Question

I’d like to implement something like below:

struct MyArray {
    void* Elements;
    int Capacity;
    int ElementsCount;
    size_t ElementSize;

    //methods
    void AddElement(void* item);
    //...
};

void* Elements should be pointer to items of any type. Every element should have particular size (ElementSize variable) and AddElement(void*) method should add item to existing array.
The problem is that I can’t do pointer arythmetic with my Array, I know I need to use casting every time I want to use it but I completely don’t know how to do it.
And I know template would be better solution for it, but in this case I’d like to practise with pointers 🙂

Thanks for help in advance.

Answer 1

Yes, you cannot do pointer arithmetic on void*, you will have to cast to char* to do the arithmetic e.g. something like:

void MyArray::AddElement( void * item )
{
    // verify that ElementsCount is not already Capacity and if so, reallocate or throw
    void * insertionPoint = static_cast<char *>(Elements) + (ElementSize * ElementsCount );
    memcpy( insertionPoint, item, ElementSize );
    ++ElementsCount;
}

Note that you need static_cast to cast from void* to char* and you do not need to explicity cast at all back to void* which is why I can assign it to insertionPoint.