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    <p>In theory, the compiler can lay out base classes differently. C++03 §10 paragraph 5 says:</p> <blockquote> <p>A base class subobject might have a layout (3.7) different from the layout of a most derived object of the same type.</p> </blockquote> <p>As <a href="https://stackoverflow.com/questions/7113422/why-can-it-be-dangerous-to-use-this-pod-struct-as-a-base-class/7113474#7113474">StackedCrooked mentioned</a>, this might happen by the compiler adding padding to the end of the base class <code>A</code> when it exists as its own object, but the compiler might not add that padding when it's a base class. This would cause <code>A::clear()</code> to overwrite the first few bytes of the members of the subclass.</p> <p>However in practice, I have not been able to get this to happen with either GCC or Visual Studio 2008. Using this test:</p> <pre><code>struct A { void clear() { memset(this, 0, sizeof(A)); } int age; char type; }; struct B : public A { char x; }; int main(void) { B b; printf("%d %d %d\n", sizeof(A), sizeof(B), ((char*)&amp;b.x - (char*)&amp;b)); b.x = 3; b.clear(); printf("%d\n", b.x); return 0; } </code></pre> <p>And modifying <code>A</code>, <code>B</code>, or both to be 'packed' (with <a href="http://msdn.microsoft.com/en-us/library/2e70t5y1%28v=vs.80%29.aspx" rel="nofollow noreferrer"><code>#pragma pack</code></a> in VS and <a href="http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Type-Attributes.html" rel="nofollow noreferrer"><code>__attribute__((packed))</code></a> in GCC), I couldn't get <code>b.x</code> to be overwritten in any case. Optimizations were enabled. The 3 values printed for the sizes/offsets were always 8/12/8, 8/9/8, or 5/6/5.</p>
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    1. POWhy can it be dangerous to use this POD struct as a base class?
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    1. USAdam Rosenfield
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    1. COHere is my testcase. The base is not a POD though: http://ideone.com/GHATf . Initially I didn't notice that GCC would behave different for PODs, so I worded the question in a way that makes the issue stand out stronger. But as you say, the possibility still exists. And I bet many people will think that a silly `private` there has no effect. Also worth noting 3.9p3 and 3.9p2 where it contains the remarks "... where neither obj1 nor obj2 is a base-class subobject ..." to support the different layouting.
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      1. USJohannes Schaub - litb
 

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