In C++, how can I use a constructor to initialize a structure that is already declared?

Suppose I have one structure called LinkedList:

struct LinkedList
{

private:
    int curNode;
    
public:
    node* head, * tail;
    LinkedList()
    {
        curNode = 0;
        head = NULL;
        tail = NULL;
    }
    
    LinkedList(int val)
    {
        curNode = 1;
        node* element = (node*)malloc(sizeof(node));

        element->next = element->prev = NULL;
        element->value = val;

        head = tail = element;
    }

};

Now, I have one more structure called Stack. It is not inheriting Linked List:

struct stack
{   
private:
    LinkedList model;

public:

    stack()
    {
        //problem
    }
    stack(int value)
    {
        //problem
    }
    
    
};

I have been searching thoroughly through the C++ documentation but I can not find the solution to my problem.

Here is the problem which I seek to solve:

Inside of the constructors, stack() and stack(int value), I want to initialize the LinkedList model, by using the constructors in my LinkedList struct. Can someone please explain to me what I would have to put in the areas in which I temporarily put the comments //problem?

>Solution :

You need to use a member initialization list to call constructors on class data members, eg:

struct stack
{   
private:
    LinkedList model;

public:

    stack() : model()
    {
    }

    stack(int value) : model(value)
    {
    }
};

In the case of the default constructor, it is optional to call it explicitly, as the compiler will automatically default-construct any data member that is not explicitly stated in the initialization list:

struct stack
{   
private:
    LinkedList model;

public:

    stack() // : model() // <- you can omit this one
    {
    }

    stack(int value) : model(value)
    {
    }
};