{"id":2123,"date":"2025-01-27T22:22:59","date_gmt":"2025-01-27T22:22:59","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=2123"},"modified":"2025-01-28T19:08:45","modified_gmt":"2025-01-28T19:08:45","slug":"operator-for-class-templates","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/class-templates-2\/operator-for-class-templates\/","title":{"rendered":"operator<< for Class Templates"},"content":{"rendered":"\n

operator<<<\/strong> for Class Templates<\/h2>\n\n\n\n

Let’s implement operator<<<\/strong> for the Queue<\/strong> class template.<\/p>\n\n\n\n

As before, the operator<<<\/strong> needs to be a non-member function, but it’s complicated by the need to access private parts of the template class, and the need to use TYPE<\/strong>.<\/p>\n\n\n\n

So, we add friend functions to the Queue<\/strong> template class:<\/p>\n\n\n\n

\n
\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Queue.h<\/span><\/span>\n\/\/  Queue Template Class with friends and Nesting<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/17\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Queue_h<\/span><\/span>\n#define<\/span> Queue_h<\/span><\/span>\n<\/span>\n\/\/ The Queue<\/span><\/span>\ntemplate<\/span> <<\/span>typename<\/span> <\/span>TYPE<\/span>><\/span><\/span>\nclass<\/span> <\/span>Queue<\/span><\/span>\n{<\/span><\/span>\n  <\/span>friend<\/span> <\/span><\/span>\n  <\/span>std<\/span>::<\/span>ostream<\/span> <\/span>&<\/span>operator<<<\/span>(<\/span>std<\/span>::<\/span>ostream<\/span> <\/span>&<\/span>os<\/span>,<\/span><\/span>\n                           <\/span>const<\/span> <\/span>Queue<\/span><<\/span>TYPE<\/span>> <\/span>&<\/span>q<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    os << <\/span>"< "<\/span>;<\/span><\/span>\n    <\/span>for<\/span> (<\/span>Queue<\/span><<\/span>TYPE<\/span>>::QueueItem *qi = <\/span>q<\/span>.<\/span>m_front<\/span>;<\/span><\/span>\n         qi != <\/span>0<\/span>; qi = <\/span>qi<\/span>-><\/span>getNext<\/span>())<\/span><\/span>\n    {<\/span><\/span>\n      os << *qi << <\/span>' '<\/span>;<\/span>    \/\/ Use QueueItem's operator<<<\/span><\/span>\n    }<\/span><\/span>\n    os << <\/span>">"<\/span>;<\/span><\/span>\n    <\/span>return<\/span> os;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>typedef<\/span> <\/span>void<\/span> <\/span>func_t<\/span>(TYPE value);<\/span> \/\/ Note change of signature<\/span><\/span>\n  <\/span><\/span>\n  <\/span>Queue<\/span>()<\/span><\/span>\n    : <\/span>m_front<\/span>(<\/span>0<\/span>), <\/span>m_back<\/span>(<\/span>0<\/span>)<\/span><\/span>\n  {}<\/span><\/span>\n  <\/span><\/span>\n  <\/span>~Queue<\/span>()<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>while<\/span> (!<\/span>IsEmpty<\/span>())<\/span><\/span>\n      <\/span>RemoveFromFront<\/span>();<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>AddToBack<\/span>(<\/span>const<\/span> <\/span>TYPE<\/span> <\/span>&<\/span>value<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    QueueItem *qiPtr<\/span><\/span>\n      = <\/span>new<\/span> <\/span>QueueItem<\/span>(value);<\/span> \/\/ Note removal of <TYPE><\/span><\/span>\n    <\/span>if<\/span> (<\/span>IsEmpty<\/span>())<\/span><\/span>\n      m_front = m_back = qiPtr;<\/span><\/span>\n    <\/span>else<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>m_back<\/span>-><\/span>setNext<\/span>(qiPtr);<\/span><\/span>\n      m_back = qiPtr;<\/span><\/span>\n    }<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\n  <\/span>const<\/span> <\/span>TYPE<\/span> <\/span>RemoveFromFront<\/span>()<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>if<\/span>(<\/span>IsEmpty<\/span>())<\/span><\/span>\n      <\/span>return<\/span> <\/span>0<\/span>;<\/span>  \/\/ Nothing to remove<\/span><\/span>\n<\/span>\n    QueueItem *qiPtr = m_front;<\/span> \/\/ Note removal of <TYPE><\/span><\/span>\n    m_front = <\/span>m_front<\/span>-><\/span>getNext<\/span>();<\/span><\/span>\n    <\/span>const<\/span> TYPE theValue = <\/span>qiPtr<\/span>-><\/span>getValue<\/span>();<\/span><\/span>\n    <\/span>delete<\/span> qiPtr;<\/span>  \/\/ Deallocate the QueueItem<\/span><\/span>\n    <\/span>return<\/span> theValue;<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\n  <\/span>bool<\/span> <\/span>IsEmpty<\/span>() <\/span>const<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>return<\/span> m_front == <\/span>0<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>ForEachItem<\/span>(<\/span>func_t<\/span> <\/span>*<\/span>func<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>for<\/span> (QueueItem *qi = m_front;<\/span> \/\/ Note removal of <TYPE><\/span><\/span>\n        qi != <\/span>0<\/span>; qi = <\/span>qi<\/span>-><\/span>getNext<\/span>())<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>func<\/span>(<\/span>qi<\/span>-><\/span>getValue<\/span>());<\/span><\/span>\n    }<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\nprivate:<\/span><\/span>\n  \/\/ A Queue item, now private<\/span><\/span>\n  <\/span>class<\/span> <\/span>QueueItem<\/span> <\/span>\/\/ Note removal of <TYPE><\/span><\/span>\n  {<\/span><\/span>\n    <\/span>friend<\/span><\/span>\n    <\/span>std<\/span>::<\/span>ostream<\/span> <\/span>&<\/span>operator<<<\/span>(<\/span>std<\/span>::<\/span>ostream<\/span> <\/span>&<\/span>os<\/span>,<\/span><\/span>\n                             <\/span>const<\/span> <\/span>QueueItem<\/span> <\/span>&<\/span>qi<\/span>)<\/span><\/span>\n    {<\/span><\/span>\n      os << <\/span>qi<\/span>.