C++ Comments and C-style Comment in C++

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In C++ programming, comments play a crucial role in enhancing the readability, maintainability, and overall understanding of the code. They are special lines that are not executed by the compiler, serving as annotations to explain the purpose, functionality, or any other significant details about the code. Comments are particularly beneficial for both the original developers and others who may need to work with the code in the future.

A comment in C++ is a programmer-readable explanation or annotation embedded directly within the source code. Its sole purpose is to be read by someone who is looking at the code. Comments are completely ignored by the C++ compiler, meaning that they have no impact on the final executable program.


Before diving into comments and how to use them in C++, there are a few prerequisites that one should be familiar with:

  1. Basic Syntax of C++: Understanding how to write and structure C++ code is essential. This includes knowledge of declarations, statements, functions, and more.
  2. Code Editors: Familiarity with code editors (such as Visual Studio Code, Eclipse, or any other IDE that supports C++ programming) as they often provide features for easier comment management.
  3. C++ Compiler: An understanding of how to compile and run C++ programs to ensure that the written code, including comments, is correct and functional.

Understanding these basic components of C++ programming will provide a strong foundation for utilizing comments effectively to make your code more understandable and maintainable.

Why We Use Comments in C++

Comments play a crucial role in C++ programming for various reasons, enhancing both the development process and the understandability of code. Below are some of the key reasons why we use comments in C++:

  • Code Explanation: Comments provide clear explanations for complex sections of code, making it easier for others (and yourself) to grasp the underlying logic and functionality. They help in breaking down the purpose of specific lines or blocks of code into simpler terms.
  • Code Documentation: They act as an inline form of documentation, elucidating how particular functions or classes operate, the kind of parameters they expect, and what values or objects they return. This is especially helpful for anyone who is new to the codebase or for developers revisiting their own code after a significant period.
  • Debugging Aid: Comments are invaluable tools for debugging. By temporarily commenting out specific sections of code, developers can isolate and troubleshoot problematic areas, thereby streamlining the debugging process.
  • Enhanced Maintainability: Well-commented code significantly improves the maintainability of a software project. Whether it’s you revisiting your own code months later or a new developer trying to understand the workflow, comments act as a comprehensive guide, making the process of updating or modifying the code much smoother.
  • Learning and Onboarding: For educational purposes or onboarding new team members, comments in the code serve as learning aids, providing context and explanations that can accelerate the learning curve.
  • Code Review and Collaboration: During code reviews, comments help reviewers understand the intentions behind certain coding decisions, facilitating more effective communication and collaboration among team members.
  • To-Do and Future Work: Comments can also be used to leave notes for future work or enhancements, often labeled as “TODO” or “FIXME”, acting as reminders for pending tasks or areas that require attention.
  • Inclusion of External Resources: Sometimes, comments are used to include links to external resources, such as documentation or related articles, providing additional context or background information relevant to the code.

Types of Comments in C++

In C++, there are primarily two types of comments used to annotate the code: single-line comments and multi-line comments. Each serves a unique purpose and is used in different contexts.

Single-Line Comment

Single-line comments are used to comment out a single line of code or to write a short note above a line of code. They are initiated with two forward slashes //. Anything following // on the same line will be ignored by the compiler.


In the above example, the compiler will ignore the text after // and the line that is completely commented out. As a result, the program will only output the value of 'a'.

Multi-Line Comment

Multi-line comments, also known as block comments, allow you to comment out multiple lines of code or write detailed explanations spanning several lines. They start with /* and end with */. Everything between these delimiters will be ignored by the compiler.


In this example, the multi-line comment provides a detailed explanation, but does not affect the execution of the program. The program will output "Hello, World!".

How the Compilers Process Comments in C++

Compilers play a crucial role in the execution of a program, and understanding how they handle comments is essential for every C++ programmer. Comments are meant for the programmer and are used to make the code more understandable, but they have no impact on the execution of the program. Here’s how compilers process comments in C++:

Ignoring Comments

When a compiler encounters comments in the code, it simply ignores them. They do not take up any memory space in the final executable file, and they have no impact on the performance of the program. Whether you write a single line of comment or multiple lines, they are all treated the same way — ignored.

