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A common practice is to use a macro in a preprocessor conditional which 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.

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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

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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);

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

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

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

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

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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 std:size_t:.

#include <cinttypes> // for int_fast16_t

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

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This compliant solution avoids redefining reserved names:.

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

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

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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:.

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

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:.

// 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;
};

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Using reserved identifiers can lead to incorrect program operation.

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

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Bibliography

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