When to use initializer list

const int x;
Base(): x{0} {}
//also this works
Base(int y): x(y) {}

int& _x;
base(int x) : _x{x} {}
//base(int x): {_x = x};

class base{
int _x;
public: 
base(){}
base(int x): _x{x} {}
};

class derive{
base b; //we are here calling default constructor of base as no //parameter passed
public:
derived(base x) {b = x;} //called first, copy constructor called from here

};

class base{
int _x;
public: 
base(int x): _x{x} {}
}

class derived : public base{
int _y;
public:
derived(int x,int y): base{x}, _y{y} {}
//base{x} calls constructor of base class
/* this is not gonna work, _x is private here
so you must use intializer list
derived(int x, int y){
_x = x;
_y = y;
}  
*/


};

int main(){
derived d(4,5);
return 0;
}

class base{
int _x;
public:
base(int _x): _x{_x} {}

//base(int _x) { _x = _x; } there is errorneous

};