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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 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:

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

Compliant Solution (User-defined Literal)

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

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Note that the name of the user-defined literal is operator"" _x and not _x, which would have otherwise been reserved to the global namespace.

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:

...

// The following declaration of a reserved identifier exists in the libc++ implementation of
// std::basic_string as a public member. The original source code may be found at:
// http://llvm.org/svn/llvm-project/libcxx/trunk/include/string
 
template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
class basic_string
{
  // ...
 
  bool __invariants() const;
};

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

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