Passing class field to member function - c++

Question

I have a class which has a few large fields (say, some big matrices) and has member functions to compute these matrices (the actual number of the matrices is bigger, of course)
class MyClass {
protected:
MatrixType m_11, m_12;
public:
void compute_m_11(double x);
void compute_m_12(double x);
}
Now, the computation code is very similar, and the most complex part is correct indexing of the matrix elements (which is the same for all the matrices involved). So I was thinking about splitting the indexing and the computation into separate functions: a compute_matrix function which will perform the indexing and call a compute_element function for each set of indexes in the matrix. This would greatly improve code readability and ease debugging.
So the compute_matrix function would take a MatrixType reference to the class field which I need to fill in and a std::function which would perform the actual computation. I obviously want to avoid writing anything that will involve additional copying of the matrices, since they can be quite large.
So, the questions are:
Is it legal/efficient to pass a reference to a class field to a class member function?
If so, do I need to use std::bind to pass the computation member functions? The compute_elements functions need to access some other fields of MyClass.
This is what I have in mind:
class MyClass {
protected:
MatrixType m_11, m_12;
double compute_elements_m11(int i, int j, double x);
double compute_elements_m12(int i, int j, double x);
void compute_matrix(MatrixType &m, double x, std::function<double(int, int, double) > f);
public:
void compute_m_11(double x) {compute_matrix(m_11, x, compute_elements_m11);};
void compute_m_12(double x) {compute_matrix(m_12, x, compute_elements_m12);};
}

Answer 1

It is legal (and not that uncommon) to pass a member reference, but your function type is wrong.
You could use std::bind, or you could use a plain pointer-to-member:
class MyClass {
protected:
MatrixType m_11, m_12;
double compute_elements_m11(int i, int j, double x);
double compute_elements_m12(int i, int j, double x);
void compute_matrix(MatrixType &m, double x, double (MyClass::*f) (int, int, double);
public:
void compute_m_11(double x) {compute_matrix(m_11, x, &MyClass::compute_elements_m11);};
void compute_m_12(double x) {compute_matrix(m_12, x, &MyClass::compute_elements_m12);};
};
std::bind and std::function gives a more flexible implementation though.

Answer 2

Sure it's not uncommon to pass a class attribute to an internal member function and you can also use std:bind to call the member function, but the question is do you really need that or can you just use a simple "if" or something similar to decide what to use? I'd say it depends on how many choices your code path has to decide that better.

Answer 3

Why would you pass a reference to a class field to a class member function? A better solution would be to implement a get method and use that in the compute_matrix function. Your class will then look something like this:
class MyClass {
protected:
MatrixType m_11, m_12;
double compute_elements_m11(int i, int j, double x);
double compute_elements_m12(int i, int j, double x);
void compute_matrix(double x, std::function<double(int, int, double) > f);
public:
void compute_m_11(double x) {compute_matrix(x, compute_elements_m11);};
void compute_m_12(double x) {compute_matrix(x, compute_elements_m12);};
MatrixType& getMatrixType_11( return m_11 );
MatrixType& getMatrixType_12( return m_12 );
}