On VTable pointers and malloc

问题

Is there any compiler independent and syntactically elegant way to set a vtable pointer in an object allocated with malloc?

I cannot use new directly as I need to be able to control the flow of memory releases on demand which requires the use of void ptrs to hold memory locations in a memory manager until there is ample time to release.

class AbstractData
{
public:
   AbstractData() {}
   virtual ~AbstractData() {}

protected:
   virtual void SetData(int NewData) =0;
   virtual int GetData() const =0;
};

class ConcreteData : public AbstractData
{
protected:
   int Data;

public:
   ConcreteData() {}
   ~ConcreteData() {}

   void SetData(int NewData);
   int GetData() const;

};

void ConcreteData::SetData(int NewData) {Data=NewData;}

int ConcreteData::GetData() const 
{return Data;}

int main(int argc, char* argv[])
{
    int OBJ_NUMBER = 4;
    ConcreteData* Test = (ConcreteData*)malloc(OBJ_NUMBER*sizeof(ConcreteData));

    if (!Test)
        return -1;        

    for (int x = 0; x < OBJ_NUMBER; x++)
       Test[x] = ConcreteData();

    Test[0]->GetData(); //Constructor was never called, vptr never initialized, crash

    free(Test);
    Test = NULL;
}

I was hoping the local copy would have an initialized vtable pointer, but alas it does not. I know you can do a compiler dependent dereference to the offset of the vptr if you know where it is, but this solution is compiler dependent and inelegant to use across many allocations. Example works with MSVC++ 8.0

int main(int argc, char* argv[])
{
    int OBJ_NUMBER = 4;
    ConcreteData* Test = (ConcreteData*)malloc(OBJ_NUMBER*sizeof(ConcreteData));

    if (!Test)
        return -1;        

    ConcreteData StealVPtr();

    int* VPtr = *(int**)StealVPtr; 

    for (int x = 0; x < OBJ_NUMBER; x++)
       *(int**)Test[x] = VPtr;

    Test[0]->GetData(); //VPtr initialized in compiler dependent way

    free(Test);
    Test = NULL;
}

Alternatively, placement new could be used, but once again it looks syntactically inelegant and can cause array offsetting problems of types with destructors when it adds the array count in front of the ptr.

int main(int argc, char* argv[])
{
    int OBJ_NUMBER = 4;
    ConcreteData* Test = (ConcreteData*)malloc(OBJ_NUMBER*sizeof(ConcreteData));

    if (!Test)
        return -1;        

    for (int x = 0; x < OBJ_NUMBER; x++)
    {
        if (!(ConcreteData* PlcTest = new(Test[x]) ConcreteData()))
        {
           free(Test);
           Test = NULL;
           return -1;
        }
    }

    PlcTest[0]->GetData(); //Constructor was invoked and VPtr was initialized

    for (int x = OBJ_NUMBER-1; x >= 0; x--)
        PlcTest[x].~ConcreteData();

    PlcTest = NULL;

    free(Test);
    Test = NULL;
}

Are these really the only ways to initialize the VTable ptr/call constructors on objects using malloc?

回答1:

You may not know that you can override global new and delete. Pay attention, you also need to override new[] and delete[] to be complete

Here an example:

void * operator new( size_t size ) 
{
    return super_malloc( size );
}

回答2:

You could use the placement new operator to construct class instances in preallocated memory.

来源：https://stackoverflow.com/questions/7702791/on-vtable-pointers-and-malloc

标签

c++

memory-management

malloc

vtable