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C++ does not allow you to change the value of a reference type, effectively treating all references as being const qualified. The C++ Standard, [dcl.ref], paragraph 1 [ISO/IEC 14882-2014], states the following:

Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef-name (7.1.3, 14.1) or decltype-specifier (7.1.6.2), in which case the cv-qualifiers are ignored.

Thus, C++ prohibits or ignores the cv-qualification of a reference type. Only a value of non-reference type may be cv-qualified.

When attempting to const-qualify a type as part of a declaration that uses reference type, a programmer may accidentally write

char &const p;

instead of

char const &p; // Or: const char &p;

Do not attempt to cv-qualify a reference type because it results 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. This results in undefined behavior.

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

Implementation Details (MSVC)

With Microsoft Visual Studio 2015, this code compiles successfully with a warning diagnostic.

warning C4227: anachronism used : qualifiers on reference are ignored

When run, the code outputs the following.

p

Implementation Details (Clang)

With Clang 3.9, this code produces a fatal diagnostic.

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

Noncompliant Code Example

This noncompliant code example correctly declares p to be a reference to a const-qualified char. The subsequent modification of p makes the program ill-formed.

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

Compliant Solution

This compliant solution removes the const qualifier.

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

Risk Assessment

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

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

DCL52-CPP

Low

Unlikely

Low

P3

L3

Automated Detection

Tool

Version

Checker

Description

Axivion Bauhaus Suite

6.9.0

CertC++-DCL52
Parasoft C/C++test

10.4.2

CERT_CPP-DCL52-a

Never qualify a reference type with 'const' or 'volatile'

Polyspace Bug Finder

R2019b

CERT C++: DCL52-CPP

Checks for:

  • const-qualified reference types
  • Modification of const-qualified reference types

Rule fully covered.

PRQA QA-C++
4.3

0014


Clang
3.9

 Clang checks for violations of this rule and produces an error without the need to specify any special flags or options.
SonarQube C/C++ Plugin
4.10
S3708

Related Vulnerabilities

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

Bibliography

[Dewhurst 2002]Gotcha #5, "Misunderstanding References"
[ISO/IEC 14882-2014]Subclause 8.3.2, "References"