DCL32-CPP. Do not declare or define a reserved identifier

The C++ Standard, [reserved.names], specifies the following rules regarding reserved names [ISO/IEC 14882-2014]:

• A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.
• A translation unit shall not #define or #undef names 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 std and 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 std and 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" and extern "C++" linkage, or as a name of namespace scope in the global namespace.
• For each type T from the Standard C library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.
• Literal suffix identifiers that do not start with an underscore are reserved for future standardization.

The identifiers and attribute names mentioned above are: override, final, alignas, carries_dependency, deprecated, and noreturn.

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, but noncompliant, practice is to choose a reserved name for a macro used in a preprocessor conditional guarding against multiple inclusions of a header file (see also PRE06-C. Enclose header files in an inclusion guard). The 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.

#ifndef _MY_HEADER_H_
#define _MY_HEADER_H_

// Contents of <my_header.h>

#endif // _MY_HEADER_H_

Compliant Solution (Header Guard)

This compliant solution avoids using leading or trailing underscores in the name of the header guard:

#ifndef MY_HEADER_H
#define MY_HEADER_H

// Contents of <my_header.h>

#endif // MY_HEADER_H

Noncompliant Code Example (User-defined Literal)

In this noncompliant code example, a user-defined literal n::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:

#include <cstddef>
 
namespace n {
unsigned int operator"" x(const char *, std::size_t);
}

Compliant Solution (User-defined Literal)

In this compliant solution, the user-defined literal is named n::_x, which is not a reserved identifier:

#include <cstddef>

 
namespace n {
unsigned int operator"" _x(const char *, std::size_t);
}

Noncompliant Code Example (File Scope Objects)

In this noncompliant code example, the names of the file scope objects _max_limit and _limit both begin with an underscore. Since 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 or not 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.

#include <cstddef> // std::for size_t

static const std::size_t _max_limit = 1024;
std::size_t _limit = 100;

unsigned int getValue(unsigned int count) {
  return count < _limit ? count : _limit;
}

Compliant Solution (File Scope Objects)

In this compliant solution, file scope identifiers do not begin with an underscore:

#include <cstddef> // for size_t

static const std::size_t max_limit = 1024;
std::size_t limit = 100;

unsigned int getValue(unsigned int count) {
  return count < limit ? count : limit;
}

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, the name MAX_SIZE conflicts with the name of the <cstdint> header macro used to denote the upper limit of std:size_t:

#include <cinttypes> // for int_fast16_t

void f(std::int_fast16_t val) {
  enum { MAX_SIZE = 80 };
  // ...
}

Compliant Solution (Reserved Macros)

This compliant solution avoids redefining reserved names:

#include <cinttypes> // for std::int_fast16_t

void f(std::int_fast16_t val) {
  enum { BUFSIZE = 80 };
  // ...
}

Exceptions

DCL32-CPP-EX1: For compatibility with other compiler vendors or language standard modes, it is acceptable to create a macro identifier the same as a reserved identifier so long as the behavior is idempotent, as in this example:

// Sometimes generated by configuration tools such as autoconf
#define const const
 
// Allowed compilers with semantically equivalent extension behavior
#define inline __inline

Risk Assessment

Using reserved identifiers can lead to incorrect program operation.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography