The fragile base class problem is a fundamental architectural problem of object-oriented programming

Object-oriented programming

Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...

 systems where base classes (superclasses) are considered "fragile" because seemingly safe modifications to a base class, when inherited by the derived classes, may cause the derived classes to malfunction. The programmer cannot determine whether a base class change is safe simply by examining in isolation the methods of the base class.

One possible solution is to make instance variables private to their defining class and force subclasses to use accessors to modify superclass states. A language could also make it so that subclasses can control which inherited methods are exposed publicly. These changes prevent subclasses from relying on implementation details of superclasses and allow subclasses to expose only those superclass methods that are applicable to themselves.

Solutions

• Objective-C
  Objective-C
  Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
  
   has Categories as well as Non-fragile instance variables
• Component Pascal
  Component Pascal
  Component Pascal is a programming language in the tradition of Niklaus Wirth's Pascal, Modula-2, Oberon and Oberon-2. It bears the name of the Pascal programming language but is incompatible with it. Instead, it is a minor variant and refinement of Oberon-2, designed and supported by a small ETH...
  
   deprecates superclass calls
• Java
  Java (programming language)
  Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
  
   allows inheritance to be prohibited by labeling the class declaration with the keyword "final". In the book Effective Java, author Joshua Bloch
  Joshua Bloch
  Joshua J. Bloch is a software engineer, currently employed at Google, and a technology author. He led the design and implementation of numerous Java platform features, including the Java Collections Framework, the java.math package, and the assert mechanism...
  
   writes (in item 17) that programmers should "Design and document for inheritance or else prohibit it".
• C# and VB.NET like Java have "sealed" and "Not Inheritable" class labels to prohibit inheritance.

See also

• Fragile binary interface problem
  Fragile binary interface problem
  The fragile binary interface problem or FBI is a shortcoming of certain object-oriented programming language compilers, in which internal changes to an underlying class library can cause descendant libraries or programs to cease working...
  
  
• Implementation inheritance
• Inheritance semantics
• Software brittleness
  Software brittleness
  In computer programming and software engineering, the term software brittleness refers to the increased difficulty in fixing older software that may appear reliable, but fails badly when presented with unusual data or altered in a seemingly minor way...
  
  
• Virtual Inheritance (object-oriented programming)
  Virtual inheritance
  Virtual inheritance is a topic of object-oriented programming. It is a kind of inheritance in which the part of the object that belongs to the virtual base class becomes common direct base for the derived class and any next class that derives from it...
  
  

External links

• A Study of The Fragile Base Class Problem - Scientific description and analysis of the fragile base class problem by Leonid Mikhailov and Emil Sekerinski
• Why Extends is Evil - An example in Java
  Java (programming language)
  Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...