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調(diào)用完成后，成員的Destructor調(diào)用前，this的狀態(tài)是這樣的：

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如果在析構(gòu)成員B時，使用了成員A，那么需要手動獲取A的指針，而不是使用this捕獲。因為這時的this已經(jīng)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
    });