When do we use the Initializer List in C++?

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Answer:

In the above question, we had seen, what is the initializer list in C++. Now let us see the situation where we have to use the Initializer List in C++.

1. In the initialization of reference members:

A reference member must be initialized using Initializer List.

#include<iostream>
using namespace std;
class Test
{
    int &m_rData;
public:
    //Initializer list must be used
    Test(int & rData):m_rData(rData) {}
    int getData()
    {
        return m_rData;
    }
};
int main()
{
    int data = 27;
    Test obj(data);
    cout<<"m_rData is " << obj.getData()<<endl;
    data = 06;
    cout<<"m_rData is " << obj.getData()<<endl;
    return 0;
}

Output:

m_rData is 27
m_rData is 6

------------------------------------------------------------------------------------------

2. In the initialization of non-static const data members:

const data members must be initialized using Initializer List.

#include<iostream>
using namespace std;
class Test
{
    const int m_data;
public:
    //Initializer list must be used
    Test(int x):m_data(x) {}
    int getData()
    {
        return m_data;
    }
};
int main()
{
    int data = 27;
    Test obj(data);
    cout<<"m_data is " << obj.getData()<<endl;
    return 0;
}

Output: m_data is 27

---------------------------------------------------------------------------------------------------------------

3. In the initialization of member objects which do not have default constructor:

See the below example, an object “a” of class “A” is a data member of class “B”, and “A” doesn’t have a default constructor. Initializer List must be used to initialize “a”.

#include <iostream>
using namespace std;
//Class A
class A
{
    int i;
public:
    A(int );
};
//Class A constructor
A::A(int arg)
{
    i = arg;
    cout << "A's Constructor called: Value of i: " << i << endl;
}
//Class B
class B
{
//obj of class A
    A a;
public:
    B(int );
};
//Class B constructor.
//Initializer list must be used for a
B::B(int x):a(x)  
{
    cout << "B's Constructor called";
}
int main()
{
    B obj(10);
    
    return 0;
}

Output:

A’s Constructor called: Value of i: 10
B’s Constructor called

------------------------------------------------------------------------------------------------------

4. In the initialization of base class members :

You have to initialize the base class members using the initialization list.

#include <iostream>
using namespace std;
//Class A
class A
{
    int i;
public:
    A(int );
};
//Class A constructor
A::A(int arg)
{
    i = arg;
    cout << "A's Constructor called: Value of i: " << i << endl;
}
//Class B
class B
{
//obj of class A
    A a;
public:
    B(int );
};
//Class B constructor.
//Initializer list to initialize base class member
B::B(int x):a(x)
{
    cout << "B's Constructor called";
}
int main()
{
    B obj(10);
    return 0;
}

Output:

A’s Constructor called: Value of i: 10
B’s Constructor called

---------------------------------------------------------------------------------------------------

5. When the constructor’s parameter name is the same as the data member:

If the constructor’s parameter name is the same as the data member name then the data member must be initialized either using this pointer or Initializer List.

#include <iostream>
using namespace std;
class Test
{
    //member name same as class constructor parameter
    int data;
public:
    Test(int data):data(data) { }
    
    int getData() const
    {
        return data;
    }
};
int main()
{
    Test obj(27);
    
    cout<<obj.getData();
    
    return 0;
}

Output: 27

-------------------------------------------------------------------------------------------------------

6. To increase performance:

It is better to initialize all class variables in the Initializer List instead of assigning values inside the constructor body.

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