What is the most efficient way to manipulate a 2D dynamic array of std::strings in memory?

Question

I currently use

std::vector<std::vector<std::string> > MyStringArray

But I have read several comments here on SO that discourage the use of nested vectors on efficiency grounds.
Unforunately I have yet to see examples of alternatives to nested vector for a situation like this.

Answer 1

Here's a simple dynamic 2D array with runtime-configurable column number:

class TwoDArray
{
  size_t NCols;
  std::vector<std::string> data;

public:
  explicit TwoDArray(size_t n) : NCols(n) { }

  std::string & operator()(size_t i, size_t j) { return data[i * NCols + j]; }
  const std::string & operator()(size_t i, size_t j) const { return data[i * NCols + j]; }

  void set_number_of_rows(size_t r) { data.resize(NCols * r); }

  void add_row(const std::vector<std::string> & row)
  {
    assert(row.size() == NCols);
    data.insert(data.end(), row.begin(), row.end());
  }
};

Usage:

TwoDArray arr(5); // five columns per row
arr.set_number_of_rows(20);
arr(0, 3) = "hello";
arr(17,2) = "world";

This is just a completely arbitrary and random example. Your real class would obviously have to contain interface methods that are suitable to what you're doing; or you might decide not to have a wrapping class at all and address the naked vector directly.

The key feature is the two-dimensional accessor operator via (i,j), which replaces the nested vectors' [i][j].