ptr<Base> bp(make_shared<Derived1>());     // shared_ptr中的 template参数与make_shared中的tmeplate参数可以不一样（继承关系）

使用 weak_ptr

void do_it(weak_ptr<Thing> wp){
shared_ptr<Thing> sp = wp.lock(); // get shared_ptr from weak_ptr
if(sp)
sp->defrangulate(); // tell the Thing to do something
else
cout << "The Thing is gone!" << endl;
}

也可以直接从weak_ptr构建shared_ptr，这个时间如果weak_ptr过期（通过 weak_ptr::expired() 可以查询），则抛出异常：

void do_it(weak_ptr<Thing> wp){
shared_ptr<Thing> sp(wp); // construct shared_ptr from weak_ptr
// exception thrown if wp is expired, so if here, sp is good to go
sp->defrangulate(); // tell the Thing to do something
}

标签：count,std,make,weak,shared,ptr
来源： https://www.cnblogs.com/vaughnhuang/p/16511072.html