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copied!<p>Here's an excerpt from Section 6.7.1 (footnote 101) of the <a href="http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1124.pdf" rel="nofollow noreferrer">C99 standard (pdf)</a>:</p> <blockquote> <p>The implementation may treat any <code>register</code> declaration simply as an <code>auto</code> declaration. However, <strong>whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed</strong>, either explicitly (by use of the unary <code>&</code> operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.3.2.1). Thus, the only operator that can be applied to an array declared with storage-class specifier <code>register</code> is <code>sizeof</code>.</p> </blockquote> <p>And from Section 7.1.1, Paragraph 3 of the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf" rel="nofollow noreferrer">C++ standard (pdf)</a>:</p> <blockquote> <p>A <code>register</code> specifier has the same semantics as an <code>auto</code> specifier together with a hint to the implementation that the object so declared will be heavily used. <strong>[Note: the hint can be ignored and in most implementations it will be ignored if the address of the object is taken. —end note]</strong></p> </blockquote> <h2>Fun tidbits about <code>register</code></h2> <p>The C++ group (WG21) <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#809" rel="nofollow noreferrer">wants to deprecate <code>register</code></a>:</p> <blockquote> <p>The <code>register</code> keyword serves very little function, offering no more than a hint that a note says is typically ignored. It should be deprecated in this version of the standard, freeing the reserved name up for use in a future standard, much like <code>auto</code> has been re-used this time around for being similarly useless. </p> <p><strong>Notes from the March, 2009 meeting:</strong></p> <p>The consensus of the CWG was in favor of deprecating <code>register</code>.</p> </blockquote> <p>Look what the C99 group (WG14) <a href="http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1300.pdf" rel="nofollow noreferrer">said about <code>register</code> (pdf)</a> at a meeting:</p> <blockquote> <p>General agreement to deprecate the “<code>auto</code>” keyword. Should we ask WG21 to go back to the previous use of “<code>register</code>” (no address)? No, this will not fly with WG21.</p> </blockquote>

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