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POC++ object size with virtual methods
 

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    <p>I have some questions about the object size with virtual.</p> <p><strong>1) virtual function</strong></p> <pre><code>class A { public: int a; virtual void v(); } </code></pre> <p>The size of class A is 8bytes....one integer(4 bytes) plus one virtual pointer(4 bytes) It's clear!</p> <pre><code>class B: public A{ public: int b; virtual void w(); } </code></pre> <p>What's the size of class B? I tested using sizeof B, it prints 12</p> <p>Does it mean that only one vptr is there even both of class B and class A have virtual function? Why there is only one vptr?</p> <pre><code>class A { public: int a; virtual void v(); }; class B { public: int b; virtual void w(); }; class C : public A, public B { public: int c; virtual void x(); }; </code></pre> <p>The sizeof C is 20........</p> <p>It seems that in this case, two vptrs are in the layout.....How does this happen? I think the two vptrs one is for class A and another is for class B....so there is no vptr for the virtual function of class C? </p> <p>My question is, what's the rule about the number of vptrs in inheritance?</p> <p><strong>2) virtual inheritance</strong></p> <pre><code> class A { public: int a; virtual void v(); }; class B: virtual public A{ //virtual inheritance public: int b; virtual void w(); }; class C : public A { //non-virtual inheritance public: int c; virtual void x(); }; class D: public B, public C { public: int d; virtual void y(); }; </code></pre> <p>The sizeof A is 8 bytes -------------- 4(int a) + 4 (vptr) = 8</p> <p>The sizeof B is 16 bytes -------------- Without virtual it should be 4 + 4 + 4 = 12. why there is another 4 bytes here? What's the layout of class B ?</p> <p>The sizeof C is 12 bytes. -------------- 4 + 4 + 4 = 12. It's clear!</p> <p>The sizeof D is 32 bytes -------------- it should be 16(class B) + 12(class C) + 4(int d) = 32. Is that right?</p> <pre><code> class A { public: int a; virtual void v(); }; class B: virtual public A{ //virtual inheritance here public: int b; virtual void w(); }; class C : virtual public A { //virtual inheritance here public: int c; virtual void x(); }; class D: public B, public C { public: int d; virtual void y(); }; </code></pre> <p>sizeof A is 8</p> <p>sizeof B is 16</p> <p>sizeof C is 16</p> <p>sizeof D is 28 Does it mean 28 = 16(class B) + 16(class C) - 8(class A) + 4 ( what's this? )</p> <p>My question is , why there is an extra space when virtual inheritance is applied? </p> <p>What's the underneath rule for the object size in this case? </p> <p>What's the difference when virtual is applied on all the base classes and on part of the base classes?</p>
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    1. COAny answer here is pure speculation. Each compiler can and does do it slightly differently. Since the standard does not specify anything about how it is implemented it is pointless wondering about it (unless you plan to write a compiler, in which case this is the wrong place to ask the question).
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    2. COMartin, writing a compiler is inherently a programming topic, and so this is *not* the wrong place to ask. Even people who aren't writing compilers can be curious about how things work, in particular if they were hoping to manage the size of an object and were under the mistaken impression that the size would be proportional to the number of virtual methods. Any answer here is free to make reference to one or more specific compilers, in which case it wouldn't be speculation at all.
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