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copied!<p>Remember that C++ separates <em>memory allocation</em> and <em>object expression</em>. The default array-new expression <code>T * p = new T[N];</code> both allocates enough memory for <code>N</code> objects <em>and</em> constructs those objects. At the other end, <code>delete[] p;</code> must call the destructor of all those elements, and then free the memory.</p> <p>While allocating and freeing memory is handled by the platform, and freeable memory is sufficiently well identified to the OS by a single pointer value, constructing and destroying objects is more complicated. The number of actual objects must be stored somewhere, and to that end, the standard permits a C++ implementation to request more memory than <code>N * sizeof(T)</code>. It is true that the pointer <code>p</code> will always point to the beginning of the array of <code>N</code> objects, but <code>p</code> does not have to be the same pointer that was returned by the underlying memory allocator. (In fact, <code>p</code> is guaranteed to be precisely the value of the underlying allocation result offset by the excess amount of memory.)</p> <p>The details are entirely left up to the implementation. Some platforms provide additional guarantees, though; for example, the <a href="http://refspecs.linuxbase.org/cxxabi-1.83.html#array-cookies">Itanium ABI</a> (which calls the extra data "array cookie") says that the memory for <code>new T[N]</code> will be laid out as follows:</p> <pre><code>+- alignof(T) --+-- sizeof(T) --+-- sizeof(T) --+-- sizeof(T) --+-- ... | ***** [8B: N] | 1st element | 2nd element | 3rd element | ..... +---------------+---------------+---------------+---------------+-- ... A A | |_ result of "new T[N]" | |_ value returned "operator new[]()" </code></pre>

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