//field.h
class Field {
    std::vector<std::vector<double>> data;

    public:
    Field(int, int);
};
Field::Field (int dim0, int dim1) :: data(dim0, std::vector<double>(dim1, 0)) { }

//approx.h
class Field;
class Approx {
    Field SWAP;

    public:
    Approx(int, int);
};
Approx::Approx (int size, int dim) { }
/*I want to initialise SWAP like this:
     if (size > dim) SWAP(size, dim)
     else SWAP(dim, size)
*/