0 
using namespace std;

class Layer
{
protected:
    Layer *lower;
    Layer *upper;
public:
    Layer(Layer *lo,Layer *up):lower(lo),upper(up)
    {}
    virtual void send()=0;
    virtual void receive()=0;
};
class Physical_Layer:public Layer
{
public:
    Physical_Layer(Layer *p):Layer(NULL,p)
    {
        cout<<"Physical_Layer constructed"<<endl;
    }
    virtual void send()
    {
        cout<<"Data send from Physical_Layer"<<endl;
        receive();
    }
    virtual void receive()
    {
        cout<<"Physical_Layer calling receive of DataLink_Layer"<<endl;
        upper->receive();
    }
};
class DataLink_Layer:public Layer
{
public:
    DataLink_Layer(Layer *p):Layer(new Physical_Layer(this),p)
    {
        cout<<"DataLink_Layer Constructed"<<endl;
        lower->send();
    }
    virtual void send()
    {
        cout<<"Data send from DataLink_Layer"<<endl;
        lower->send();
    }
    virtual void receive()
    {
        cout<<"DataLink_Layer calling receive of Application_Layer"<<endl;
        cout<<typeid(upper).name()<<endl;



        upper->receive();



    }
};
class Application_Layer:public Layer
{
public:
    Application_Layer():Layer(new DataLink_Layer(this),NULL)
    {
        cout<<"Application_Layer Constructed"<<endl;
        send();
    }
    virtual void send()
    {
        cout<<"Sending data from Application_Layer"<<endl;
        lower->send();
    }
    virtual void receive()
    {
        cout<<"Receiving data at Application_Layer"<<endl;
    }
};

int main()
{
    Layer *l=new Application_Layer();
}

我试图使用协议设计模式来模拟三层协议栈。但是，在取消引用 DataLink_Layer 接收中的上层->接收时，我得到了一个运行时异常：System.AccessViolationException。为什么我会得到它？

4

1 回答 1

3

的构造函数试图在构建基类之前DataLink_Layer回调到Application_Layervia a （此时您仍在评估）。Layer*LayerApplication_Layernew DataLink_Layer(this)

通过简单地调用构造函数upper->receive()，您可以更清楚地看到这一点。DataLink_Layer

此常见问题解答解释了有关this在构造函数中使用的更多信息。

这个更简单的例子可能更清楚地说明了这个问题：

struct C;
struct A
{
    A(C* c) {};
    virtual void Foo() = 0;
};

struct C
{
    C(A* a)
    {
        a->Foo();
    }
};

struct B : public A
{
    B() : A(new C(this)) {}
    void Foo() {}
};

int main()
{
    B b;
}

通常，您不应该使用构造函数对部分构造的对象执行复杂的调用堆栈。只需在构造后显式调用send()orreceive()函数即可。

于 2013-01-09T04:51:09.397 回答