Working code throwing exception when new lines added. Debugging tips?

Question

I'm working on a proprietary code base, so I have to abstract this.

I'm trying to set the value of DataType_T*** myData in MyApplication. I'm using a shared c++ library (which I denote library A) to set the value. The shared c++ library is just a simple wrapper class around a C API.The C API is included in library A as a shared library (which I denote library B).

So MyApplication calls GetData(myData) in A which calls GetData(myData) in B.

MyApplication has the following code:

void OnButtonPress(){
    const DataType*** myData;
    GetData(myData);
    DataTypeVal1 val1 = (*myData)[0]->val1; // just grabbing some info.
}

GetData(myData): works, and properly sets myData.

Me: types some new code

void OnButtonPress(){
    const DataType*** myData;
    GetData(myData);
    const void* strData = (*myData)[0]->strData; // just grabbing some info now that we have the pointer.

    //Add lots more new code that does this over and over for each member of myData
    String^ str = gcnew String(static_cast<const char*>(strData));
}

GetData(myData): throws a write access violation.

Me: ". . . . . . .what."

• Could the exception be getting thrown because of some sort of dll unloading?

• Is there a possibility that the linking process changes when I type new code?

I haven't encountered an issue like this before, so I don't really know how to debug this.

Got suggestions?

Thanks.

---

Solved. I found my undefined behavior.

Answer 1

I think you want something like this:

const DataType** myData;
GetData(&myData);
const void* strData = myData[0]->strData; 

Because in the original code you're just passing a pointer by value; an uninitialized pointer, and then accessing that same uninitialized value.

ed: fixed up the third line