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POwhere did my 4 bytes go?
 

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  1. POwhere did my 4 bytes go?
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    <pre><code>#include &lt;iostream&gt; #include &lt;cstdlib&gt; using std::cout; class A { public : A() { cout &lt;&lt; "A()" &lt;&lt; this &lt;&lt; "\n";} ~A() { cout &lt;&lt; "~A()" &lt;&lt; this &lt;&lt; "\n";} //void func() { } virtual void debug(int a) { cout &lt;&lt; "A::debug";} private : int a; }; class A1 : public A { public : A1() { cout &lt;&lt; "A1()"&lt;&lt; this &lt;&lt; "\n";} ~A1() { cout &lt;&lt; "~A1()"&lt;&lt; this &lt;&lt; "\n";} private : int a1; }; class A2 : public A { public : A2() { cout &lt;&lt; "A2()"&lt;&lt; this &lt;&lt; "\n";} ~A2() { cout &lt;&lt; "~A2()"&lt;&lt; this &lt;&lt; "\n";} private : int a2; }; class B : public A1, public A2 { public : B() { cout &lt;&lt; "B()"&lt;&lt; this &lt;&lt; "\n";} ~B() { cout &lt;&lt; "~B()"&lt;&lt; this &lt;&lt; "\n";} void debug() { cout &lt;&lt; "B::debug()"; } private : int a3; }; int main() { cout &lt;&lt; "sizeof(int)" &lt;&lt; sizeof(int) &lt;&lt; "\n"; cout &lt;&lt; "sizeof(void*)" &lt;&lt; sizeof(void*) &lt;&lt; "\n"; cout &lt;&lt; "sizeof(A): " &lt;&lt; sizeof(A) &lt;&lt; "\n"; cout &lt;&lt; "sizeof(A1): " &lt;&lt; sizeof(A1) &lt;&lt; "\n"; cout &lt;&lt; "sizeof(A2): " &lt;&lt; sizeof(A2) &lt;&lt; "\n"; cout &lt;&lt; "sizeof(B): " &lt;&lt; sizeof(B) &lt;&lt; "\n"; B b; b.debug(); } </code></pre> <p>output :</p> <pre><code>sizeof(int)4 sizeof(void*)4 sizeof(A): 8 sizeof(A1): 12 sizeof(A2): 12 **sizeof(B): 28** A()0x28fef4 A1()0x28fef4 **A()0x28ff00** A2()0x28ff00 B()0x28fef4 B::debug()~B()0x28fef4 ~A2()0x28ff00 ~A()0x28ff00 ~A1()0x28fef4 ~A()0x28fef4 </code></pre> <p>Both A1 and A2 are 4(vtbl) + 4(A'sint) + 4(respective int) = 12 bytes but B is 28 bytes I know its not guaranteed but what could be the possible use of those 4 bytes...I dont see any padding issues ? Can anyone point out what am I missing ?</p>
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    1. USDrew Dormann
    1. USmhk
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    1. PO
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      1. POwhere did my 4 bytes go?
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      1. VTUpMod
    2. VO
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      1. POwhere did my 4 bytes go?
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    3. VO
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      1. POwhere did my 4 bytes go?
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    1. CODespite you don't see them, padding issues are there and looking for further victims.
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      1. POwhere did my 4 bytes go?
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    2. COI must be missing something. :shrug: But you've just said that each of classes A1 and A2 are 12 bytes each. You've then shown B to inherit from each of them AND add another int. My simple mind is doing 12 + 12 + 4 = 28 and asking 'huh? I don't get it, what missing 4 bytes?'
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      1. POwhere did my 4 bytes go?
      1. USenhzflep
    3. COoops I completely missed out that I added an int in B also...and I was also wrongly suspecting the A2's address is 4 bytes ahead...
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      1. POwhere did my 4 bytes go?
      1. USmhk
 

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