How to Use the Non-Default Constructor for a Member

How do I invoke non-default constructor for a member variable?

If you are using C++11 or later, you can write

class X{
    MyClass<10> mcTen = {1, 5};
}

Demo 1.

Prior to C++11 you would need to do it in a constructor's initializer list:

class X{
    MyClass<10> mcTen;
    X() : mcTen(1, 5) {
    }
}

Demo 2.

c++ calling non-default constructor as member

Use initialization list.

class B {
  public:
    B(int i) {}
};

class A {
    B m_B;
  public:
    A() : m_B(17) {}
};

BTW, to reset m_B somewhere outside of the constructor, the correct syntax is:

m_B = B(17);

How do you use the non-default constructor for a member?

You need to call a(int) explicitly in the constructor initializer list:

b() : aInstance(3) {}

Where 3 is the initial value you'd like to use. Though it could be any int. See comments for important notes on order and other caveats.

Initializing a member class of an object using a non-default constructor in C++

What you want is an initializer list. For example:

Tree::Tree(int fruit_num) 
    : apples(fruit_num) // Initializes "apple" with "fruit_num"
{
}

You simply add a colon (:) after the constructor parameters and before the opening brace {. You can separate different member constructors with commas (,). Example:

Tree::Tree(int fruit1, int fruit2) : apples(fruit1), bananas(fruit2) {
}

Using default and non default constructors in my application

When you use

 BOOK1 = new BOOKItem();

You are calling the default constructor (Construction without having any arguments)

After taking user input you would call the Non Default Constructor

bookID = keyboard.nextInt();
numberInStock = keyboard.nextInt();
code = keyboard.nextInt();
price = keyboard.nextDouble();

BOOK2 = new BOOKItem(bookID, numberInStock, price, code);

Use the above code to use the Parameterized Constructor (Non Default constructor)

This is called constructor overloading.

So for the objects you want to assign default values you call the default constructor
For the object you have information available with variables you sent them in the constructor that you have defined with parameters, To assign that value

Using default constructor in non-default constructor

Yes it is possible with the help of delegating constructor.
This feature, called Constructor Delegation, was introduced in C++ 11. Take a look at this,

#include<iostream>  
using namespace std;
class Test{
    private:
        int age;
        int createdAt;
    public:
        //Here is the defualt constructor.
        Test(){            
            createdAt = 0;
        };

        //Non-default constructor calling default constructor.
        Test(int age): Test(){ // delegating constructor
            this->age = age;
        };

        int getAge(){
            return age;
        }

        int getCreatedAt(){
            return createdAt;
        }
};

int main(int argc, char *argv[]) {
    Test t(28);
    cout << t.getCreatedAt() << "\n";
    cout << t.getAge() << "\n";
    return 0;
}

Call non-default constructor as member initialization

Default member initializer (since C++11) only supports brace or equals initializer; you could use brace initializer here.

class A
{
    B b{parameters...};
    Other thing = 3;
};