<\/span>getValue<\/span>();<\/span><\/span>\n      <\/span>return<\/span> os;<\/span><\/span>\n    }<\/span><\/span>\n<\/span>\n  <\/span>public:<\/span><\/span>\n    <\/span>QueueItem<\/span>(<\/span>const<\/span> <\/span>TYPE<\/span> <\/span>&<\/span>item<\/span>)<\/span><\/span>\n      : <\/span>m_item<\/span>(item), <\/span>m_next<\/span>(<\/span>0<\/span>)<\/span><\/span>\n    {}<\/span><\/span>\n<\/span>\n    <\/span>const<\/span> <\/span>TYPE<\/span> <\/span>getValue<\/span>() <\/span>const<\/span><\/span>\n    { <\/span>return<\/span> m_item; }<\/span><\/span>\n    <\/span>QueueItem<\/span>*<\/span> <\/span>getNext<\/span>()<\/span><\/span>\n    { <\/span>return<\/span> m_next; }<\/span><\/span>\n    <\/span>void<\/span> <\/span>setNext<\/span>(<\/span>QueueItem<\/span>*<\/span> <\/span>next<\/span>)<\/span><\/span>\n    { m_next = next; }<\/span><\/span>\n    <\/span><\/span>\n  <\/span>private:<\/span><\/span>\n    <\/span>const<\/span> TYPE  m_item;<\/span><\/span>\n    QueueItem  *m_next;<\/span><\/span>\n  };<\/span><\/span>\n<\/span>\n  QueueItem *m_front;<\/span> \/\/ Note removal of <TYPE><\/span><\/span>\n  QueueItem *m_back;<\/span>  \/\/ Note removal of <TYPE><\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* Queue_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
\n
… and some small changes to the main program:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Queue Template Class with friends and Nesting<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/17\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Queue.h"<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>displayItem<\/span>( <\/span>int<\/span> <\/span>value<\/span> )<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>' '<\/span> << value;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[])<\/span><\/span>\n{<\/span><\/span>\n  Queue<<\/span>int<\/span>> queue;<\/span><\/span>\n<\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>5<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>queue<\/span>.<\/span>AddToBack<\/span>(i*i);<\/span><\/span>\n            \/\/ The square of i<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>queue<\/span>.<\/span>ForEachItem<\/span>(displayItem);<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>std<\/span>::cout << queue << <\/span>std<\/span>::endl;<\/span> \/\/ Use operator<< for Queue<\/span><\/span>\n  <\/span><\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>5<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>const<\/span> <\/span>int<\/span> value<\/span><\/span>\n          = <\/span>queue<\/span>.<\/span>RemoveFromFront<\/span>();<\/span><\/span>\n    <\/span>std<\/span>::cout << <\/span>"["<\/span> << i << <\/span>"] : "<\/span><\/span>\n              << value << <\/span>std<\/span>::endl;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
which outputs:<\/p>\n\n\n\n
 0 1 4 9 16\n< 0 1 4 9 16 >\n[0] : 0\n[1] : 1\n[2] : 4\n[3] : 9\n[4] : 16\nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
<\/div>\n\n\n\n
\n
Next: Member Templates<\/a><\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
operator<< for Class Templates Let’s implement operator<< for the Queue class template. As before, the operator<< needs to be a non-member function, but it’s complicated by the need to access private parts of the template class, and the need to use TYPE. So, we add friend functions to the Queue template class: … and some […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":2103,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-2123","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2123","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/comments?post=2123"}],"version-history":[{"count":4,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2123\/revisions"}],"predecessor-version":[{"id":2148,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2123\/revisions\/2148"}],"up":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2103"}],"wp:attachment":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/media?parent=2123"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}