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POAny reason to prefer static_cast over a chain of implicit conversions?
 

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  1. POAny reason to prefer static_cast over a chain of implicit conversions?
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    <p>Suppose I have a class implementing several interfaces</p> <pre><code>class CMyClass : public IInterface1, public IInterface2 { }; </code></pre> <p>and in a member function of that class I need to obtain a <code>void*</code> pointer to one of those interfaces (typical situation in <a href="https://stackoverflow.com/questions/1742848/why-exactly-do-i-need-an-explicit-upcast-when-implementing-queryinterface-in-an"><code>IUnknown::QueryInterface()</code></a>.</p> <p>The typical solution is to use a <code>static_cast</code> to achieve pointer adjustment:</p> <pre><code>void* pointer = static_cast&lt;IInterface2*&gt;( this ); </code></pre> <p>and it is safe in this case if there's no known class inherited from <code>CMyClass</code>. But if such class exists:</p> <pre><code>class CDerivedClass : public CUnrelatedClass, public CMyClass {}; </code></pre> <p>and I accidentially do</p> <pre><code>void* pointer = static_cast&lt;CDerivedClass*&gt;( this ); </code></pre> <p>and <code>this</code> is actually a pointer to <code>CMyClass</code> instance the compiler won't catch me and the program might run into undefined behavior later - <code>static_cast</code> becomes unsafe.</p> <p>The suggested solution is to use implicit conversion:</p> <pre><code>IInterface2* interfacePointer = this; void* pointer = interfacePointer; </code></pre> <p>Looks like this will solve both problems - pointer adjustment and risk of invalid downcast.</p> <p>Are there any problems in the second solution? What could be the reasons to prefer the first one?</p>
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      1. POAny reason to prefer static_cast over a chain of implicit conversions?
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    1. COIts interesting that `CMyClass` has knowledge of `CDerivedClass` here... Not impossible, not even a real sign of horrible design, but in the general case, `CMyClass` should not have any knowledge of its descendants. I can imagine having the definitions of both classes in headers and the translation unit where `CMyClass` methods are defined including both, more so in VS with its tendency to advocate precompiled headers... still, food for thought.
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      1. POAny reason to prefer static_cast over a chain of implicit conversions?
      1. USDavid Rodríguez - dribeas
 

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