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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]. 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 become still more necessary. This phenomenon is sometimes called const poisoning, which can frequently lead to violations of EXP05-C. Do not cast away a const qualification. Although const qualification is a good idea, the costs may outweigh the value in the remediation of existing code.

A macro or an enumeration constant may also be used instead of a const-qualified object. 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.

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Related Vulnerabilities

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

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ISO/IEC 9899:2011 Section 6.7.3, "Type qualifiers"

Bibliography

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

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