Calling member function from another member function's implementation

Question

So I'm defining a class sqmatrix of square matrices for fun and learning, and I've successfully defined a function submat that outputs a submatrix of the object constructed in a certain way:

sqmatrix sqmatrix::submat (unsigned int row, unsigned int col)
{ /* code */   return smat;    }

Now I want to define another function that takes the submatrix constructed by submat and outputs, say, the matrix where all elements have been multiplied by 42. To do so, I wrote

sqmatrix sqmatrix::cofact (unsigned int srow, unsigned int scol)
{
   sqmatrix cfac = 42 * m_mat.submat(srow, scol);
   return cfac;
}

where I'd previously overloaded * to work with my objects, and m_mat has been declared in the header of the class as a vector of vectors containing long long ints. However, this did not compile, so I went for member function pointers and wrote:

sqmatrix sqmatrix::cofact (unsigned int srow, unsigned int scol)
{
   sqmatrix (sqmatrix::*point)(unsigned int, unsigned int);
   point = &sqmatrix::submat;
   sqmatrix cfac = 42 * (m_mat.*point)(srow, scol);
   return cfac;
}

However, this does not compile either. Here are the relevant lines in the header file:

private:
 // ...
 std::vector< std::vector<long long int> > m_mat;
public:
 // ...
 sqmatrix submat(unsigned int row, unsigned int col);
 sqmatrix cofact(unsigned int srow, unsigned int scol);

Compiler says:

Error: pointer to member type sqmatrix (sqmatrix::)(unsigned int,unsigned int) incompatible with object type std::vector< std::vector<long long int> >

What am I getting wrong?

Answer 1

Mmm. I think you're wanting:

sqmatrix sqmatrix::cofact (unsigned int srow, unsigned int scol)
{
   sqmatrix cfac = 42 * submat(srow, scol);
   return cfac;
}

No idea what kind of matrix operation you're actually trying to do, but if you're trying to take the submatrix of this and then multiply it by a constant 42, then you just need to call submat(srow, scol).

The way you've written it, you're trying to call a member function of a vector, instead of a member function of your class which contains the vector.

C++ allows you to also call this->submat(srow, scol) which may make it more clear what you're actually doing, but most of the time you see people call member functions with no reference to this, as its perfectly valid C++ and also shorter.