The C++ Standard, [reserved.names] [ISO/IEC 14882-2014], specifies the following rules regarding reserved names:
- A translation unit that includes a standard library header shall not
#undefnames declared in any standard library header.
- A translation unit shall not
#undefnames lexically identical to keywords, to the identifiers listed in Table 3, or to the attribute-tokens described in 7.6.
- Each name that contains a double underscore
__or begins with an underscore followed by an uppercase letter 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.
- Each name declared as an object with external linkage in a header is reserved to the implementation to designate that library object with external linkage, both in namespace
stdand in the global namespace.
- Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage.
- Each name from the Standard C library declared with external linkage is reserved to the implementation for use as a name with
extern "C"linkage, both in namespace
stdand in the global namespace.
- Each function signature from the Standard C library declared with external linkage is reserved to the implementation for use as a function signature with both
extern "C++"linkage, or as a name of namespace scope in the global namespace.
- For each type
Tfrom the Standard C library, the types
std::Tare reserved to the implementation and, when defined,
::Tshall be identical to
- Literal suffix identifiers that do not start with an underscore are reserved for future standardization.
The identifiers and attribute names referred to in the preceding excerpt are
No other identifiers are reserved. Declaring or defining an identifier in a context in which it is reserved results in undefined behavior. Do not declare or define a reserved identifier.
Noncompliant Code Example (Header Guard)
A common practice is to use a macro in a preprocessor conditional that guards against multiple inclusions of a header file. While this is a recommended practice, many programs use reserved names as the header guards. Such a name may clash with reserved names defined by the implementation of the C++ standard template library in its headers or with reserved names implicitly predefined by the compiler even when no C++ standard library header is included.
Compliant Solution (Header Guard)
This compliant solution avoids using leading or trailing underscores in the name of the header guard.
Noncompliant Code Example (User-Defined Literal)
In this noncompliant code example, a user-defined literal operator"" x is declared. However, literal suffix identifiers are required to start with an underscore; literal suffixes without the underscore prefix are reserved for future library implementations.
Compliant Solution (User-Defined Literal)
In this compliant solution, the user-defined literal is named
operator"" _x, which is not a reserved identifier.
The name of the user-defined literal is
operator"" _x and not
_x, which would have otherwise been reserved for the global namespace.
Noncompliant Code Example (File Scope Objects)
In this noncompliant code example, the names of the file scope objects
_limit both begin with an underscore. Because it is
static, the declaration of
_max_limit might seem to be impervious to clashes with names defined by the implementation. However, because the header
<cstddef> is included to define
std::size_t, a potential for a name clash exists. (Note, however, that a conforming compiler may implicitly declare reserved names regardless of whether any C++ standard template library header has been explicitly included.) In addition, because
_limit has external linkage, it may clash with a symbol with the same name defined in the language runtime library even if such a symbol is not declared in any header. Consequently, it is unsafe to start the name of any file scope identifier with an underscore even if its linkage limits its visibility to a single translation unit.
Compliant Solution (File Scope Objects)
In this compliant solution, file scope identifiers do not begin with an underscore.
Noncompliant Code Example (Reserved Macros)
In this noncompliant code example, because the C++ standard template library header
<cinttypes> is specified to include
<cstdint>, as per [c.files] paragraph 4 [ISO/IEC 14882-2014], the name
MAX_SIZE conflicts with the name of the
<cstdint> header macro used to denote the upper limit of
Compliant Solution (Reserved Macros)
This compliant solution avoids redefining reserved names.
DCL51-CPP-EX1: For compatibility with other compiler vendors or language standard modes, it is acceptable to create a macro identifier that is the same as a reserved identifier so long as the behavior is semantically identical, as in this example.
DCL51-CPP-EX2: As a compiler vendor or standard library developer, it is acceptable to use identifiers reserved for your implementation. Reserved identifiers may be defined by the compiler, in standard library headers, or in headers included by a standard library header, as in this example declaration from the libc++ STL implementation.
Using reserved identifiers can lead to incorrect program operation.
|Axivion Bauhaus Suite
-Wreserved-id-macro flag is not enabled by default or with
-Wall, but is enabled with
-Weverything. This flag does not
catch all instances of this rule, such as redefining reserved names.
Macro name is C keyword
Declaration of reserved name
|LDRA tool suite
86 S, 218 S, 219 S, 580 S
Do not #define or #undef identifiers with names which start with underscore
|Polyspace Bug Finder
|CERT C++: DCL51-CPP
|Checks for redefinitions of reserved identifiers (rule partially covered)
|SonarQube C/C++ Plugin
|SEI CERT C++ Coding Standard
|DCL58-CPP. Do not modify the standard namespaces
|SEI CERT C Coding Standard
|DCL37-C. Do not declare or define a reserved identifier
PRE06-C. Enclose header files in an include guard
Subclause 18.104.22.168, "Reserved Names"
|Subclause 7.1.3, "Reserved Identifiers"