What are the rules about using an underscore in a C++ identifier?

It's common in C++ to name member variables with some kind of prefix to denote the fact that they're member variables, rather than local variables or parameters. If you've come from an MFC background, you'll probably use m_foo. I've also seen myFoo occasionally.

C# (or possibly just .NET) seems to recommend using just an underscore, as in _foo. Is this allowed by the C++ standard?

The rules (which are still unchanged in C++23):

• 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 std namespace is reserved. (You are allowed to add template specializations, though.)

• User-defined literal suffix identifiers must start with an underscore (since C++11) and must not contain double underscore (reserved since C++23)

From the 2003 C++ Standard (appears in section 5.10.3 Identifiers [lex.name] since C++17):

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).

From the 2023 C++ Standard:

16.4.5.3.6 User-defined literal suffixes [usrlit.suffix]

Literal suffix identifiers ([over.literal]) that do not start with an underscore are reserved for future standardization.

Literal suffix identifiers that contain a double underscore __ are reserved for use by C++ implementations.

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, and va_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 E followed a digit or uppercase letter:

• may be used for additional error code names.

• Names that begin with either is or to followed 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 f or l are reserved:

• for corresponding functions that operate on float and long double arguments, respectively.

• Names that begin with SIG followed 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, or wcs followed by a lowercase letter are reserved:

• for additional string and array functions.

• Names beginning with PRI or SCN followed by any lowercase letter or X are reserved:

• for additional format specifier macros

• Names that end with _t are 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

Stack Overflow Q#228783, score: 979