What is this Weird Colon-Member (” : “) Syntax in the Constructor?

Are you still using the constructor to assign values instead of using the member initializer list in C++? If yes, then you might have been missing out on some great performance improvement and secure coding practices. This avoids unnecessary assignments and hidden type conversions and causes cleaner and more optimized code. In this article, let’s explore what this Weird Colon-Member (” : “) is, its syntax in the Constructor, and its uses in detail.

Table of Contents:

• What is a Weird Colon-Member (” : “)?
• What is The Use of Initializer Lists?
• How the Member Initializer List Works (With Examples)
  • Without Member Initializer List (Inefficient Way)
  • With Member Initializer List (Efficient Way)
• When is a Member Initializer List Required?
  • Required for const Data Members
  • Required for Reference(&) Data Members
  • Required for Member Objects Without a Default Constructor
• Conclusion

What is a Weird Colon-Member (” : “)?

In C++, the colon(:) syntax in a constructor is called a member initializer list. This is used to initialize the member variables before the constructor body executes. This allows direct initialization, which is very efficient and important for certain types of members like const variables and references. 

What is The Use of Initializer Lists? 

• Required for const and reference(&) members: These members must be initialized during object construction and cannot be modified later. Therefore, they must be initialized using a member initializer list. 
• Better performance: Without the member initializer list, the member variables go through the default initialization first, and after that, the values are assigned inside the constructor body. The initializer list makes sure of direct initialization and performance improvement. 
• Required for Member Objects Without a Default Constructor: An initializer list is used when a class contains an object of another class that does not have a default constructor.
• Prevents Naming Conflicts Between Constructor Parameter and Class Member: An initializer list is required in some cases, like if a constructor parameter name is the same as the data member name.

How the Member Initializer List Works (With Examples)

1. Without Member Initializer List (Inefficient Way)

Unnecessary assignments may occur if the constructor assigns the values inside the constructor body. 

Example:

Output: 

2. With Member Initializer List (Efficient Way)

In this approach, to initialize the members directly, we use: name(n), and age(a) in the constructor.

Example:

Output: 

In the above code, the class Person with two data members, name and num, is initialized using a member initializer function (: name(n), num(a)). This approach initializes the members directly at the time of object creation, and thus, it helps the efficiency not only by reducing unnecessary assignments but also by executing the corresponding code blocks sequentially. From the main() function, an object p1 is created with “Intellipaat” and 25, and the display() function will print “Name: Intellipaat, Num: 25”.

When is a Member Initializer List Required? 

1. Required for const Data Members 

The data members of const should be initialized at the time of object creation, and this initialization can only be done using an initializer list. 

Example: 

Explanation: The Test class has a const int value that requires using a member initializer list (: value(v)) to be initialized because objects of this type cannot be directly assigned after creation.

2. Required for Reference(&) Data Members 

In C++, the reference members should be initialized only when the object is created, the same as the previous approach. The initialization can also be done using an initializer list. 

Example: 

Explanation: The Test class has a reference member ref that needs to be initialized using a member initializer list (: ref(x)) to get it done because references cannot be reassigned after their creation.

3. Required for Member Objects Without a Default Constructor 

An initializer list is used when a class contains an object of another class that does not have a default constructor.

Example: 

Explanation: Here, Class B includes an object obj of class A without a default constructor, so obj should be initialized using a member initializer list (: obj(x)) in B’s constructor.

Conclusion 

The colon: syntax (member initializer list) is an effective way to initialize class members before running the constructor body in C++11. It is especially useful for const variables, references, and member objects that don’t have default constructors, as it helps us initialize these variables correctly and better. This avoids unnecessary assignments and hidden type conversions and causes cleaner and more optimized code by avoiding both. Adopted in object-oriented programming, a best practice that makes your code readable and efficient is to make use of an initializer list.

You can learn more about C++ in the C++ programming language and also explore C++ Interview Questions prepared by industry experts.

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