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Wiki MarkupImmutable objects should be {{const}}\-qualified. Enforcing object immutability using {{const}}\ qualification helps ensure the correctness and security of applications. ISO/IEC PDTR TR 24772, for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments \ [[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 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 become necessary. This phenomenon is sometimes called "const poisoning." Const poisoning , which can frequently lead to violations of EXP05-C. Do not cast away a const qualification. While Although const qualification is a good idea, the costs may outweigh the value in the remediation of existing code.

Macros, 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 in program logic 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 the following 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;

...

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;

...

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

Automated Detection

Compass/ROSE can detect most violations of this recommendation.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.3, "Type qualifiers"
\[[Saks 00|AA. C References#Saks 00]\]

Related Guidelines

 Bibliography

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