C++ template implementation to store 2D nested vector data continuously?

Question

Storing a two-dimensional grid as std::vector<std::vector<double>> is rather inefficient because each of the inner vectors contains separately allocated heap memory. I am seeking a template implementation to let me do the things like:

nested_vector<double, 2> nv;
nv.push_back({0, 1, 2});   // row1
nv.push_back({3, 4});      // row2
nv.push_back({5, 6, 7, 8});// row3

Ideal storage in memory:

-------------------------------------------------
|     row 1     |     row 2     |     row 3     |
-------------------------------------------------

instead of

std::vector<std::vector<double>>  v = {{0, 1, 2}, {3, 4}, {5, 6, 7, 8}};

--------------
|     row 1     |
-------------
|     row 2     |
-------------
|     row 3     |
--------------

Is there any template implementation to do this? Any tips will be appreciated, thanks in advance.

Answer 1

You do not need a template to pass different number of parameters to a function. You can use a initializer list:

#include <iostream>
#include <vector>

using namespace std;

struct foo {
    std::vector<int> data;
    void add(std::initializer_list<int> a){
        std::copy(a.begin(),a.end(),std::back_inserter(data));
    }
};

int main() 
{
    foo f;
    f.add({1,2,3});
    f.add({1,2,3,4});
    f.add({1,2,3,4,5});
    for (const auto& e : f.data) std::cout << e;
}

However, as the "rows" are of different size, you will need to do more to know where a row ends. If row sizes are dynamic and can change during runtime then a std::vector<std::vector<int>> is probably just fine. Using a flat 1d container and resizing rows would require to move around elements very often. Also you cannot resize when you do not know the size of a row before hand. Things are different when the size of the rows is fixed, then you'd need to do the index transformations and place added elements at the right position.