Constructor

Same named function of class. Ctr is called when object of class is created. Used to provide values to data members.
Return type: None
Cannot access to address of ctr
Inheriting ctr: Cannot
PRIVATE SCOPED CTR*: Can ctr be inside private section(Yes. Single DP)

Ctr calling Hierarchy (Base Class > Derv Class Ctr)

Derived class object also have inherited properties of Base class,
and only base class constructor can properly initialize base class members 

class A{
    public:
        A() { cout<<"base ctr";  }
        ~A() { cout<<"base dtr";  }
};
    
class B: public A {
    public:
        B() { cout<<"Derv ctr";  }
        ~B() { cout<<"Derv dtr";  }
};
int main(){
    B obj;
}
$ a.out
base ctr
Derv ctr
Derv dtr
Base dtr

Base class constructor should be defined 

class A{
    public:
        A();
        ~A();
};
class B: public A {
public:
    B() { cout<<"B ctr";  }
    ~B() { cout<<"~B";  }
};
int main(){
    B obj;
}
# ./a.out
Error cannot find A::A()

Ctr Initializer list

In the initializer list, the order of execution takes place according to the order of declaration of member variables. 

class test{
    int a; //Declared 1st
    int b; //Declared 2nd
public:
    test (int x): b(x), a(b * 2) {
        cout << b << a;       //b=10, a=garbage
    }
};
int main() {
    test obj(10);
    return 0;
}
$ ./a.exe
b = 10
a = garbage value
This is because, a is declared before b. Initialization of a will happen before b.
Since a = b*2, and b does not have value hence a is initialized to garbage.