What are the '@' and '$' for in C/C++? [duplicate]

Question

So far, in MinGW, the '$' seems to be just a basic character (like 'a'), that can be used in names (variables, functions, etc.) But it doesn't look like one. Is there some hidden feature of using '$' as part of a name that I'm missing?

int $m = 2;
printf("$m = %i", $m);

Console Output:

$m = 2

However the '@' symbol produces an error when used as a character. error: stray '@' in program I assume that mean's that it's used for something? Wikipedia, and MSDN don't mention either character. And Google gives plenty of matches for "symbol" or "C".

Answer 1

Neither $ or @ are part of standard C's character set (C11 5.2.1 Character sets, paragraph 3):

Both the basic source and basic execution character sets shall have the following members: the 26 uppercase letters of the Latin alphabet

    A B C D E F G H I J K L M
    N O P Q R S T U V W X Y Z

the 26 lowercase letters of the Latin alphabet

    a b c d e f g h i j k l m
    n o p q r s t u v w x y z

the 10 decimal digits

    0 1 2 3 4 5 6 7 8 9

the following 29 graphic characters

    ! " # % & ' ( ) * + , - . / :
    ; < = > ? [ \ ] ^ _ {  | } ~

the space character, and control characters representing horizontal tab, vertical tab, and form feed.

The C++ standard says about the same (2.2 Character sets, paragraph 1):

The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters:

a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9
_ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ∼ ! = , \ " ’

So if you can or can't use them (at all or even for a specific purpose) it's up to your implementation.

In your case it sounds like you're probably using GCC, which allows $ in identifiers as an extension, but doesn't allow @ - probably because GCC also compiles Objective-C code, where @ has special meaning.

From the GCC documentation:

In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.