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POA class rewritten with templates makes a program slower (in run-time)
 

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    <p>I have a class for a serial-memory 2D array that was initially an array of <code>int</code>s. Now that I need a similar array with another type, I've rewritten the class with templates; the only difference is in the type of stored objects:</p> <pre><code>template &lt;class T&gt; class Serial2DArray { ... T ** Content; } </code></pre> <p>I have a few test functions that deal with Content, for example, a one that nullifies all elements in the array (they're not class members, they're outside functions working with <code>Serial2DArray&lt;int&gt;</code> objects. I've noticed that now it works 1-2% slower - all the other code in the class is untouched, the only difference is that earlier it was just a regular class with <code>int ** Content</code> and now it's a template.</p> <p>A sort of similar question: <a href="https://stackoverflow.com/questions/2442358/do-c-templates-make-programs-slow">Do c++ templates make programs slow?</a> - has opinions that only compilation becomes slower (and I can see why, the compiler generates classes for each that it finds in the code), but here I see the program becoming slower in run-time - is there any rational explanation?</p> <p>Upd: issue narrowed down a little bit here: <a href="https://stackoverflow.com/a/11058672/1200000">https://stackoverflow.com/a/11058672/1200000</a></p> <p>Upd2: as mentioned in the comments, here's the function that became slower:</p> <pre><code>#include &lt;windows.h&gt; #include &lt;mmsystem.h&gt; ... int Size = G_Width * G_Height * sizeof(int); DWORD StartTime = timeGetTime(); for(int i=0; i&lt;100; ++i) { FillMemory(TestArray.Content[0], Size, 0); } MeasuredTime = timeGetTime() - StartTime; </code></pre> <p>And here's the actual class template:</p> <pre><code>#include &lt;malloc.h&gt; template &lt;class T&gt; class Serial2DArray { public: Serial2DArray() { Content = NULL; Width = 0; Height = 0; } Serial2DArray(int _Width, int _Height) { Initialize(_Width, _Height); } ~Serial2DArray() { Deinitialize(); } T ** Content; int GetWidth() { return Width; } int GetHeight() { return Height; } int Initialize(int _Width, int _Height) { // creating pointers to the beginning of each line if((Content = (T **)malloc(_Height * sizeof(T *))) != NULL) { // allocating a single memory chunk for the whole array if((Content[0] = (T *)malloc(_Width * _Height * sizeof(T))) != NULL) { // setting up line pointers' values T * LineAddress = Content[0]; for(int i=0; i&lt;_Height; ++i) { Content[i] = LineAddress; // faster than Content[i] = LineAddress += _Width; // Content[0] + i * _Width; } // everything went ok, setting Width and Height values now Width = _Width; Height = _Height; // success return 1; } else { // insufficient memory available // need to delete line pointers free(Content); return 0; } } else { // insufficient memory available return 0; } } int Resize(int _Width, int _Height) { // deallocating previous array Deinitialize(); // initializing a new one return Initialize(_Width, _Height); } int Deinitialize() { // deleting the actual memory chunk of the array free(Content[0]); // deleting pointers to each line free(Content); // success return 1; } private: int Width; int Height; }; </code></pre> <p>As requested, binaries size comparison.</p> <p>Code with the following: </p> <pre><code>Serial2DArray&lt;int&gt; TestArray; Serial2DArray&lt;int&gt; ZeroArray; </code></pre> <ul> <li>1 016 832 bytes. </li> </ul> <p>Code with the following: </p> <pre><code>Serial2DArray TestArray; // NOT-template class with ints Serial2DArray ZeroArray; // methods are in class declaration </code></pre> <ul> <li>1 016 832 bytes </li> </ul> <p>Code with the following: </p> <pre><code>Serial2DArray&lt;int&gt; TestArray; Serial2DArray&lt;int&gt; ZeroArray; Serial2DArray&lt;double&gt; AnotherArray; Serial2DArray&lt;double&gt; YetAnotherArray; </code></pre> <ul> <li>1 017 344 bytes</li> </ul>
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    1. CORegarding the answers to the post you provided a link to, from my point of view they are all plain false. Take care of the false sense of .. performace the c++ template gives you. As well as for inlining everything in your code. It is a false widespread common idea... Take some time to read the Bruce Eckel's book "thinking in C++" Vol1 or 2 (don't remember in which one the template code bloat topic is tackled) and read his way to deal with the issue. Very interesting.
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      1. USyves Baumes
    2. CO(originally posted as a comment to your answer where you say that the real issue comes from putting all code in a header) What compiler is this? It seems that this would only give the compiler more information (and would not force any pessimization such as always inlining).
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      1. USDavid Stone
    3. COThanks for adding code, but it's not enough. Post a [_self-contained_](http://sscce.org/) repro.
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      1. USildjarn
 

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