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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 std namespace 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 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.

• 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.

• 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.

Because C++ is based on the C standard (1.1/2, C++03) and C99 is a normative reference (1.2/1, C++03) these also apply, from the 1999 C Standard:

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):