Which of the following are valid C++ variable names?

Question

9index, break, user_name, CONSTANT, _member

Got this wrong on a test and I'm wondering where I can find the right answer. Would be highly appreciated. Thanks!

Open your IDE, assign variables to each variable name, then you can get your answer. — Yang HG, Mar 20 '19 at 02:57
[Here's where you can find the right answer to this question](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Sam Varshavchik, Mar 20 '19 at 02:57
https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier — drescherjm, Mar 20 '19 at 03:04

Willian Gaspar · Answer 1 · 2019-03-20T03:10:47.340

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'9index' is wrong. You never start a variable name with numbers. After the first digit is okay.

'break' is a reserved word

edited Mar 20 '19 at 03:10

answered Mar 20 '19 at 03:00

Willian Gaspar

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break is a keyword – hookenz Mar 20 '19 at 03:02
You right, I missed that – Willian Gaspar Mar 20 '19 at 03:04

score 2 · Answer 2 · answered Mar 20 '19 at 04:18

9index is not a valid identifier because it starts with a digit.

break is not a valid identifier because it is a language keyword.

user_name is a valid identifier.

CONSTANT is a valid identifier

_member may or may not be a valid identifier. The standard explicitly reservies a number of identifiers for use by the implementation (e.g. the compiler or standard library). Identifiers starting with an underscore are reserved at global scope, but not in other scopes (e.g. to name variables of automatic storage duration within a function). The danger with using reserved identifiers is that no diagnostic is required (i.e. the code can successfully compile) and the code has undefined behaviour.

Anything that is not a valid identifier cannot be used as a variable name (among other things).

Mike Hayes · Answer 3 · 2019-03-20T03:12:01.097

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All variable names must begin with a letter of the alphabet or an underscore( _ ). You also cannot use reserved keywords. user_name and _member are valid, the rest aren't. -edit- missed the CONSTANT not being CONST as mentioned in another answer.

edited Mar 20 '19 at 03:12

answered Mar 20 '19 at 03:01

Mike Hayes

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`_member` is not valid in global scope, though. Global identifiers are not allowed to begin with an underscore, that is reserved by the language implementation only – Remy Lebeau Mar 20 '19 at 04:01
@RemyLebeau good to know, thanks! – Mike Hayes Mar 20 '19 at 06:01

hookenz · Answer 4 · 2019-03-20T10:07:06.397

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9index - NO. Variables cannot begin with numbers.
break - NO. This is a keyword so cannot be a variable name
user_name - YES.
CONSTANT - YES. If the variable is called CONSTANT and not const
_member - YES. Member variables are allowed to begin with underscore

edited Mar 20 '19 at 10:07

answered Mar 20 '19 at 03:05

hookenz

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"*Variables are allowed to begin with underscore*" - except in global scope, they are not. – Remy Lebeau Mar 20 '19 at 04:01
Thanks Remy, I hadn't thought about using _ in global scope before. Changed answer to reflect that. – hookenz Apr 01 '19 at 20:48

Which of the following are valid C++ variable names?

4 Answers4