Question

I have a function that takes in the number of rows and columns and initialises a vector of vector with default values of the object ‘cell’ and return the pointer to that vector.

//Cell class
class cell{
public:
    int cost, parent;
    cell(int cost = 0, int parent = 0) : cost(cost), parent(parent){}
}

//The initialisation function
vector<vector<cell> >*  init_table(int n_rows, int n_cols){
    //Error line
    vector<vector<cell> >* table = new vector<vector<cell>(n_cols)> (n_rows);

    //Some(very few) special cells need a different value so I do that here

    return table; //Return the pointer
}

It seems the compiler resolves (n_cols)> (n_rows) like a > operation and not create n_cols copies of cell objects and n_rows copies of vector objects. How can I initialise the vector without manually looping through and pushing default valued cells in the vector?

Answer 1

Since C++ compilers usually have return value optimization, you can just do simply

vector<vector<cell> >  init_table(int n_rows, int n_cols)
{
    return vector<vector<cell> >(n_rows, vector<cell>(n_cols));
}

and writing

vector<vector<cell> > my_table = init_table(int n_rows, int n_cols);

will be as efficient as “new”-ing a vector, but this is safer.