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<p class="MsoNormal">Hi Team,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We are using Clang 11 for our product that has common C++ code base for Windows, Linux, Macintosh, ….<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We observed that <span style="background:yellow;mso-highlight:yellow">
sizeof</span> operator gives different value on Windows and Linux/OSX, when the inheritance is from a common base class:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Example Sample:<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:8.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">#include <cassert><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">struct Base {}; // empty class<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">struct Derived1 : Base {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> int i;<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">};<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">struct Derived2 : Base {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> Base c; // Base, occupies 1 byte, followed by padding for i<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> int i;<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">};<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">struct Derived3 : Base {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> Derived1 c; //
<span style="background:yellow;mso-highlight:yellow">Derived1 is too derived from same Base class</span><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> int i;<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">};<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">int main()<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">{<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">
<a href="http://en.cppreference.com/w/cpp/error/assert"><span style="color:blue">assert</span></a>(sizeof(Derived2) == 2*sizeof(int));<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New""> <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">
<a href="http://en.cppreference.com/w/cpp/error/assert"><span style="color:blue">assert</span></a>(<span style="background:yellow;mso-highlight:yellow">sizeof(Derived3)</span> == 3*sizeof(int));<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:45.8pt"><span style="font-size:8.0pt;font-family:"Courier New"">}<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> When we compile above program using Clang 11 and run it on windows and Linux sizeof(<span style="font-size:10.0pt;font-family:"Courier New";background:yellow;mso-highlight:yellow">Derived3</span>) give different value.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We don’t want a work-around via making first data member of derived class of type different from class that also is derived from same empty base class.<o:p></o:p></p>
<p class="MsoNormal">Is there any flag that we can use so that it gives same result on all platform (Windows/Linux/OSX/Android/iOS/….)<o:p></o:p></p>
<p class="MsoNormal">Or another way to solve this gracefully. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thank you!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Best Regards,<o:p></o:p></p>
<p class="MsoNormal">Vivek<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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