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    <p>For me it seems quite acceptable solution if X's destructor will be called before placement new. It's syntactically allowed even the destructor is not actually specified for the class.</p> <pre><code>struct X { int val; int* pVal; X(int v) : val(v), pVal(&amp;val) {} }; X x(1); void foo() { doStuffWith(x); x.~X(); new (&amp;x) X(2); doStuffWith(x); } </code></pre> <p>In this form it is a correct way to reuse memory for any object (but only if the object's ctor can't throw! otherwise UB may happen on program shutdown, i.e. double call of the destructor).</p> <p>Indeed, the equality of pointers passed and returned from placement new in it non-array form is guaranteed by the standard:</p> <blockquote> <h3>18.6.1.3 Placement forms</h3> <p>...</p> <p>void* operator new(std::size_t size, void* ptr) noexcept;</p> <p>Returns: ptr.</p> <p>Remarks: Intentionally performs no other action.</p> </blockquote> <p>(and the result of the conversion to void* and then back to the same pointer type is also guaranteed to be the same as the source pointer)</p> <p>However, to avoid non-proper use of the class it would be more safe either define copy assignment and copy constructor or declare this class as noncopyable (with deleted ones)</p> <p>And only the last (noncopyable) case may be regarded as a reason for using placement new.</p> <p>Although I'm far from promoting placement new for general use, it express the intention to reuse object memory directly and doesn't rely on any optimization. Copy constructor and copy assignment are, of course, more safe, but don't express this intention excactly: no "copy" actually needed, the new object should be constructed in place of the old one.</p>
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    1. POPlacement new to avoid copy constructor
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    1. USEric
    1. USuser396672
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    1. CO-1 There are constructs in C++ to do this properly; why rely on the user instead?
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      1. USAlex Chamberlain
    2. CO@Alex: other constructs such copy ctor/assignments burden the class with unnecessary things.The class shouldn't be responsible for reusing of object's memory. It may be noncopyable at all for some reasons.
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