Immutable objects should be const-qualified. Enforcing object immutability using const-qualification helps ensures the correctness and security of applications. ISO/IEC PDTR 24772 [[ISO/IEC PDTR 24772]], for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments. [STR05-A. 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-A. 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-A. Use meaningful symbolic constants to represent literal values in program logic describes the relative merits of using const-qualified object, 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.
Non-Compliant Code Example
In the following non-compliant 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.
float const pi = 3.14159f; float degrees; float radians; /* ... */ radians = degrees * pi / 180;
Risk Assessment
Failing to const-qualify immutable objects can result in a constant being modified at runtime.
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
DCL00-A |
low |
unlikely |
high |
P1 |
L3 |
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]] Dan Saks. Numeric Literals
. Embedded Systems Programming. September, 2000.
02. Declarations and Initialization (DCL) 02. Declarations and Initialization (DCL) DCL01-A. Do not reuse variable names in subscopes