C++ Classes within classes as a custom vector (how to init?)

Question

Fine, now i want to create my own basic vector and i do:

template <typename T>
class Vector {
    T * vector;
    int size;
public:
    Vector(int pSize = 10) : size(pSize)
    {
        vector = new T[size];
    }
    T& operator[](int i)
    {
        return vector[i];
    }
};

So for a one-dimensional vector everything is fine. I can do:

Vector<int> vectorInt(20);
for (int i = 0; i < 20; i++)
    vectorInt[i] = i;
for (int i = 0; i < 20; i++)
    std::cout << vectorInt[i] << std::endl;

All right, but the problem states when i want to put my vector inside the same vector class, as this:

Vector<Vector<int>> xy(5);
for (int i = 0; i < 5; i++)
{
    for (int j = 0; j < 10; j++)
    {
        xy[i][j] = j;
        std::cout << xy[i][j] << " ";
    }
    std::cout << std::endl;
}

The thing is i cannot initialize to a requested size the insider vector (it defaults to 10 as the constructor says). Only the outsider one to "5".

Is there anyway to initialize the insider vector at declaration?

You will have to write more code to be able to do this. [See this question about the analogous initialization for std::vector](https://stackoverflow.com/questions/40887674/initializing-2d-vector-using-only-1-line-c) and implement the same for your template. — Sam Varshavchik, May 17 '20 at 00:54
@Sam that link doesn't answer my question. I know i have to write more code and i don't know how to do it. — Lucas, May 17 '20 at 01:16
That link certainly answers your question. It demonstrates exactly how to do what you're trying to do, but with `std::vector`. `std::vector` has an overloaded constructor that takes an optional parameter that specifies the default new value, which gets used to copy-construct each value in the new vector, with an example that passes the default value consisting of `std::vector` of size 2 to the outer constructor of a vector of size 3, thus initializing a 2x3 two-dimensional vector. You simply have to do the same thing: implement an overloaded constructor, plus Rule Of 3 compliance. — Sam Varshavchik, May 17 '20 at 01:40
@Sam, sorry for the inconvinence, you pointed me in the right direction. It's just not much knowlagde from me. — Lucas, May 17 '20 at 02:47

score 0 · Answer 1 · answered May 17 '20 at 03:00

Well, to do my own basic Vector class i did the following: I created a constructor like this one:

Vector(int pSize, T twoDim) : size(pSize)
{
    vector = new T[size];
}

With this constructor the same class takes another instance of Vector<int> and creates the one like the std lib does.

Here is my final code that dinamically initialize a 2d 5x10 matrix:

#include <iostream>
template <typename T>
class Vector {
    T * vector;
    int size;
public:
    Vector(int pSize, T twoDim) : size(pSize)
    {
        vector = new T[size];
    }
    Vector(int pSize = 10) : size(pSize)
    {
        vector = new T[size];
    }
    T& operator[](int i)
    {
        return vector[i];
    }
};

int main()
{
    Vector<Vector<int>> xy(5, Vector<int>(10,0));
    for (int i = 0; i < 5; i++)
    {
        for (int j = 0; j < 10; j++)
        {
            xy[i][j] = j;
            std::cout << xy[i][j] << " ";
        }
        std::cout << std::endl;
    }
    return EXIT_SUCCESS;
}

C++ Classes within classes as a custom vector (how to init?)

1 Answers1