Extracting a reference from a c++ vector

Question

Hello folks,

I have a vector< vector< vector< int>>> and I would like to extract from it a vector< vector< int>> to process it individually.

The problem is that when I write :

myMatrix = myCube[anIndex];

the matrix is copied but I only want a reference in order to save memory. Can you please help me out ?

Thanks a lot!

Answer 1

Use an Iterator of type vector< vector< int> >::const_iterator. Hope this helps.

Answer 2

vector::operator[] returns a reference - as long as you store that result into a reference instead of a full-blown object you should avoid a copy.

vector< vector<int > >& myMatrix = myCube[anIndex];

Answer 3

Just use

vector<vector<int> >& myMatrix = myCube[anIndex]; 

Answer 4

If myMatrix is a class member and you want to be able to initialise it outside the constructor, your only choice is using a pointer:

class Whatever
{
  //...
  vector<vector<int>>* myMatrix;
  //...
}

  myMatrix = &myCube[anIndex]

EDIT: If you can live with initialising myMatrix in the constructor, you can use a reference:

class Whatever
{
  //...
  vector<vector<int> >& myMatrix;
  //...
}

Whatever::Whatever(vector<vector<vector<int> > >& myCube), int anIndex)
: myMatrix(myCube[anIndex])
{
  //...
}

Incidentally, you have to separate the > in your code, unless you are using a C++0x conformant compiler; otherwise, the compiler will parse the >> as operator>> and give you an error.

Answer 5

Instead of using operator[] use functions returning iterators as advance, and declare the matrix as follows

vector< vector< int> >::iterator myMatrixPtr;

myMatrixPtr = std::advance(myCube, anIndex);

Now you are able to work with myMatrixPtr as if it were a pointer. If you prefer a reference you can initialize one after this initialization

vector< vector<int > >& myMatrix = *myMatrixPtr;

Answer 6

Be careful with member references! If the referred-to object is cleaned up before the instance of MyClass is, you will have an invalid reference. Often (but not always), if you find that you need to use a member reference, it is an early warning sign that your design could be improved. I rarely use member references for anything other than as a handle to an object's owner, where I can be absolutely certain that the referred-to object will outlive the reference.

It might help if you change slightly your class' responsibilities. Right now, you are building up a set of data, and then creating an object to which you pass that set of data. Instead, why not first build the object, then put the data directly into it.

// To start, define your class to allow you to build up the data
class MyMatrix
{
public:
  MyMatrix ()
    { }

  void  setData (int x, int y, int data)
    { this->myData [x][y] = data; }
  int   getData (int x, int y)
    { return this->myData [x][y]; }

private:
  vector<vector<int> > myData; 
} 

// Then, create a vector of 6 (or however many you need) matrices
int numberOfMatrices = 6;
vector<MyMatrix> myCube (numberOfMatrices);

// Then, fill in the vector
for (int matrixIndex = 0; matrixIndex < numberOfMatrices; matrixIndex++)
  {
  // Fill in the data for this matrix
  MyMatrix &currentMatrix = myCube [matrixIndex];

  currentMatrix.setData (0 /* x */, 0 /* y */, 0 /* data */);
  // ... rest of data
  }

Then, you have all the data already in your objects, ready to be processed as you need. Plus, each object owns its data, so as long as the object is around, its member data will be valid. Best of all, using objects with usefully named member functions will make your code much more self-documenting than tons of nested containers.