SINGLETON Object Diagram for  Singleton  using Presidency Example The  Singleton  pattern ensures that a class has only on...
SINGLETON <ul><li>Intent </li></ul><ul><li>Ensure a class has only one instance, and provide a global point of access to i...
<ul><li>Solution </li></ul><ul><li>Solution is to make the class itself responsible for keeping track of its sole instance...
<ul><li>Applicability </li></ul><ul><li>Use singleton pattern when </li></ul><ul><li>there must be exactly one instance of...
<ul><li>Structure </li></ul><ul><li>Collaborations </li></ul><ul><li>Clients access a Singleton instance solely through Si...
<ul><li>Consequences </li></ul><ul><li>Controlled access to sole instance </li></ul><ul><li>Reduced name space </li></ul><...
<ul><li>Implementation </li></ul>
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Singleton

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Singleton

  1. 1. SINGLETON Object Diagram for Singleton using Presidency Example The Singleton pattern ensures that a class has only one instance, and provides a global point of access to that instance. The Singleton pattern is named after the singleton set, which is defined to be a set containing one element. The office of the President of the United States is a Singleton . The United States Constitution specifies the means by which a president is elected, limits the term of office, and defines the order of succession. As a result, there can be at most one active president at any given time. Regardless of the personal identity of the active president, the title, &quot;The President of the United States&quot; is a global point of access that identifies the person in the office.
  2. 2. SINGLETON <ul><li>Intent </li></ul><ul><li>Ensure a class has only one instance, and provide a global point of access to it </li></ul>Problem Application needs one, and only one, instance of an object and that the instance is easily accessible. Additionally, lazy initialization and global access are necessary.
  3. 3. <ul><li>Solution </li></ul><ul><li>Solution is to make the class itself responsible for keeping track of its sole instance. </li></ul><ul><li>The class can ensure that no other instance can be created ( by intercepting request to create new objects ), and it provide a way to access the instance. </li></ul><ul><li>1. Make the class of the single instance object responsible for creation, initialization, access, and enforcement. </li></ul><ul><li>2. Declare the instance as a private static data member. </li></ul><ul><li>3. Provide a public static member function that encapsulates all initialization code, and provides access to the instance. </li></ul>
  4. 4. <ul><li>Applicability </li></ul><ul><li>Use singleton pattern when </li></ul><ul><li>there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. </li></ul><ul><li>When the sole instance should be extensible by sub-classing, and clients should be able to use and extended instance without modifying their code. </li></ul>
  5. 5. <ul><li>Structure </li></ul><ul><li>Collaborations </li></ul><ul><li>Clients access a Singleton instance solely through Singleton’s Instance operation. </li></ul>
  6. 6. <ul><li>Consequences </li></ul><ul><li>Controlled access to sole instance </li></ul><ul><li>Reduced name space </li></ul><ul><li>Permits refinement of operations and representation </li></ul><ul><li>Permits a variable number of instances </li></ul><ul><li>More flexible than class operations </li></ul>
  7. 7. <ul><li>Implementation </li></ul>
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