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POWhen S is a trivial subclass of T, is it safe to use an array of S where an array of T is expected?
 

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  1. POWhen S is a trivial subclass of T, is it safe to use an array of S where an array of T is expected?
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    <p>Consider the following declarations of a pair of related structs. The descendant class adds no member variables, and the only member function is a constructor that does nothing but forward all its arguments to the base class's constructor.</p> <pre><code>struct Base { Base(int a, char const* b): a(a), b(b) { } int a; char const* b; }; struct Descendant: Base { Descendant(int a, char const* b): Base(a, b) { } }; </code></pre> <p>Now consider the following code using those types. Function <code>foo</code> expects to receive an array of <code>Base</code>. However, <code>main</code> defines an array of <code>Descendant</code> and passes that to <code>foo</code> instead.</p> <pre><code>void foo(Base* array, unsigned len) { /* &lt;access contents of array&gt; */ } int main() { Descendant main_array[] = { Descendant(0, "zero"), Descendant(1, "one") }; foo(main_array, 2); return 0; } </code></pre> <p>Is behavior of this program defined? Does the answer depend on the body of <code>foo</code>, like whether it writes to the array or only reads from it?</p> <p>If <code>sizeof(Derived)</code> is unequal to <code>sizeof(Base)</code>, then behavior is undefined according to the answers to a previous question about a <a href="https://stackoverflow.com/q/7197677/33732">base pointer to an array of derived objects</a>. Is there any chance the objects in <em>this</em> question will have differing sizes, though?</p>
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    1. COFor whoever you may trust' sake, this question was asked in my test yesterday. I was fully lost then, and I'm still fully lost about this question now. (The question addressed the assignment of a vector of inherited classes to a vector of base classes in java. It was said there would be a break on the soundness of the operations after the assignment. The assignemnt: `class Mammal ...; class Dog extends Mammal ...; Dog x[] = new Dog[5]; Mammal y[] = x;`).
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    2. CO@Rubens Java is a very different matter from C++ though. For starters, concerns about the size of the elements (as in the last paragraph of this question) don't apply because all classes are reference types. The soundness problem is visible in code like `y[0] = new Cat; Dog fido = x[0];` would result in the dog `fido` actually being a `Cat`. (This does not actually work, the `y[0] = new Cat` part throws an exception at run time.) *This* problem does apply to C++, but let's assume we're not doing that.
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    3. COI think [this](http://stackoverflow.com/questions/17845128/struct-alignment-c-c]) question gets us some of the way in showing this is OK.
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      1. USuser786653
 

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