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POStrange C++ reference invalidation on return
 

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    <p>Have a look at the following code. The goal here is to return a reference through two functions (from <code>ReferenceProvider::getReference()</code> to <code>getRef()</code> to <code>main()</code>):</p> <pre><code>#include &lt;tchar.h&gt; #include &lt;assert.h&gt; #include &lt;string&gt; class BaseClass { public: virtual void write() const { printf("In base class\n"); } }; typedef BaseClass* BaseClassPointer; class ChildClass : public BaseClass { public: virtual void write() const { printf("In child class\n"); } }; typedef ChildClass* ChildClassPointer; ////////////////////////////////////////////////////////////////////////// ChildClass* g_somePointer = new ChildClass(); class ReferenceProvider { public: const BaseClassPointer&amp; getReference() { const BaseClassPointer&amp; val = g_somePointer; return val; } }; ReferenceProvider g_provider; const BaseClassPointer&amp; getRef() { std::string test; const BaseClassPointer&amp; val = g_provider.getReference(); return val; } int _tmain(int argc, _TCHAR* argv[]) { BaseClass* child = getRef(); assert(child == g_somePointer); child-&gt;write(); return 0; } </code></pre> <p>Now, when debugging this code (in Visual C++), breaking at <code>return val;</code> in <code>getRef()</code> will give you a screen like this:</p> <p><img src="https://i.stack.imgur.com/5YZNQ.png" alt="Breaking a return"></p> <p>Notice how the values of <code>g_somePointer</code> and <code>val</code> are the same. Now, step over the return statement and you'll get a screen like this:</p> <p><img src="https://i.stack.imgur.com/bTAKU.png" alt="Breaking after return"></p> <p>Notice how <code>val</code> has become invalid (<code>0xcccccccc</code>). This is probably because the stack of <code>getRef()</code> has been cleared and <code>val</code> is no longer available. </p> <p>The problem now is that <code>child</code> in <code>_tmain()</code> will get this invalid value (<code>0xcccccccc</code>) rendering <code>child</code> unusable. So my first (and main) question is: <em>How to do this correctly?</em> </p> <p>(Please note that this is just an boiled down example from some other code I've been working on. It needs to be structured like with, including using references to pointers.)</p> <p>What's making this whole thing very strange (and hard to debug) is that the function <code>getRef()</code> works under some conditions:</p> <ol> <li>If you change the type of <code>g_somePointer</code> to <code>BaseClass*</code> (from <code>ChildClass*</code>)</li> <li>If you remove the local variable in <code>getRef()</code> (i.e. the line <code>std::string test;</code>)</li> </ol> <p>In both cases the reference variable <code>val</code> (in <code>getRef()</code>) will <em>not</em> become invalid and the function will return the correct pointer address. Can anybody explain this to me?</p>
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    1. USSebastian Krysmanski
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    1. COLooks like someone is trying to write Java in C++. I hope you're not doing it by choice. ;)
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      1. USjalf
    2. COUnfortunately I'm stuck with C++; I can't switch to Java (although I'd love to as C++ is driving me crazy ;) )
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    3. COWell, your other option is to learn C++ *the right way*. Treating it as Java is a recipe for pain and suffering. But when used "normally", it's not too bad
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