Question

In c++, I can create a 2D array with fixed number of columns, say 5, as follows:

char (*c)[5];

then I can allocate memory for rows as follows

c = new char[n][5];

where n can be any variable which can be assigned value even at run time. I would like to know whether and how can I dynamically allocate variable amount of memory to each row with this method. i.e. I want to use first statement as such but can modify the second statement.

Answer 1

Instead of a pointer to an array, you’d make a pointer to a pointer, to be filled with an array of pointers, each element of which is in turn to be filled with an array of chars:

char ** c = new char*[n];  // array of pointers, c points to first element

for (unsigned int i = 0; i != n; ++i)
    c[i] = new char[get_size_of_array(i)]; // array of chars, c[i] points to 1st element

A somewhat more C++ data structure would be a std::vector<std::string>.

As you noticed in the comment, dynamic arrays allocated with new[] cannot be resized, since there is no analogue of realloc in C++ (it doesn’t make sense with the object model, if you think about it). Therefore, you should always prefer a proper container over any manual attempt at dynamic lifetime management.

In summary: Don’t use new. Ever. Use appropriate dynamic containers.