Question

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();
}

I was trying to simulate a three layer protocol stack using the Protocol Design Pattern. But while dereferencing the upper->receive in DataLink_Layer’s receive i am getting a run time exception: System.AccessViolationException. Why am i getting it?

Answer 1

The constructor of DataLink_Layer is trying to call back into the Application_Layer via a Layer* before the Layer base class of Application_Layer is even constructed (you are still evaluating the new DataLink_Layer(this) at this time).

You can see this more clearly by simply calling upper->receive() in the DataLink_Layer constructor.

This FAQ explains a little more about using this in constructors.

This simpler example may more clearly illustrate the issue:

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;
}

In general, you shouldn’t use the constructor to execute a complicated call stack on partially constructed objects. Just call the send() or receive() functions explicitly after construction.