Problem with references

[fdm]

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

 

[Victor Bazarov]

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() = 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;
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

 

[fdm]

Hm actually this is also wrong:

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]
...

It should print:

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

etc. I am missing something very basic here...

 

[fdm]

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() = parameters[j];

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

Ah yes I should not expect the local copy to update the vector. This does
what I want:

m_ControlPoints[ctrlCount].getDeformation() = parameters[j];

 

[Victor Bazarov]

In a class I have the following functions:
[..]
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() = parameters[j];

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

Ah yes I should not expect the local copy to update the vector. This
does what I want:

m_ControlPoints[ctrlCount].getDeformation() = parameters[j];

Good!

Another way to do what you need is to make your 'cp' a reference:

ControlPointType &cp = m_ControlPoints[ctrlCount];
... // everything else the same as previously

(it makes 'cp' an alias for the element of the vector). Note that the
declaration of 'cp' now contains the '&' (ampersand) in front of the
name of the variable.

V