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.
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.
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.
In this compliant solution,
pi is declared as a
const-qualify immutable objects can result in a constant being modified at runtime.
|Axivion Bauhaus Suite|
Cast removes const qualifier
Variable Could Be const
Returned Pointer Not Treated as const
C3204, C3227, C3232, C3673, C3677
|LDRA tool suite|
Declare local variable as const whenever possible
|Polyspace Bug Finder|
|CERT C: DCL00-C||Checks for unmodified variable not const-qualified (rule fully covered).|