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PO
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Id
1580981
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0
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CreationDate
2009-10-17T00:06:20.733
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LastActivityDate
2009-10-17T00:10:38.340
LastEditDate
2009-10-17T00:10:38.340
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23283
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23283
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1580960
PostTypeId
2
Score
6
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text
Body
<p>I'm not an expert in placement new but there are a couple of issues with how you are defining the macro. </p> <h2>Issue 1</h2> <p>The most obvious problem is the use of the cast <code>(TYPE*)STORAGE</code> for the storage location. This is incorrect. Placement new is just another C++ function and it participates in operations like overload resolution. Arbitrarily casting the memory to a specific type could cause the placement new to bind to a different operator new that the user expected. </p> <p>For example, it's valid to have the following two definitions of placement new. Your macro would potentially cause the wrong one to be called. </p> <pre><code>void * _cdecl operator new(size_t cbSize, void* pv); void * _cdecl operator new(size_t cbSize, SomeType* pv)- </code></pre> <p>...</p> <pre><code>// These two call different overloads void* p = malloc(sizeof(SomeType)); SomeType* f1 = CONSTRUCT_INPLACE(SomeType, p,()) SomeType* f2 = new (p) SomeType(); </code></pre> <p>I wrote a blog post awhile back on how you can use this type of overload resolution to implement custom allocators. </p> <ul> <li><a href="http://blogs.msdn.com/jaredpar/archive/2007/10/17/c-placement-new-and-allocators.aspx" rel="nofollow noreferrer">http://blogs.msdn.com/jaredpar/archive/2007/10/17/c-placement-new-and-allocators.aspx</a></li> </ul> <h2>Issue 2</h2> <p>The expression STORAGE in the macro should be wrapped in parens to prevent evil macro expansion bugs. </p> <pre><code>::new((TYPE*)(STORAGE)) TYPE INIT </code></pre>
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