For both user-defined or implicitly-defined destructors, after the body of the destructor is executed, the compiler calls the destructors for all non-static non-variant members of the class, in reverse order of declaration, then it calls the destructors of all direct non-virtual base classes in reverse order of construction (which in turn call the destructors of their members and their base classes, etc), and then, if this object is of most-derived class, it calls the destructors of all virtual bases.
在Destructor调用完成后，成员的Destructor调用前，this的状态是这样的：
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如果在析构成员B时，使用了成员A，那么需要手动获取A的指针，而不是使用this捕获。因为这时的this已经是失效的了：
std::shared_ptr<A> dump_a = this->a;
this->b = std::shared_ptr<B>(this->a->CreateB(),
[this, dump_a](B* p)
{
dump_a->ReleaseB(p); // OK
this->a->ReleaseB(p); // Error
});
