Objects can only be returned from factory functions as pointers or pointers wrapped by smart pointers. Slower speed than static polymorphism templates and overloaded functions or methods. Delegation a kind of OO composition Use another object for implementing some functionality and forward a method call to it instead of inheriting the object class. Examples: store items in a std::vector, instead of storing them directly in the class with heap-allocated arrays; store an heap-allocated object using smart pointers rather than storing it using raw pointer and freeing it on the destructor.
Static Polymorphism It is templates and overloaded functions or methods. Templates can eliminate virtual method call overhead by resolving methods at compile-time rather than at runtime. Type Erasure Combination of static and dynamic polymorphism which gets the best of both sides with minimal overhead.
So, functions can be used to extending classes without modifying them by taking objects as arguments and applying operations on them. L singleton1. Same object? Definitions: Factory method or factory functions are any methods or functions which returns an instance of a class or derived class.
Disadvantage: Adding new derived classes requires code modification. A static method with if-else statements. This is mostly used on languages where there are not free functions or ordinary functions. Disadvantage: Requires modifying the code when new classes are added to the hierarchy. Registry-based factory mostly known as Polymorphic Factory - A class contains a factory method for instantiating derived classes based on some input and a hash table for registration of derived classes.
Disadvantage: Possibly requires manual registration during the program initialization. Example: Factory. It is equivalent to the old C static keyword. Example: File: builder. Decorators are also known as wrappers due to it wrap itself around anohter object. All member objects are allocated on the heap and deleted manually.
All objects in the main function are allocated on the hep free-store. All objects in the main functions are allocated on the stack. In this example, there is any problem over using raw pointers since they don't own memory. Note: Also shows the decorator implemented with Mixins. Design Patterns: Decorator Pattern - Uses raw pointer for wrapped object. All objects in main functions are allocated on the heap. Parts: Context : Object that has a reference to an strategy objects and sets the strategy at runtime.
Resposibilities: Set strategy Change strategy Invoke strategy IStrategy : Algorithm interface Strategt interface define the algorithm operations. Concrete strategy. Strategies objects or implementation of IStrategy class. CharacterSuperMutant mutant1 ; mutant1. The parent abstract class has four different types of methods: Concrete methods : Methods implemented in the abstract class. Abstract methods : Methods without implementation that must be implemented by subclasses.
Hook methods : Methods with default implementation that can be overriden by subclasses. Template methods : Method that calls concrete methods, abstract methods or hook methods. Participants: Base Class : Defines an algorithm which calls primitive methods aka hook methods that will be defined by the derived classes. Derived Class : Implements primitive methods virtual methods or methods that can be overriden defined in the base class. Features: The base class defines an algorithm stub, however some steps are required to be implemented by the derived class. This pattern provides an inverse control structure , the algorithm defined in the base class which calls the primitive methods which are algorithm steps declared in the base class.
This pattern can be the base of an framework. Alternatives: Throw an exception when there is the absense of an object. Return a pointer that is set to null for denoting the absence of an object. However it is prone for the infamous null pointer exception problems. Features and applications: Allow client code to ignore differences between individual objects and collection composite objects and treat them uniformly. Leaf Overview: Represents a primitive object which doesn't contain any child object. Composite Overview As the name implies, the composite object contains leaf children objects.
Design Patterns (C++): Introduction -
Implements child operations in the component interface. Can add or remove leaf objects or composite object as children. The composite object forwards the messages related to child operations to the children elements. Add Component. Remove Component. Intent: GOF : "Represent an operation to be performed on the elements of an object structure.
Visitor lets you define a new operation without changing the classes of the elements on which it operates. Decouple data structure and algorithms.
Decouple two class hierarchies. The code to perform the operation is centralized in the visitor object.
C++ Programming: Code patterns design
If the operation were implemented as methods, the code would be scattered through all derived classes. Drawbacks: The class hierarchy must be stable as adding a new derived class requires modifying all visitors implementations.
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Possible in Common Lisp, Clojure and so on. Parts: Interface: Element The element is an element of the class hirarchy which contains the method. Interface: Visitor - Perform the operations the class hierarchy.
Every operation is an implementation of the visitor interface. Methods: The visitor interface requires a method visit for derived class of the interface Element. Note: Element1, Element2 and Element3 are derived classes of interface Element.
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Variant - 1. It should define a specific version operation method visit for each derived class. L visitor2. Python has the with statement and C has the using statement. Using the resource Releasing the resource in the object's destructor member function which is executed when the object goes out of escope.
Problems solved by this technique. Avoid memory leak Avoid resource leak Save and restore context.
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The same technique could be used to save and restore std::cout or std::cerr flags. Example: Naive code - If an exception happens, the resource will not be released and the code will be vulnerable to memory leak or resource leak. All STL containers such as std::vector, std::map, std::deque and so on uses RAII technique for avoiding memory leak by deleting the heap-allocated objects when no longer needed.
L raai1. Makes the class data member in the public header really private and obfuscated. Changing of private members of the class which are encapsulated in the opaque pointer class does not require recompilation of client code. This feature is very important for library development as it avoid client code recompilation as the public headers are not changed. Note: Changing class member variables or member functions methods breaks the ABI and requires the recompilation of client code.
The pimpl maintains the binary compatibility by not changing class' private members. Drawbacks: More complexity and work for API implementators. Not convenient when there are protected members which needs to be accessed by subclasses.
Runtime performance overhead due to the pointer indirection. Note: resource means anything that needs to be disposed when no longer needed such as heap memory, database handlers, socket handlers, file descriptors and etc. If the following class used the default copy constructor and copy-assignment operator, the socket handler would be copied, but the initial class and the copy would still use the same resource, in this case connection.
Then, if one of the classes went out of scope, the destructor would be invoked disposing the resource.