What are the Rules for Using an Underscore in a C++ Identifier?

Last Updated : 29 May, 2024

In C++, an identifier is a unique name that is used to define a variable, function, class or object, and etc in a program. It can consist of numbers, letters, and underscores and must start with an underscore or letter (can be uppercase or lowercase). No special character is allowed in identifiers. In this article, we will learn the rule about using an underscore in a C++ identifier.

Rules for Using Underscore (_) in C++ Identifier

The following are the rules that we should handle while using an underscore in a C++ identifier:

1. Leading Underscore Followed by a Lowercase Letter

In C++, an identifier can start with an underscore followed by a lowercase letter but such identifiers are reserved for implementations in the global namespace.

Example:

 int _variable;   // Incorrect usage in global namespace

2. Trailing Underscore

Identifiers ending with a single underscore are not specifically reserved, but it's a common convention to use them for private member variables in classes. It’s a good practice to avoid ending identifier names with an underscore for general purposes.

Example:

int variable_;   // Valid, commonly used for private members

3. Double Underscore Anywhere

Identifiers containing adjacent double underscores are reserved for the compiler and its libraries in all scopes. Using these can interfere with compiler operations and library functionality.

Example:

int __variable;  // Invalid: Reserved for implementation

4. Leading Underscore Followed by an Uppercase Letter

Identifiers starting with an underscore followed by an uppercase letter are reserved for use in all scopes. This is done to avoid conflicts with macros and other identifiers in the standard library.

Example:

int _Variable;   // Invalid: Reserved for implementation

5. Single Underscore

Identifiers consisting solely of a single underscore are allowed, especially in local scopes, but their use is discouraged. They are typically used as placeholders or in very short code segments. However, they can be unclear in their purpose and can potentially conflict with compiler extensions.

Example:

 int _;       // Discouraged: Potentially unclear and error-prone

Conclusion

All of the above rules and conventions regarding the use of the underscores in C++ identifiers are based on the guidelines established by the C++ standard to ensure that the identifiers you use are portable, understandable, and free of conflicts with compiler or system-level names. It's always best practice to follow these rules strictly, especially when writing library code or code that needs to be highly portable.