Re: Problem with references

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 08 Sep 2009 10:21:28 -0400

Message-ID:

<h85p97$d0m$1@news.datemas.de>

fdm wrote:

In a class I have the following functions:

template <typename PointType>
class ControlPoint
{
public:
   typedef typename PointType::ValueType ValueType;

   // Used for read only
   PointType getDeformation() const {
     return m_deformation;
   }

   // Used when updating m_deformation field
   PointType & getDeformation(){
     return m_deformation;
   }

   PointType m_deformation;

};

In some other code I run through a list of empty controlPoints and
initializes the m_deformation field:

int a = 0;
int params = m_ControlPoints.size(); // std::vector with controlpoints.
for (int i=0; i<NDimensions; i++) {
   int ctrlCount = 0;
   for (int j=a; j<params+a; j++) {
     ControlPointType cp = m_ControlPoints[ctrlCount];

So, here you make a copy of the 'ctrlCount'th element of that vector.

cp.getDeformation()[i] = parameters[j];

And here you change the deformation *for that copy*. Do you expect the
vector element to change as well? Why?

It's the same as:

        std::vector<int> v(3); // three zeros
        int a = v[0]; // make a copy of the 0th element
        a = 42; // change the copy
        std::cout << "v[0] = " << v[0]; // what will it print?

What do you expect that code to change in the 'v' vector?

     std::cout<< "cp.getDeformation() = " << cp.getDeformation() <<
std::endl;
     ctrlCount++;
   }
   a = a + params;
}

The line:

     std::cout<< "cp.getDeformation() = " << cp.getDeformation() <<
std::endl;

prints the expected result:

...
cp.getDeformation() = [1058, 0]
cp.getDeformation() = [1059, 0]
cp.getDeformation() = [1060, 0]
cp.getDeformation() = [1061, 0]
cp.getDeformation() = [1062, 0]
cp.getDeformation() = [1063, 0]
cp.getDeformation() = [0, 7000]
cp.getDeformation() = [0, 7001]
cp.getDeformation() = [0, 7002]
cp.getDeformation() = [0, 7003]
cp.getDeformation() = [0, 7004]
cp.getDeformation() = [0, 7005]
...

Just after this loop I create another loop that runs through the same
controlpoints and prints the m_deformation filed to verify that its
correct:

for (int i=0; i<params; i++) {
   ControlPointType cp = m_ControlPoints[i];
   std::cout << "cp.getDeformation() = " << cp.getDeformation() <<
std::cout;
}

but now they no longer exists:

cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]
cp.getDeformation() = [0, 0]

I have verified that the function:

   // Used when updating m_deformation field
   PointType & getDeformation(){
     return m_deformation;
   }

is called, but why are the m_deformation field 0 when I read it afterwards?

Because you never change them.

V
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