CODEWITHSUNDEEP
c++pointersdeep-copy
Xerunix
Xerunix

Reputation: 431

C++ Pointer Deep Copy

I am trying to make a deep copy of a pointer to an array of integer, and am having trouble deciding what code is unnecessarily verbose and what is needed.

The pointer I am trying to copy is just a simple integer array.

int* vertexArray = new int[G->size()];

It contains the numbers 0 to size()-1 as its values. My first though was to make a new int* where equal to vertex array,

int* shortestTour = vertexArray

but I believe that will make the shortestTour change every time I permute the vertexArray. Is the only way to make a deep copy of this with a loop like this

for(int i=0; i<G->size(); i++){
    shortestTour[i] = vertexArray[i];
}

and then to run that loop every time I find a tour/path shorter than the current shortest?

EDIT: This is for a simple, brute force implementation of the travelling salesman problem.

Upvotes: 9

Views: 37458

Answers (1)

vsoftco
vsoftco

Reputation: 56577

As you may have guessed, a deep copy is not copying the pointer (after all, a pointer is just an address that points to some memory location), but copying the memory that is being pointed by the pointer.

So, if you have

int* vertexArray = new int[G->size()];

a deep copy is

int* deepCopy = new int[G->size()];
for(size_t i = 0; i < G->size(); ++i)
    deepCopy[i] = vertexArray[i]; // copy the allocated memory

Of course, there are smarter and faster ways of copying the memory, like

std::memcpy(deepCopy, vertexArray, sizeof(int)*G->size()); // should be faster

A shallow copy is just copying the pointer, not the memory,

int* shallowCopy = vertexArray;

Any modifications you'll make will be reflected by both shallowCopy and vertexArray pointers, as both now point to the same memory chunk.

You should think whether you need a deep copy or not. If you just want to share data around, you probably don't. If the data must be "local", then probably you do.

Upvotes: 20

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