Initialize custom collection with list of values

Question

I have created a class that should act as a collection (with some custom behavior of mine). Inside, the class contains an array, to store the values.

class MyCollection {

private:         
    int m_array[N];

public:

    int operator [] (int idx) const {
        return m_array[idx];
    }
    
    int operator [] (TKey k) {
        return m_array[idx];
    }
};

I would like to be able to initialize an instance of this class like it can be done with arrays, but I don't know how to do it, or what this kind of initialization is called ot then look on the web.

std::array<int, 3> a2 = {1, 2, 3};

How can this be achieved? Please note that I may change the type of the elements of the collection (even template it), the referred int is just an example.

You can add a constructor with a `std::initializer_list` I will show you some code below. — A M, Nov 18 '22 at 09:57
See [Why use initializer_list instead of vector in parameters?](https://stackoverflow.com/questions/14414832/why-use-initializer-list-instead-of-vector-in-parameters) and [Why do we use initializer_list in C++11?](https://stackoverflow.com/questions/39512381/why-do-we-use-initializer-list-in-c11) — Jason, Nov 18 '22 at 10:38
@463035818_is_not_a_number I'm not using an array, because I want to introduce additional behavior that an std::array does not provide. — AmiguelS, Nov 18 '22 at 11:48
but the c-array does not provide this addtional behavior either. I was suggesting to use `std::array` as member of your `MyCollection`, it avoids a lot of problems — 463035818_is_not_an_ai, Nov 18 '22 at 12:01
@AmiguelS what is slow? A `std::array` is just a `int[N]` nothing more, only some quirk removed ;) — 463035818_is_not_an_ai, Nov 18 '22 at 12:07

score 1 · Accepted Answer · answered Nov 18 '22 at 10:29

The std::initializer_list will be your friend.

We can add it in a constructor, but also in an assignment operator, or any other member function.

Please read about it here.

Code could look like:

#include <iostream>
#include <initializer_list>

class MyCollection {
private:
    int m_array[10]{};
public:
    MyCollection(const std::initializer_list<int> il) {
        if (il.size() <= 10) {
            size_t i{};
            for (const int& t : il) m_array[i++] = t;
        }
    }
    int operator [] (size_t idx) const {
        return m_array[idx];
    }
};
int main() {
    MyCollection mc {1,2,3,4,5,6,7,8,9,10};

    for (size_t i{}; i < 10; ++i)
        std::cout << mc[i] << '\n';
}

Initialize custom collection with list of values

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