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Do not attempt to cv-qualify a reference type as it can result in undefined behavior. A conforming compiler is required to issue a diagnostic message. However, if the compiler does not emit a fatal diagnostic, the program may produce surprising results, such as allowing the character referenced by p to be mutated.

Noncompliant Code Example

In this noncompliant code example, a const-qualified reference to a char is formed instead of a reference to a const-qualified char, resulting in undefined behavior:

#include <iostream>
 
void f(char c) {
  char &const p = c;
  p = 'p';
  std::cout << c << std::endl;
}

Implementation Details

With Microsoft Visual Studio 2013, this code compiles successfully with a warning diagnostic (warning C4227: anachronism used : qualifiers on reference are ignored) and outputs:

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error: 'const' qualifier may not be applied to a reference

Compliant Solution

This compliant solution assumes the programmer intended for the previous example to fail to compile due to attempting to modify a const-qualified char reference:

#include <iostream>
 
void f(char c) {
  const char &p = c;
  p = 'p'; // error, read-only variable is not assignable
  std::cout << c << std::endl;
}

Risk Assessment

const and volatile reference types may result in undefined behavior instead of a fatal diagnostic, causing unexpected values to be stored and leading to possible data integrity violations.

Automated Detection

Related Vulnerabilities

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

Bibliography

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