With gcc 4.7.2 this compiles just fine for me:
int main()
{
int _ = 1;
return 0;
}
Can I expect this to compile in general? I've read the answers about underscores as prefixes. But what if the underscore isn't prefixing anything?
C++ – Validity of Single Underscore as Variable Name
c++standardsvariable-names
Related Solutions
From Programming in C++, Rules and Recommendations :
The use of two underscores (`__') in identifiers is reserved for the compiler's internal use according to the ANSI-C standard.
Underscores (`_') are often used in names of library functions (such as "_main" and "_exit"). In order to avoid collisions, do not begin an identifier with an underscore.
The rules (which did not change in C++11):
- Reserved in any scope, including for use as implementation macros:
- identifiers beginning with an underscore followed immediately by an uppercase letter
- identifiers containing adjacent underscores (or "double underscore")
- Reserved in the global namespace:
- identifiers beginning with an underscore
- Also, everything in the
stdnamespace is reserved. (You are allowed to add template specializations, though.)
From the 2003 C++ Standard:
17.4.3.1.2 Global names [lib.global.names]
Certain sets of names and function signatures are always reserved to the implementation:
- Each name that contains a double underscore (
__) or begins with an underscore followed by an uppercase letter (2.11) is reserved to the implementation for any use.- Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.165
165) Such names are also reserved in namespace
::std(17.4.3.1).
The C++ language is based on the C language (1.1/2, C++03), and C99 is a normative reference (1.2/1, C++03), so it's useful to know the restrictions from the 1999 C Standard (although they do not apply to C++ directly):
7.1.3 Reserved identifiers
Each header declares or defines all identifiers listed in its associated subclause, and optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers.
- All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
- All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.
- Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included; unless explicitly stated otherwise (see 7.1.4).
- All identifiers with external linkage in any of the following subclauses (including the future library directions) are always reserved for use as identifiers with external linkage.154
- Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.
No other identifiers are reserved. If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.
If the program removes (with
#undef) any macro definition of an identifier in the first group listed above, the behavior is undefined.154) The list of reserved identifiers with external linkage includes
errno,math_errhandling,setjmp, andva_end.
Other restrictions might apply. For example, the POSIX standard reserves a lot of identifiers that are likely to show up in normal code:
- Names beginning with a capital
Efollowed a digit or uppercase letter: - may be used for additional error code names.
- Names that begin with either
isortofollowed by a lowercase letter - may be used for additional character testing and conversion functions.
- Names that begin with
LC_followed by an uppercase letter - may be used for additional macros specifying locale attributes.
- Names of all existing mathematics functions suffixed with
forlare reserved - for corresponding functions that operate on float and long double arguments, respectively.
- Names that begin with
SIGfollowed by an uppercase letter are reserved - for additional signal names.
- Names that begin with
SIG_followed by an uppercase letter are reserved - for additional signal actions.
- Names beginning with
str,mem, orwcsfollowed by a lowercase letter are reserved - for additional string and array functions.
- Names beginning with
PRIorSCNfollowed by any lowercase letter orXare reserved - for additional format specifier macros
- Names that end with
_tare reserved - for additional type names.
While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of that standard.
Personally I just don't start identifiers with underscores. New addition to my rule: Don't use double underscores anywhere, which is easy as I rarely use underscore.
After doing research on this article I no longer end my identifiers with _t
as this is reserved by the POSIX standard.
The rule about any identifier ending with _t surprised me a lot. I think that is a POSIX standard (not sure yet) looking for clarification and official chapter and verse. This is from the GNU libtool manual, listing reserved names.
CesarB provided the following link to the POSIX 2004 reserved symbols and notes 'that many other reserved prefixes and suffixes ... can be found there'. The POSIX 2008 reserved symbols are defined here. The restrictions are somewhat more nuanced than those above.
Best Answer
Yes, from The C++ Programming Language, 4th Edition: