CODEWITHSUNDEEP
c++pointers
Rudy
Rudy

Reputation: 7044

Copying the contents from one pointer to another

I have 2 class: 

    class Request
    {
      KeyValuePair* dimension;
    };

    class Response
    {
      KeyValuePair* dimension;
    };

    class KeyValuePair
    {
        public:
        hash_map<string, string> hmap;
    };

in one of the method, I want to populate the Response.dimension with Request.dimension but I want the Request.dimension pointing to a different address from Response.dimension?

I don't want any change Response.dimension which also affect Request.dimension.

Is there a way to do that? 

void Transformation::applyTransformations(const Request& req, Response& res)
{
   res.dimension = req.dimension; // WRONG. Pointing the same address.
}

Upvotes: 1

Views: 7405

Answers (5)

m0skit0
m0skit0

Reputation: 25863

Copy constructor

More on C++ copy constructors

Upvotes: 0

CapelliC
CapelliC

Reputation: 60004

You could implement a 'copy on write' pointer, to avoid unneeded overhead.

Upvotes: 0

Ivaylo Strandjev
Ivaylo Strandjev

Reputation: 70931

You need to allocate new memory. Also you need to have copy constructor(or use the default one if appropriate).

res.dimension = new KeyValuePair(req.dimension);

Also you will have to handle the memory i.e. define a copy constructor, copy operator, empty constructor and destructor.

Upvotes: 0

Alok Save
Alok Save

Reputation: 206508

You can overload the =operator of your Response class to perform a deep copy of the request.dimension, this will make your Response.dimension have same value as that of Request.dimension and yet will be independent of any changes to Request.dimension.

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258558

Yes, of course:

*(res.dimension) = *(req.dimension);

Note that if KeyValuePair is not a POD type, you should define the assignment operator (operator =), as it will get called during the assignment. If you don't, the compiler-generated one will be called.

You should also check if res.dimension is properly allocated, if not, use the copy constructor:

if ( res.dimension )
   *(res.dimension) = *(req.dimension);
else
   res.dimension = new KeyValuePair(*(req.dimension));

Upvotes: 2

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