Using Constructor of base class for constructor in derived?

[ced]

Hi!

I'm trying to understand how the inheritance works but ran into some issues. I've been able to solve most of them with other forum threads but I have a feeling that I'm missing something important ;).

I have a base class called "Matrix" with a .cpp and .h file.
There, the constructor Matrix(int n, int m) creates a matrix with n rows and m columns.

In addition, I would like to make an "Eye" class for the Identity Matrix with a separate .cpp and .h file.
My idea was to have a constructor Eye(int n) which would look something like this:

Eye::Eye (int n) {
    Matrix temp(n,n);
    for (int i=0; i<n ; i++) {temp.set(i,i,1);} 
    //set(int a, int b, int c) is a method of base class which assigns the value c to the spot (a,b) in the Matrix
    (*this)=temp; // I know this doesn't work, but that's sort of the idea
}

Issues:
1. My values are saved within a "protected" array of numbers (I'm using vector< vector< double>>). It looks like I cannot change a protected value from a member of the base class in a method of the derived class, therefore I need to use this public method "set(...)". Is that correct?
2. The assignment operator doesn't work because it doesn't know how to convert "Matrix" to "Eye".. or vice-versa. That makes sense... but how can I achieve that effect?

Thanks for some help!

Ced

[TheDestroyer]

1. My values are saved within a "protected" array of numbers (I'm using vector< vector< double>>). It looks like I cannot change a protected value from a member of the base class in a method of the derived class, therefore I need to use this public method "set(...)". Is that correct?

Yes, if the Eye is not inherited from Matrix. But you may define Eye as a friend class if you want to access its private or protected memebers.

2. The assignment operator doesn't work because it doesn't know how to convert "Matrix" to "Eye".. or vice-versa. That makes sense... but how can I achieve that effect?

You have to define an operator= that tells the compiler how those classes are related:

Eye& operator=(const Matrix& src)
{
    //for example
    this->Matrix.matrix = src.matrix; //copies the internal vector<vector<double> > called matrix.
}

[Athar]

1. protected allows you to access base members in the derived class (as opposed to private).
2. It does work. An Eye is a Matrix if it derives from Matrix, so static_cast<Matrix&>(*this)=temp; or Matrix::operator=(temp); works.
3. Both approaches aren't the way to go. You can call a base constructor as follows:
Eye::Eye (int n) : Matrix(n,n) {}

4. Inheritance isn't appropriate here. An indentity matrix is a matrix, but it is no longer an identity matrix once you change it (however, the type remains). Instead, create a static Matrix method that returns an identity matrix.

[ced]

Thanks for the replies!

@The Destroyer:
Eye is inherited from Matrix as class Eye : public Matrix. But when I tried to call temp.mat[n][n] = 1; (mat is the name of my number array in the Matrix class), I got the following error:

matrix.h:13:34: error: ‘std::vector<std::vector<double> > Matrix::mat’ is protected
eye.cpp:6:10: error: within this context

Accessing "mat" for a member of Eye from within Eye works, but I couldn't access "mat" belonging to "temp", which is a "Matrix", from within "Eye".

@Athar:
to 2: Maybe the error lies somewhere else then. But my compiler was complaining:

eye.cpp:8:15: error: no match for ‘operator=’ in ‘*(Eye*)this = temp’
eye.cpp:8:15: note: candidate is:
eye.h:7:7: note: Eye& Eye::operator=(const Eye&)
eye.h:7:7: note:   no known conversion for argument 1 from ‘Matrix’ to ‘const Eye&’

to 3: Worked great, thanks!
to 4: Seems obvious now that you say it, thanks. I haven't really looked at static methods yet, will do.

[Athar]

Accessing "mat" for a member of Eye from within Eye works, but I couldn't access "mat" belonging to "temp", which is a "Matrix", from within "Eye".

Yeah, you can access your own protected Matrix internals, but not those of other objects.

About 2.:
Yeah, to avoid calling Eye's assignment operator, explictly call the one of the base class.