C++ 2D Vector: How to Create and Use a Vector of Vectors

A regular std::vector holds a list of values. A 2D vector is a vector whose values are themselves vectors — giving you a grid, matrix, or table that you can resize at runtime. It’s the flexible, modern alternative to fixed-size 2D arrays.

Declaring a 2D Vector

The type is vector<vector<T>> where T is the element type:

#include <iostream>
#include <vector>

int main() {
    // A 3x4 grid of integers, all initialised to 0
    int rows = 3, cols = 4;
    std::vector<std::vector<int>> grid(rows, std::vector<int>(cols, 0));

    std::cout << "Rows: " << grid.size() << "\n";
    std::cout << "Cols: " << grid[0].size() << "\n";

    return 0;
}

Output:

Rows: 3
Cols: 4

The constructor std::vector<int>(cols, 0) creates one row of cols zeros. Passing it as the second argument to the outer vector creates rows copies of that row.

Accessing and Modifying Elements

Use double bracket notation grid[row][col]:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::vector<int>> grid(3, std::vector<int>(3, 0));

    // Set some values
    grid[0][0] = 1;
    grid[1][1] = 5;
    grid[2][2] = 9;

    // Print the grid
    for (int r = 0; r < 3; r++) {
        for (int c = 0; c < 3; c++) {
            std::cout << grid[r][c] << " ";
        }
        std::cout << "\n";
    }

    return 0;
}

Output:

1 0 0
0 5 0
0 0 9

Using Range-Based for Loops

The modern C++ way to iterate is with range-based for loops. Use auto& to avoid copying each row:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::vector<int>> matrix = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    for (const auto& row : matrix) {
        for (int val : row) {
            std::cout << val << "\t";
        }
        std::cout << "\n";
    }

    return 0;
}

Output:

1	2	3
4	5	6
7	8	9

Initializing with { {1,2,3}, {4,5,6}, {7,8,9} } uses an initializer list — a clean way to set up small grids.

Adding Rows Dynamically

Because this is a vector, you can push_back new rows at runtime:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::vector<int>> table;

    // Add rows one at a time
    table.push_back({10, 20, 30});
    table.push_back({40, 50, 60});
    table.push_back({70, 80, 90});

    for (const auto& row : table) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    return 0;
}

You can also start with an empty outer vector and fill it entirely at runtime — useful when you don’t know the dimensions until the program runs.

Jagged Vectors: Rows of Different Lengths

Unlike 2D arrays, each row in a 2D vector can have a different length:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::vector<int>> jagged;

    jagged.push_back({1});
    jagged.push_back({2, 3});
    jagged.push_back({4, 5, 6});

    for (const auto& row : jagged) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    return 0;
}

Output:

1
2 3
4 5 6

This is impossible with a traditional 2D array but trivial with a vector of vectors.

2D Vector vs 2D Array: Which to Use?

A C++ 2D array has fixed dimensions set at compile time. A 2D vector can be sized and resized at runtime. For most beginner programs, prefer the 2D vector — it’s easier to pass to functions, safer to use, and more flexible. Only prefer a 2D array when you know the exact size at compile time and want raw performance.

Passing a 2D Vector to a Function

Pass by const reference to avoid copying the entire grid:

#include <iostream>
#include <vector>

void printGrid(const std::vector<std::vector<int>>& grid) {
    for (const auto& row : grid) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }
}

int main() {
    std::vector<std::vector<int>> g = {{1,2},{3,4},{5,6}};
    printGrid(g);
    return 0;
}

Related Articles

• C++ Vector Tutorial: The Complete Guide to std::vector
• C++ 2D Arrays: How to Declare, Initialize, and Iterate
• C++ Arrays Tutorial: Store and Access Multiple Values
• C++ STL Containers Explained: Choosing the Right Container

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