C++ structure, similar to vector but which resizes automatically

Question

I'm coding a simulation of spatial cell growth in a "corridor". I start with a certain number of cells in a line (i.e. the "width" of my corridor) and let them grow in a stochastical fashion downwards along the "length" of the corridor. Because I need to be able to access all the cells via their coordinates on my corridor, I always used a 2d vector (i.e. a vector whose elements are vectors again) grid and saved my cells in there.

My problem now is that with my current simulation I don't know how far my cells need to grow along the corridor, i.e. I don't know the "length" I need for my 2d grid. (Because this will vary stochastically from run to run) In order to "grow" my cells i.e. place new objects onto my grid, I need to access specific grid points though, but if I don't know the max. length required yet I can't initiate a vector with sufficient length at the beginning.

Is there some kind of structure where I can access each slot via an index, just like in vector, but where I don't need to set out from the start how big it is? (I know with vector I don't need to specify the length right from the start, but I don't always use append, becauae sometimes there are gaps in the grid so I need to have access to specific indeces right from the start).

I'm sorry if I formulated the question not clear enough, please let me know if you have an idea or if the question is unclear. Thank you!

Answer 1

You could write something along the line of:

#include <vector>
#include <iostream>

struct foo { 
    std::vector<int> data;
    int& operator[](size_t i){
        if ( data.size() < i) data.resize(i-1);
        return data[i];
    }
};

int main() {
    foo f;
    f[100] = 10;
    std::cout << f[100];
}

However, I would not advise to actually use that. std::vector gives you fine grained control about when and where allocations happen. You can use resize and/or reserve to make sure the vector has enough elements / capacity. On the other hand, the above will cause you to resize the vector in places where one would not expect it. push_back is to add elements, while accessing an element via [] or at should either access an element or fail.

What you can do instead is reserve enough space and then push_back as many element you will have:

 std::vector<int> foo;
 // dont know how many elements exactly, but it will be less than 1000
 foo.reserve(1000);
 while (some_condition()) foo.push_back(something());