DCL00-C. Const-qualify immutable objects

Immutable objects should be const-qualified. Enforcing object immutability using const-qualification helps ensure the correctness and security of applications. ISO/IEC TR 24772, for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments [ISO/IEC TR 24772]. See recommendation STR05-C. Use pointers to const when referring to string literals describes a specialized case of this recommendation.

Adding const qualification may propagate through a program; as you add const, qualifiers still more become necessary. This phenomenon is sometimes called const poisoning, which can frequently lead to violations of recommendation EXP05-C. Do not cast away a const qualification. While const qualification is a good idea, the costs may outweigh the value in the remediation of existing code.

Macros, or an enumeration constant, may also be used instead of a const-qualified object. Reocmmendation DCL06-C. Use meaningful symbolic constants to represent literal values describes the relative merits of using const-qualified objects, enumeration constants, and object-like macros. However, adding a const qualifier to an existing variable is a better first step than replacing the variable with an enumeration constant or macro because the compiler will issue warnings on any code that changes your const-qualified variable. Once you have verified that a const-qualified variable is not changed by any code, you may consider changing it to an enumeration constant or macro, as best fits your design.

Noncompliant Code Example

In this noncompliant code, pi is declared as a float. Although pi is a mathematical constant, its value is not protected from accidental modification.

float pi = 3.14159f;
float degrees;
float radians;
/* ... */
radians = degrees * pi / 180;

Compliant Solution

In this compliant solution, pi is declared as a const-qualified object.

const float pi = 3.14159f;
float degrees;
float radians;
/* ... */
radians = degrees * pi / 180;

Exceptions

DCL00-EX1: It is acceptable to define valueless macros to serve as 'inclusion guards'. That is, the macro serves to control the multiple inclusion of header files, as in the following example:

#ifndef SOME_HEADER_H
#define SOME_HEADER_H
...  // content of header file
#endif

See recommendation PRE06-C. Enclose header files in an inclusion guard for more information on inclusion guards.

Risk Assessment

Failing to const-qualify immutable objects can result in a constant being modified at runtime.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

CERT C++ Secure Coding Standard: DCL00-CPP. Const-qualify immutable objects

ISO/IEC 9899:1999 Section 6.7.3, "Type qualifiers"

Bibliography

[Dewhurst 2002] Gotcha #25, "#define Literals"
[Saks 2000]

02. Declarations and Initialization (DCL)      02. Declarations and Initialization (DCL)