C++: how to initialize the dimensions of a matrix inside a class through parameters of its constructor?

Question

I need help in something really simple, which C++ makes very difficult. I created a class, lattice, with the aim to initialize and treat a matrix. There are the following private members:

private unsigned dim;
private double matrix [dim][dim];

I would like to initialize the variable dim in the constructor of the class through a parameter, but the compiler continue to return errors. I tried to make dim public and static and initializing it in the main program, but there are still problems. How could I create this simple class?

Moreover, I also implemented some methods in the class in order to update the values of the matrix. Is it true that, initializing an object of the class in a main program and then using its "updating" methods, the values of the matrix are stored only once?

Thank you for your help.

Answer 1

There are (at least) three ways to create such a class:

• with a template

  template<size_t dim>
  class Matrix
  {
     private:
       double matrix[dim][dim];
  }
  

• with the use of built in types such as std::vector (e.g. valarray would work too)

  #include <vector>
  class Matrix
  {
     private:
       size_t dim;
       std::vector<std::vector<double> > matrix;
     public:
       Matrix(size_t dim_) : dim(dim_), matrix()
       {
         matrix.resize(dim);
         for ( size_t i = 0; i < dim; ++i )
            matrix[i].resize(dim);
       }
  }
  

• with the use of plain arrays (I do not recommend that!)

  class Matrix
  {
     private:
       size_t dim;
       double** matrix;
     public:
       Matrix(size_t dim_) : dim(dim_), matrix()
       {
         matrix = new double*[dim];
         for ( size_t i = 0; i < dim; ++i )
            matrix[i] = new double[dim];
       }
       ~Matrix() // you need a destructor!
       {
         for ( size_t i = 0; i < dim; ++i )
            delete [] matrix[i];
         delete [] matrix;
       }
  }