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 PDTR 24772, for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments [[ISO/IEC PDTR 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, still more become necessary. This phenomenon is sometimes called "const poisoning." Const poisoning can frequently lead to violations of 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. 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 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;

Automated Detection

Compass/ROSE could detect violations of this rule by flagging any variable as a violation as long as:

• the variable is local to a function, or static (but not extern)
• It is never assigned to
• Its address is never evaluated.
  These violations will be genuine, but ROSE can't catch all violations. In particular:
• A variable whose address is taken might still be const
• Variables appearing in multiple files might still be const.
• Doesn't handle constant data on the heap

Risk Assessment

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

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 6.7.3, "Type qualifiers"
[[Saks 00]]

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