{"id":2112,"date":"2025-01-27T21:48:02","date_gmt":"2025-01-27T21:48:02","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=2112"},"modified":"2025-01-28T19:04:25","modified_gmt":"2025-01-28T19:04:25","slug":"template-class-method-definitions","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/class-templates-2\/template-class-method-definitions\/","title":{"rendered":"Template Class Method Definitions"},"content":{"rendered":"\n

<\/span>Template Class Method Definitions<\/span><\/h2>\n\n\n\n

However, where should these class template member function definitions be placed? If they are placed in a separate .cpp file (which is conventional with regular classes), how would the member function definitions be instantiated, and associated with the instantiated template class?<\/p>\n\n\n\n

From what we’ve learned about C++, so far, you’d suspect that you could separate out the class member function implementation (definition) from the declaration.<\/p>\n\n\n\n

That depends on the Template Compilation Model<\/em>:<\/p>\n\n\n\n

<\/span>Template Compilation Models<\/span><\/h3>\n\n\n\n

The answer depends on the Template Compilation Model<\/em> of your C++ compiler:<\/p>\n\n\n\n

If it uses the Inclusion Compilation Model<\/em>, then you must place the definitions in an appropriate header file, and include that header file wherever a class template needs to be instantiated.<\/p>\n\n\n\n

\n

Originally, C++ included a Separation Compilation Model<\/em>, which would allow you to separate out definitions and declaration, and added some syntax (an extra export<\/code><\/strong> keyword in the template declaration) to support it. However, that was removed in C++11. <\/p>\n\n\n\n

C++20 introduced modules<\/em> as a better alternative for separate compilation.<\/p>\n<\/blockquote>\n\n\n\n

<\/span>Template Parameters<\/span><\/h3>\n\n\n\n

As with function templates, class templates have a template parameter list<\/em> that contains one or more template parameters<\/em>. A template parameter may be one of two types:<\/p>\n\n\n\n

