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can immediately be applied to plain old memory buffers, and that there is no new syntax to learn. However, it requires an "end" iterator to test for equality, rather than allowing an iterator to know that it has reached the end. In C++ language, we say that an iterator models the iterator
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Defining access and traversal operations in the aggregate interface is inflexible because it commits the aggregate to particular access and traversal operations and makes it impossible to add new operations later without having to change the aggregate interface.
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in that language. In C++, a class can overload all of the pointer operations, so an iterator can be implemented that acts more or less like a pointer, complete with dereference, increment, and decrement. This has the advantage that C++ algorithms such as
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that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
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The essence of the
Iterator Pattern is to "Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.".
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can be implemented generally using a specified type of iterator rather than implementing it as a container-specific algorithm. This allows
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The elements of an aggregate object should be accessed and traversed without exposing its representation (data structures).
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Clients use an iterator to access and traverse an aggregate without knowing its representation (data structures).
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from containers; in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.
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Define a separate (iterator) object that encapsulates accessing and traversing an aggregate object.
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New access and traversal operations can be defined independently by defining new iterators.
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Different iterators can be used to access and traverse an aggregate in different ways.
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This C++11 implementation is based on chapter "Generalizing vector yet again".
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to be used on any container that supports the required type of iterator.
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Some languages standardize syntax. C++ and Python are notable examples.
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A sample UML class and sequence diagram for the
Iterator design pattern.
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and access the container's elements. The iterator pattern decouples
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Iterator pattern in UML and in LePUS3 (a formal modelling language)
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Erich Gamma; Richard Helm; Ralph
Johnson; John Vlissides (1994).
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Design
Patterns: Elements of Reusable Object-Oriented Software
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1295:"The Iterator design pattern - Structure and Collaboration"
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The
Iterator design pattern is one of the 23 well-known
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What solution does the
Iterator design pattern describe?
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What problems can the
Iterator design pattern solve?
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