Preprocessing Phase

The removal of comments occurs during the preprocessing phase of compilation. This is the initial stage where the source code is prepared for compilation. Here, the preprocessor takes care of directives (like #include), macros, and also removes all the comments from the code.

No Syntax Checking

Comments are not subjected to syntax checking. Since they are removed before the actual compilation starts, you can write anything inside the comments without worrying about the syntax. However, it’s a good practice to write meaningful comments.

Handling Nested Comments

It's important to note that C++ does not support nested comments. If you have a block comment inside another block comment, it will lead to a compilation error. Single-line comments can be nested inside block comments without any issues.

Example of Nested Comments Issue:

In the above example, the compiler will get confused when it encounters the second */ and will throw an error.

Inline Comments

Both single-line and multi-line comments can be used at the end of a line of code. The compiler will ignore the comment part and execute the code normally.


In this example, the compiler will only consider int a = 5; and ignore the rest of the line.

C-style Comments

C-style comments, originating from the C programming language, are also supported in C++. These comments are utilized to annotate multiple lines of text in the code. Here’s a detailed look at C-style comments:


C-style comments start with /* and end with */. Everything between these delimiters is treated as a comment.

Single Line Usage

Although designed for multi-line comments, C-style comments can also be used for single lines.

No Nesting

C-style comments cannot be nested. Attempting to nest them will result in a compilation error.

Incorrect Usage:

Use Cases

C-style comments are ideal for commenting out blocks of code during debugging or development, as well as for providing detailed explanations or documentation for chunks of code.

Compatibility with C

C-style comments maintain compatibility with C code, making it easier to integrate or migrate code between C and C++.

C++ style Comments

C++ introduces its own style of comments, which provide a concise way to annotate single lines of code. Here’s a comprehensive overview of C++-style comments.

Single-Line Comments

C++-style comments begin with //. Everything following these two slashes on the same line is treated as a comment.

Advantages of Single-Line Comments

  • Clarity: They are useful for short explanations and notes right next to the code.
  • Ease of Use: Quick to type and do not require an ending tag.
  • Flexibility: Can be easily added for temporary debugging or code clarification without disturbing multi-line comments or code structure.

Inline Comments

C++ style comments can also be used at the end of a line of code.

This form is commonly used for brief annotations to explain a particular line of code.

Temporary Code Removal

C++ style comments can be used to quickly comment out a single line of code, which can be useful for debugging or testing changes.


  • No Multi-Line Support: To comment out multiple lines, you must place // in front of each line.
  • No Inline Block Commenting: Unlike C-style comments, you cannot start a comment in the middle of a line and continue it onto the next line.

Nested Comments

C++ style comments can be nested within C-style comments, but not within other C++ style comments.

Valid Nesting:

Invalid Nesting:

/* and */ Comments

The /* ... */ comment syntax originates from the C programming language and is often referred to as C-style or block comments. In C++, these comments are used to annotate multiple lines of text or code.

Multi-Line Comments

C-style comments start with /* and end with */. Everything between these delimiters is treated as a comment, regardless of how many lines it spans.

Advantages of Multi-Line Comments

  • Versatility: Suitable for both short and lengthy explanations.
  • Ease of Use: Easy to comment out large blocks of code or text.
  • Conciseness: Provides a cleaner look for extensive descriptions compared to using multiple single-line comments.

Temporary Code Deactivation

C-style comments are commonly used to quickly disable blocks of code during debugging or development.


  • No Nesting: Unlike C++ style comments, C-style comments cannot be nested. Attempting to nest them will result in a compilation error.

Inline Block Commenting

C-style comments can be used to comment out a part of a line, a whole line, or multiple lines.


  • Comments are statements that are not executed by the compiler or the interpreter.
  • There are two types of comments: single and multi-line.
  • C-style comments or multi-line comments are used to comment on large blocks of text or code.
  • C++ style comments or single-line comments are used to comment on single lines of text or code.
  • They make our code easy to understand not only for us when we revisit it after some time but also for everyone trying to understand the code.