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.
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; } |
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 |
With Clang 3.9, this code produces a fatal diagnostic.
error: 'const' qualifier may not be applied to a reference |
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; } |
This compliant solution removes the const
qualifier.
#include <iostream> void f(char c) { char &p = c; p = 'p'; std::cout << c << std::endl; } |
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 |
Tool | Version | Checker | Description |
---|---|---|---|
Axivion Bauhaus Suite | CertC++-DCL52 | ||
Helix QAC | C++0014 | ||
Klocwork | CERT.DCL.REF_TYPE.CONST_OR_VOLATILE | ||
Parasoft C/C++test | CERT_CPP-DCL52-a | Never qualify a reference type with 'const' or 'volatile' | |
Polyspace Bug Finder | CERT C++: DCL52-CPP | Checks for:
Rule fully covered. | |
Clang | 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 | S3708 |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
[Dewhurst 2002] | Gotcha #5, "Misunderstanding References" |
[ISO/IEC 14882-2014] | Subclause 8.3.2, "References" |