CRT virtual destructor

Question

I ran into a heap corruption today caused by different CRT settings (MTd MDd) in my dll and my actual project. What I found strange is that the application only crashed when I set the destructor in the dll to be virtual. Is there an easy explanation for that? I get that I can't free memory that's not on my heap, but where exactly is the difference when I define the destructor as non-virtual.

Some Code just to make it a little clearer

The DLL

#pragma once
class CTestClass
{
public:
    _declspec(dllexport) CTestClass() {};
    _declspec(dllexport) virtual ~CTestClass() {};
};

And my project

int main(int argc, char* argv[])
{
    CTestClass *foo = new CTestClass;
    delete foo; // Crashes if the destructor is virtual but works if it's not
}

Answer 1

There is a difference between

class CTestClass
{
public:
    _declspec(dllexport) CTestClass() {}
    _declspec(dllexport) virtual ~CTestClass() {}
};

and

__declspec(dllexport) class CTestClass
{
public:
     CTestClass() {}
     virtual ~CTestClass() {}
};

In the former case you instructed a compiler to export only two member functions: CTestClass::CTestClass() and CTestClass::~CTestClass(). But in the latter case you would instruct a compiler to export the table of virtual functions as well. This table is required once you have got a virtual destructor. So it might be the cause of the crash. When your program tries to call virtual destructor, it looks for it in the associated virtual functions table, but it is not initialized properly so we do not know where it really points then. If your destructor is not virtual, then you do not need any virtual function table and everything works just fine.