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    <p>Here's what is wrong with it in C++. From 3.8 <code>[basic.life]</code>:</p> <blockquote> <p>The lifetime of an object of type <code>T</code> begins when:</p> <ul> <li>storage with the proper alignment and size for type <code>T</code> is obtained, and</li> <li>if the object has non-trivial initialization, its initialization is complete.</li> </ul> </blockquote> <p>and later</p> <blockquote> <p>For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the object occupies is reused or released; however, if there is no explicit call to the destructor or if a delete-expression (5.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined behavior.</p> </blockquote> <p>Others have already pointed out the potential alignment issues, which also exist in C. But in C++ initialization is a special problem. The public user isn't performing any, so you can only cast the pointer to the private type and use it if the private type has no initialization. There's a parallel problem with destruction -- you force the private object to have trivial destruction.</p> <p>Which clearly is why you've written <code>private_baz_t *bz;</code> when you should be using a smart pointer.</p> <p>The only "benefits" this trick buys you are memory leaks and lack of exception safety -- all the things RAII is designed to protect against. Use the p/impl pattern instead, which actually provides a compilation firewall and improves build times.</p>
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    1. POWhy one should not hide a structure implementation that way?
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    1. USBen Voigt
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    1. COThanks for this answer, that's a very interesting point.
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      1. USYann Droneaud
 

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