<\/span>A template type parameter<\/em>:<\/span><\/h4>\n\n\n\n
typename T<\/strong> or class T <\/strong><\/code><\/pre>\n\n\n\n
where T<\/code><\/strong> is simply a name for the type. The actual type is determined by the compiler when it instantiates the template class.<\/p>\n\n\n\n
<\/span>A template nontype parameter<\/em>:<\/span><\/h4>\n\n\n\n
int size<\/strong> \nor\ndouble value<\/strong>\netc...<\/code><\/pre>\n\n\n\n
The expression to which a nontype paramete<\/em>r is bound must be a constant expression<\/em> (i.e. it must be possible to evaluate it at compile time).<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
template <typename Type, int size>\nclass Buffer;<\/strong><\/code><\/pre>\n\n\n\n
Here’s an example, a character array (CharArray<\/strong>), where the nontype parameter specifies the the number of elements in the array:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  CharArray.h<\/span><\/span>\n\/\/  Template Nontype Parameters<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/17\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> CharArray_h<\/span><\/span>\n#define<\/span> CharArray_h<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><assert.h><\/span><\/span>\n<\/span>\ntemplate<\/span> <<\/span>int<\/span> <\/span>SIZE<\/span>><\/span><\/span>\nclass<\/span> <\/span>CharArray<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>size_t<\/span> <\/span>getSize<\/span>()  { <\/span>return<\/span> SIZE; }<\/span><\/span>\n<\/span>\n  <\/span>char<\/span> <\/span>&<\/span>operator[]<\/span>(<\/span>int<\/span> <\/span>i<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>assert<\/span>(i >= <\/span>0<\/span> && i < SIZE);<\/span><\/span>\n    <\/span>return<\/span> <\/span>m_buffer<\/span>[i];<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\nprivate:<\/span><\/span>\n  <\/span>char<\/span> <\/span>m_buffer<\/span>[SIZE];<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* CharArray_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Template Nontype Parameters<\/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>"CharArray.h"<\/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  CharArray<<\/span>8<\/span>> carray1;<\/span><\/span>\n<\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>carray1<\/span>.<\/span>getSize<\/span>(); i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>carray1<\/span>[i] = <\/span>'a'<\/span> + i;<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\n  <\/span>std<\/span>::cout << <\/span>"Array has "<\/span> << <\/span>carray1<\/span>.<\/span>getSize<\/span>()<\/span><\/span>\n            << <\/span>" elements"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>carray1<\/span>.<\/span>getSize<\/span>(); i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>std<\/span>::cout << <\/span>"["<\/span> << i << <\/span>"] : "<\/span> <\/span><\/span>\n              << <\/span>carray1<\/span>[i] << <\/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
Array has 8 elements\n[0] : a\n[1] : b\n[2] : c\n[3] : d\n[4] : e\n[5] : f\n[6] : g\n[7] : h\nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
How about a Queue<\/strong> class template?<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  Queue.h<\/span><\/span>\n\/\/  Queue Template Class<\/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#include<\/span> <\/span>"assert.h"<\/span><\/span>\n<\/span>\n\/\/ Forward declaration<\/span><\/span>\ntemplate<\/span> <<\/span>typename<\/span> <\/span>TYPE<\/span>><\/span><\/span>\nclass<\/span> <\/span>QueueItem<\/span>;<\/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>\npublic:<\/span><\/span>\n  <\/span>typedef<\/span> <\/span>void<\/span> <\/span>func_t<\/span>(QueueItem<TYPE> *qiPtr);<\/span><\/span>\n<\/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<TYPE> *qiPtr<\/span><\/span>\n      = <\/span>new<\/span> <\/span>QueueItem<\/span><<\/span>TYPE<\/span>>(value);<\/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<TYPE> *qiPtr = m_front;<\/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<TYPE> *qi = m_front;<\/span><\/span>\n        qi != <\/span>0<\/span>; qi = <\/span>qi<\/span>-><\/span>getNext<\/span>())<\/span><\/span>\n    {<\/span><\/span>\n      <\/span>func<\/span>(qi);<\/span><\/span>\n    }<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\nprivate:<\/span><\/span>\n  QueueItem<TYPE> *m_front;<\/span><\/span>\n  QueueItem<TYPE> *m_back;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n\/\/ A Queue item<\/span><\/span>\ntemplate<\/span> <<\/span>typename<\/span> <\/span>TYPE<\/span>><\/span><\/span>\nclass<\/span> <\/span>QueueItem<\/span><\/span>\n{<\/span><\/span>\npublic:<\/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><\/span>\n  <\/span>const<\/span> <\/span>TYPE<\/span> <\/span>getValue<\/span>() <\/span>const<\/span><\/span>\n  { <\/span><\/span>\n    <\/span>return<\/span> m_item; <\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>QueueItem<\/span>*<\/span> <\/span>getNext<\/span>()<\/span><\/span>\n  { <\/span><\/span>\n    <\/span>return<\/span> m_next; <\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>setNext<\/span>(<\/span>QueueItem<\/span>*<\/span> <\/span>next<\/span>)<\/span><\/span>\n  { <\/span><\/span>\n    m_next = next; <\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>const<\/span> TYPE  m_item;<\/span><\/span>\n  QueueItem  *m_next;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* Queue_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Queue Template Class<\/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>QueueItem<\/span><<\/span>int<\/span>> <\/span>*<\/span>qiPtr<\/span> )<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"QueueItem: "<\/span> << qiPtr<\/span><\/span>\n    << <\/span>" value = "<\/span><\/span>\n    << <\/span>qiPtr<\/span>-><\/span>getValue<\/span>() << <\/span>std<\/span>::endl;<\/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><\/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>\n\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Template Nontype Parameters<\/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>"CharArray.h"<\/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  CharArray<<\/span>8<\/span>> carray1;<\/span><\/span>\n<\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>carray1<\/span>.<\/span>getSize<\/span>(); i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>carray1<\/span>[i] = <\/span>'a'<\/span> + i;<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\n  <\/span>std<\/span>::cout << <\/span>"Array has "<\/span> << <\/span>carray1<\/span>.<\/span>getSize<\/span>()<\/span><\/span>\n            << <\/span>" elements"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>carray1<\/span>.<\/span>getSize<\/span>(); i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>std<\/span>::cout << <\/span>"["<\/span> << i << <\/span>"] : "<\/span> <\/span><\/span>\n              << <\/span>carray1<\/span>[i] << <\/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
QueueItem: 0x600000008020 value = 0\nQueueItem: 0x600000008030 value = 1\nQueueItem: 0x600000008040 value = 4\nQueueItem: 0x600000008050 value = 9\nQueueItem: 0x600000008060 value = 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: friend Declarations in Class Templates<\/a><\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
Template Class Method Definitions However, where should these class template member function definitions be placed? If they are placed in a separate .cpp file (which is conventional with regular classes), how would the member function definitions be instantiated, and associated with the instantiated template class? From what we’ve learned about C++, so far, you’d suspect […]<\/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-2112","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2112","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=2112"}],"version-history":[{"count":3,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2112\/revisions"}],"predecessor-version":[{"id":2144,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/2112\/revisions\/2144"}],"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=2112"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}