Declaring and allocating a 2d array in C++

Question

I am a Fortran user and do not know C++ well enough. I need to make some additions into an existing C++ code. I need to create a 2d matrix (say A) of type double whose size (say

Answer 1

The C++ equivalent of your Fortran code is:

int cols, rows;
if ( !(std::cin >> cols >> rows) )
     // error handling...

std::vector A(cols * rows);

To access an element of this array you would need to write A[r * rows + c] (or you could do it in a column-major fashion, that's up to you).

The element access is a bit clunky, so you could write a class that wraps up holding this vector and provides a 2-D accessor method.

In fact your best bet is to find a free library that already does this, instead of reinventing the wheel. There isn't a standard Matrix class in C++, because somebody would always want a different option (e.g. some would want row-major storage, some column-major, particular operations provided, etc. etc.)

Someone suggested boost::multi_array; that stores all its data contiguously in row-major order and is probably suitable. If you want standard matrix operations consider something like Eigen, again there are a lot of alternatives out there.

If you want to roll your own then it could look like:

struct FortranArray2D   // actually easily extensible to any number of dimensions
{
    FortranArray2D(size_t n_cols, size_t n_rows)
        : n_cols(n_cols), n_rows(n_rows), content(n_cols * n_rows) { }

    double &operator()(size_t col, size_t row) 
        { return content.at(row * n_rows + col); }

    void resize(size_t new_cols, size_t new_rows)
    { 
         FortranArray2D temp(new_cols, new_rows);
         // insert some logic to move values from old to new...
         *this = std::move(temp);
    }

private:
    size_t n_rows, n_cols;
    std::vector content;
};

Note in particular that by avoiding new you avoid the thousand and one headaches that come with manual memory management. Your class is copyable and movable by default. You could add further methods to replicate any functionality that the Fortran array has which you need.