The @ISA variable is a package variable that is used by all classes to indicate the class's parent (or parents). While this variable can be safely read to learn a class's inheritence hierarchy, it must not be modified at runtime [Conway 05].
Noncompliant Code Example (@ISA)
This noncompliant code example defines a base class and an object class with simple methods:
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This error occurs because the BEGIN block is evaluated at the beginning of run time, before the @ISA statement can be evaluated. Therefore, when the Derived::new() constructor is invoked, the Derived class has an empty parents list, and therefore fails to invoke Base::new().
Compliant Solution (base)
This compliant solution uses the base module rather than directly modifying the @ISA variable.
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| Code Block |
|---|
new Base new Derived derived_value = 2 base_value = 1 |
Risk Assessment
Modifying class inheritence at runtime can introduce subtle bugs, and is usually a sign of poor design.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
OBJ30-PL | low | unlikely | low | {*}P3 | L1 |
Automated Detection
Tool | Diagnostic |
|---|---|
Perl::Critic | ClassHierarchies::ProhibitExplicitISA |
Bibliography
[Conway 05] pg. 360 "Inheritance"
[CPAN] Shank, Elliot, Perl-Critic-1.116 ClassHierarchies::ProhibitExplicitISA
[CPAN] Dolan, Chris. base
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