C++ Access Violation

I'm working on an existing data parsing program that overlays a struct onto a buffer in order to extract values. Recently a new data format has been added to the buffer and requires a new struct. I abstracted functions into a common base class and defined a new struct like this:

struct Header
{
    Header () { }

public:
    virtual unsigned __int8 getCommonField1() const = 0;
}

struct HeaderTypeA : public Header
{
    unsigned __int8 Field1;

public:
    unsigned __int8 getCommonField1() const { return Field1; }
}

struct HeaderTypeB : public Header
{
    unsigned __int8 Field0;
    unsigned __int8 Field1;

public:
    unsigned __int8 getCommonField1() const { return Field1; }
}

The existing code which does the processing evaluates the data (this is working) and returns a pointer to the calling function... something like this:

Header* parse()
{    
    Header* parsedHeader = 0;

    if (typeADetected)
    {
        parsedHeader = (HeaderTypeA *) &buffer[offset];
        // Other logic here...
    }
    else if (typeBDetected)
    {
        parsedHeader = (HeaderTypeB *) &buffer[offset];
        // Other logic here...
    }

    return (parsedHeader);
}

The problem arises in the caller of the parsing logic. When the header is returned as a pointer the calls to the member functions result in an access violation error:

Header * hdr;
hdr = m_parser->parse();
unsigned __int8 valu开发者_C百科e = hdr->getCommonField1(); // Access Violation

I realize the snippet above is missing null pointer checks; I have excluded some of that logic for brevity. I've traced the code through and everything appears to be running smoothly until an attempt is made to call a method on the base class. In playing around with the code, I've also seen member function not present errors.

Thanks for any help.

You seem to be using the memory in buffer without constructing an object there. Since Header is polymorphic it contains a virtual table pointer and you can't initialize its data directly (legally you can't even if it's a POD but you would probably get away with it).

You should be using placement new in order to construct the Header object in buffer.

 parsedHeader = new (&buffer[offset]) HeaderTypeA;