How to copy a structure with pointers to data inside (so to copy pointers and data they point to)?

Question

so I have a structure like

struct GetResultStructure
{
  int length;
  char* ptr;
};

I need a way to make a full copy of it meaning I need a copy to have a structure with new ptr poinnting on to copy of data I had in original structure. Is It any how possible? I mean any structure I have which contains ptrs will have some fields with its lengths I need a function that would copy my structure coping all ptrs and data they point to by given array of lengthes... Any cool boost function for it? Or any way how to create such function?

Answer 1

For the specific scenario you describe, use a std::vector or some other sequence container. If you do so, then simply copying objects of type GetResultStructure will make copies of the pointed-to data as well:

struct GetResultStructure {
    std::vector<char> data;
};

GetResultStructure a = GetData();
GetResultStructure b = a; // a and b have distinct, equivalent data vectors

In general, when you do need to implement this yourself, you do so by implementing a copy constructor and a copy assignment operator. The easiest way to do that is to use the copy-and-swap idiom, covered in great detail in What is the copy-and-swap idiom?

Answer 2

It's pretty much up to you to implement that. Normally you want to do it as a copy constructor so you only have to do it in one place. Unfortunately, there's no real magic to avoid telling the computer about how to copy your structure.

Of course, that only applies if your structure really is substantially different from something that's already written. The one you've given looks a lot like a string or (possibly) vector. Unless you really need to implement something new, you're probably better off just using one of those that's already provided.

Answer 3

Both a copy constructor and assignment operator should be implemented (in the way stated above). A technique which may aid in this process, however, is using a dereference operator (*) when copying pointer data. This will copy the pointer data rather than the memory locations. If you do ptr1 = ptr2 it simply sets the memory location of ptr1 to ptr2 which is why we dereference.

For instance, I'll just show a quick example for a copy constructor:

GetResultStructure(const GetResultStructure& other)
    : length(other.length), ptr(new char[length]) // <--- VERY _important_ - initialization of pointer
{
    // Alternatively, put your initialization here like so:
    // ptr = new char[length];
    for(int i=0;i<length;++i)
    {
        ptr[i]  = new char;
        *ptr[i] = *other.ptr[i]; // Copy the values - not the memory locations
    }
}

And then obviously be sure to clean up in your destructor to prevent memory leaks.

Regards,
Dennis M.

Answer 4

GetResultStructure doCopy(GetResultStructure const& copy) {
  GetResultStructure newstruct;
  newstruct.length = copy.length;
  newstruct.ptr = new char[newstruct.length];
  memcpy(newstruct.ptr, copy.ptr, newstruct.length*sizeof(char));
  return newstruct;
}

Should be simple. Yes, the sizeof(char) isn't really necessary, but there to show what to do for other data types.

Answer 5

Since you tagged it as C++: Write a copy constructor and an assignment operator, within which you implement your deep copy code:

struct GetResultStructure
{
    GetResultStructure(const GetResultStructure& other)
    {
        // Deep copy code in here   
    }
    GetResultStructure& operator=(const GetResultStructure& other)
    {
        if (this != &other) {
            // Deep copy code in here
        }
        return *this
    }
    int length;
    char* ptr;
};