> For the complete documentation index, see [llms.txt](https://notes.tejpratapsingh.com/_/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://notes.tejpratapsingh.com/_/cpp/memory/vector-arraylist/custom-vector-part-1.md).

# Custom vector, part 1

As explained before, what us vector and how it works. In this lesson, we will  try to create vector implementation by ourself, let's begin.

Here is the algorithm:

1. Create a generic class with `template`, it has 3 things:
   1. `data` to store data pointer.
   2. `size` to store current size of vector.
   3. `capacity` to keep track of capacity of vector, vector will be reallocated once this capacity is reached by current `size` of vector.
2. A method `push_back` to add new elements to vector.
3. If `size == capacity`, then copy all current data to a different array with increased capacity.
4. Else add  new data at the end of current vector.
5. Repeat `2` to `4` on every push.

```cpp
#include <iostream>

template <typename T>
class MyVector {
public:
    MyVector() : _data(nullptr), _size(0), capacity(0) {}

    ~MyVector() {
        delete[] _data;
    }

    void push_back(const T& value) {
        if (_size == capacity) {
            resize();
        }
        _data[_size++] = value;
    }

    void pop_back() {
        if (_size > 0) {
            --_size;
        }
    }

    T& operator[](size_t index) {
        return _data[index];
    }

    const T& operator[](size_t index) const {
        return _data[index];
    }

    size_t size() const {
        return _size;
    }

private:
    T* _data;
    size_t _size;
    size_t capacity;

    void resize() {
        capacity = capacity == 0 ? 1 : capacity * 2;
        T* new_data = new T[capacity];
        for (size_t i = 0; i < _size; ++i) {
            new_data[i] = _data[i];
            // delete &_data[i];
        }
        delete[] _data;
        _data = new_data;
    }
};

int main() {
    MyVector<int> v;
    std::cout << "add fist" << std::endl;
    v.push_back(35);

    for (size_t i = 0; i < v.size(); ++i) {
        std::cout << "value: " << v[i] << ", address: " << &v[i] << std::endl;
    }

    std::cout << "add second" << std::endl;
    v.push_back(30);

    for (size_t i = 0; i < v.size(); ++i) {
        std::cout << "value: " << v[i] << ", address: " << &v[i] << std::endl;
    }

    std::cout << "add third" << std::endl;
    v.push_back(25);

    for (size_t i = 0; i < v.size(); ++i) {
        std::cout << "value: " << v[i] << ", address: " << &v[i] << std::endl;
    }

    std::cout << "Pop Back" << std::endl;

    v.pop_back();

    for (size_t i = 0; i < v.size(); ++i) {
        std::cout << "value: " << v[i] << ", address: " << &v[i] << std::endl;
    }

    std::cout << "add Fourth" << std::endl;
    v.push_back(20);

    for (size_t i = 0; i < v.size(); ++i) {
        std::cout << "value: " << v[i] << ", address: " << &v[i] << std::endl;
    }
    std::cout << std::endl;

    return 0;
}
```

> Run it [here](https://onecompiler.com/cpp/42ng6ddpa).

#### Output:

```
add fist
value: 35, address: 0x55dcb2e3eec0
add second
value: 35, address: 0x55dcb2e3eee0
value: 30, address: 0x55dcb2e3eee4
add third
value: 35, address: 0x55dcb2e3eec0
value: 30, address: 0x55dcb2e3eec4
value: 25, address: 0x55dcb2e3eec8
Pop Back
value: 35, address: 0x55dcb2e3eec0
value: 30, address: 0x55dcb2e3eec4
add Fourth
value: 35, address: 0x55dcb2e3eec0
value: 30, address: 0x55dcb2e3eec4
value: 20, address: 0x55dcb2e3eec8
```

Here, we can see, after each `push_back`, a new array is created and item is added into it, and if there is any available space after `pop_back` new item will take its place.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://notes.tejpratapsingh.com/_/cpp/memory/vector-arraylist/custom-vector-part-1.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.