Question

The following code compile without errors. Basically, the C#2005 Console application calls VC++2005 class library which in turn calls native VC++6 code. I get the following error when I run the C#2005 application:

“Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.“

What is the cause of this error? And how to go about correcting it?

Edit1: It crashes at the line StdStringWrapper ssw = w.GetNext();

Edit2: I followed the advice of Naveen and used an integer index instead of iterators and there is no more errors now. A big thanks to all who commented as well!

Code Written in C#2005 as Console Application:

class Program
{
    static void Main(string[] args)
    {
        Class1 test= new Class1();
        test.PerformAction();
        test.PerformAction();
        test.PerformAction();
        test.PerformAction();
    }
}

Code Written in VC++2005 as Class Library:

public ref class Class1
{
    public:
        void PerformAction();       
};

void Class1::PerformAction()
{
    DoSomethingClass d;
    StdStringContainer w;
    d.PerformAction(w);

    for(int i=0; i<w.GetSize(); i++)
    {
        StdStringWrapper ssw = w.GetNext();
        std::cout << ssw.CStr() << std::endl;
    }
}

Code Written in VC++6 as Dynamic Link Library:

#ifdef NATIVECODE_EXPORTS
    #define NATIVECODE_API __declspec(dllexport)
#else
    #define NATIVECODE_API __declspec(dllimport)
#endif

class NATIVECODE_API StdStringWrapper
{
    private:
        std::string _s;

    public:     
        StdStringWrapper();
        StdStringWrapper(const char *s);
        void Append(const char *s);
        const char* CStr() const;       
};

StdStringWrapper::StdStringWrapper()
{
}

StdStringWrapper::StdStringWrapper(const char *s)
{
    _s.append(s);
}

void StdStringWrapper::Append(const char *s)
{
    _s.append(s);
}

const char* StdStringWrapper::CStr() const
{
    return _s.c_str();
}

//

class NATIVECODE_API StdStringContainer
{
    private:
        std::vector<StdStringWrapper> _items;
        std::vector<StdStringWrapper>::iterator _it;

    public: 
        void Add(const StdStringWrapper& item);
        int GetSize() const;        
        StdStringWrapper& GetNext();  
};

void StdStringContainer::Add(const StdStringWrapper &item)
{
    _items.insert(_items.end(),item);
}

int StdStringContainer::GetSize() const
{
    return _items.size();
}

StdStringWrapper& StdStringContainer::GetNext()
{
    std::vector<StdStringWrapper>::iterator it = _it;
    _it++;

    return *it;
}

//

class NATIVECODE_API DoSomethingClass
{
    public:
        void PerformAction(StdStringContainer &s);
};

void DoSomethingClass::PerformAction(StdStringContainer &s)
{
    StdStringWrapper w1;
    w1.Append("This is string one");
    s.Add(w1);

    StdStringWrapper w2;
    w2.Append("This is string two");
    s.Add(w2);
}

Answer 1

The main problem from my point of view is you are storing an iterator to a vector in your stdStringContainer class. Remember that whenever vector resizes all the existing iterators are invalidated. So whenever you do insert operation into the vector it may be possible that it resizes and your existing iterator becomes invalid. If you try to to dereference it in GetNext() then it will access invalid memory location. For checking whether this really the case try to reserve the initial vector size to some relatively big number so that the resizing doesn’t happen. You can reserve the size using reserve() method, in which case it is guaranteed that the capacity() of the vector is greater than or equal to the reserved value.