{"id":2102,"date":"2025-01-27T21:33:49","date_gmt":"2025-01-27T21:33:49","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=2102"},"modified":"2025-01-28T19:02:05","modified_gmt":"2025-01-28T19:02:05","slug":"whats-the-problem","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/class-templates-2\/whats-the-problem\/","title":{"rendered":"What’s The Problem?"},"content":{"rendered":"\n

<\/span>What’s the Problem?<\/span><\/h2>\n\n\n\n

Just as for function templates, which address the issue of dealing with functions where the algorithm is the same, but the need for supporting different types, class templates address the same issue, but for structures\/classes. <\/p>\n\n\n\n

Consider a Stack<\/strong> class, which implements a stack of int<\/strong>s:<\/p>\n\n\n\n

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\/\/<\/span><\/span>\n\/\/  Stack.h<\/span><\/span>\n\/\/  Class Templates<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/16\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Stack_h<\/span><\/span>\n#define<\/span> Stack_h<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>Stack<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>Stack<\/span>(<\/span>size_t<\/span> <\/span>s<\/span>)<\/span><\/span>\n    : <\/span>m_base<\/span>(<\/span>new<\/span> <\/span>int<\/span>[s]), <\/span>m_size<\/span>(s)<\/span><\/span>\n  {<\/span><\/span>\n    m_top = m_base;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>~Stack<\/span>()<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>delete<\/span> <\/span>[]<\/span> m_base;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Push<\/span>(<\/span>int<\/span> <\/span>a<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    *m_top++ = a;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>int<\/span> <\/span>Pop<\/span>()<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>return<\/span> *--m_top;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>size_t<\/span> <\/span>Size<\/span>() <\/span>const<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>return<\/span> m_top - m_base;<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\nprivate:<\/span><\/span>\n  <\/span>int<\/span>*    m_base;<\/span> \/\/ The stack<\/span><\/span>\n  <\/span>int<\/span>*    m_top;<\/span>  \/\/ The current top<\/span><\/span>\n  <\/span>size_t<\/span>  m_size;<\/span>  \/\/ The size of the stack<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* Stack_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Class Templates<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/16\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"Stack.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  Stack <\/span>mystack<\/span>(<\/span>100<\/span>);<\/span><\/span>\n<\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>10<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>mystack<\/span>.<\/span>Push<\/span>(i);<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Size = "<\/span> << <\/span>mystack<\/span>.<\/span>Size<\/span>() << <\/span>std<\/span>::endl<\/span><\/span>\n            << <\/span>"Values: "<\/span>;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>10<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>std<\/span>::cout << <\/span>mystack<\/span>.<\/span>Pop<\/span>() << <\/span>' '<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>std<\/span>::endl;<\/span><\/span>\n<\/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
Size = 10\nValues: 9 8 7 6 5 4 3 2 1 0 \nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
That’s all well and good for int<\/strong>s, but what about stacks of other types?<\/p>\n\n\n\n
We could write new classes:<\/p>\n\n\n\n
    \n
  • StackOfInts<\/strong><\/li>\n\n\n\n
  • StackOfLongs<\/strong><\/li>\n\n\n\n
  • StackOfChars<\/strong><\/li>\n\n\n\n
  • StackOfMumblefratzes<\/strong><\/li>\n<\/ul>\n\n\n\n
    etc., but this rapidly becomes repetitive, tedious, and error-prone. (Not to mention polluting the namespace!)<\/p>\n\n\n\n
    And it doesn’t help with user-defined types, where a vendor doesn’t know what types will exist for customers.<\/p>\n\n\n\n
    The implementation of something like a Stack<\/strong> is largely independent of what type the stack supports<\/em>.  The algorithms and structure remain identical; only the types change<\/em>. <\/strong><\/p>\n\n\n\n
    So what to do?<\/strong><\/p>\n\n\n\n
    <\/span>One Solution: Use typedefs<\/span><\/h3>\n\n\n\n
    Note that the following is identical to the original, except that every int<\/strong> that relates to the type of stack has been changed to TYPE<\/strong>.<\/p>\n\n\n\n
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    \/\/<\/span><\/span>\n\/\/  GenStack.h<\/span><\/span>\n\/\/  GenStack<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/16\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> GenStack_h<\/span><\/span>\n#define<\/span> GenStack_h<\/span><\/span>\n<\/span>\n\/\/  A class Stack of TYPEs<\/span><\/span>\n\/\/ (no error checking, for simplicity)<\/span><\/span>\n\/\/<\/span><\/span>\nclass<\/span> <\/span>Stack<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>Stack<\/span>(<\/span>size_t<\/span> <\/span>s<\/span>)<\/span><\/span>\n    : <\/span>m_base<\/span>(<\/span>new<\/span> <\/span>TYPE<\/span>[s]), <\/span>m_size<\/span>(s)<\/span><\/span>\n  {<\/span><\/span>\n    m_top = m_base;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>~Stack<\/span>()<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>delete<\/span> <\/span>[]<\/span> m_base;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Push<\/span>(<\/span>const<\/span> <\/span>TYPE<\/span> <\/span>a<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    *m_top++ = a;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>TYPE<\/span> <\/span>Pop<\/span>()<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>return<\/span> *--m_top;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span><\/span>\n  <\/span>size_t<\/span> <\/span>Size<\/span>() <\/span>const<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>return<\/span> m_top - m_base;<\/span><\/span>\n  }<\/span><\/span>\n<\/span>\nprivate:<\/span><\/span>\n  TYPE*   m_base;<\/span> \/\/ The stack<\/span><\/span>\n  TYPE*   m_top;<\/span>  \/\/ The current top<\/span><\/span>\n  <\/span>size_t<\/span>  m_size;<\/span> \/\/ The size of the stack<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* GenStack_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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    \/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  GenStack<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/16\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n<\/span>\n\/\/ Choose type for Stack<\/span><\/span>\n\/\/ (before #include)<\/span><\/span>\ntypedef<\/span> <\/span>const<\/span> <\/span>char<\/span>* TYPE;<\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"GenStack.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  <\/span>const<\/span> <\/span>char<\/span>* values[] = {<\/span>"Mabel"<\/span>,<\/span>"George"<\/span>,<\/span>"Joe"<\/span>};<\/span><\/span>\n<\/span>\n  Stack <\/span>mystack<\/span>(<\/span>100<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>3<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>mystack<\/span>.<\/span>Push<\/span>(<\/span>values<\/span>[i]);<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Size = "<\/span><\/span>\n  << <\/span>mystack<\/span>.<\/span>Size<\/span>() << <\/span>std<\/span>::endl<\/span><\/span>\n       << <\/span>"Values: "<\/span>;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>for<\/span> (<\/span>int<\/span> i = <\/span>0<\/span>; i < <\/span>3<\/span>; i++)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>std<\/span>::cout << <\/span>mystack<\/span>.<\/span>Pop<\/span>() << <\/span>' '<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>std<\/span>::endl;<\/span><\/span>\n<\/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
    Size = 3\nValues: Joe George Mabel \nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n\n\n\n
    Note:<\/strong> We have to specify what TYPE<\/code><\/strong> actually is before<\/em> we include the definition of Stack<\/code><\/strong>.  <\/p>\n\n\n\n
    We could have used a macro instead of a typedef<\/code><\/strong>, but a typedef<\/code><\/strong> is safer.<\/p>\n<\/div>\n<\/div>\n\n\n\n
    This solution is less than perfect:<\/p>\n\n\n\n