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| 1 | +<?xml version='1.0' encoding='utf-8' standalone='no'?> |
| 2 | +<!DOCTYPE issue SYSTEM "lwg-issue.dtd"> |
| 3 | + |
| 4 | +<issue num="4149" status="New"> |
| 5 | +<title>User defined macros without standard headers (294 redux)</title> |
| 6 | +<section><sref ref="[macro.names]"/></section> |
| 7 | +<submitter>Jonathan Wakely</submitter> |
| 8 | +<date>05 Sep 2024</date> |
| 9 | +<priority>99</priority> |
| 10 | + |
| 11 | +<discussion> |
| 12 | +<p> |
| 13 | +Issue <iref ref="294"/> changed <sref ref="[macro.names]"/> from: |
| 14 | +<blockquote> |
| 15 | +A translation unit that includes a header shall not contain any macros that |
| 16 | +define names declared or defined in that header. Nor shall such a translation |
| 17 | +unit define macros for names lexically identical to keywords. |
| 18 | +</blockquote> |
| 19 | +to: |
| 20 | +<blockquote> |
| 21 | +A translation unit that includes a standard library header shall not |
| 22 | +`#define` or `#undef` names declared in any standard library header. |
| 23 | +<p> |
| 24 | +A translation unit shall not `#define` or `#undef` names lexically identical |
| 25 | +to keywords. |
| 26 | +</p> |
| 27 | +</blockquote> |
| 28 | +</p> |
| 29 | +<p> |
| 30 | +Note that the second sentence of the original says "such a translation unit" |
| 31 | +when prohibiting things like `#define while`. This means the prohibition only |
| 32 | +applies to "a translation unit that includes a header". |
| 33 | +The replacement has the prohibition in a separate paragraph and does not |
| 34 | +clearly say that it only applies when a header is included. |
| 35 | +</p> |
| 36 | +<p> |
| 37 | +The issue discussion seems clear that the concern is about C++ headers |
| 38 | +including other unspecified headers, which is allowed in C++ (though not in C). |
| 39 | +There is no justification for broadening the second sentence to apply |
| 40 | +unconditionally. Such a rule would belong in <sref ref="[cpp]"/> anyway, |
| 41 | +not in library wording. That overreach doesn't appear to have been intended, |
| 42 | +and we should clarify what the library is prohibiting. |
| 43 | +</p> |
| 44 | +<p> |
| 45 | +It was pointed out on the reflector that it's not enough to only prohibit |
| 46 | +defining such macros <i>after</i> including headers, because in some cases |
| 47 | +that could still break the library header. For example, the library macro |
| 48 | +`assert` typically uses `void` and so `#define void ...` would break `assert` |
| 49 | +even if it happens after including <code><cassert></code>. |
| 50 | +</p> |
| 51 | +</discussion> |
| 52 | + |
| 53 | +<resolution> |
| 54 | +<p> |
| 55 | +This wording is relative to <paper num="N4986"/>. |
| 56 | +</p> |
| 57 | + |
| 58 | +<ol> |
| 59 | +<li><p>Modify <sref ref="[macro.names]"/> as indicated:</p> |
| 60 | + |
| 61 | +<blockquote> |
| 62 | +<p> |
| 63 | +-1- A translation unit that includes a standard library header shall not |
| 64 | +`#define` or `#undef` names declared in any standard library header. |
| 65 | +</p> |
| 66 | +<p> |
| 67 | +-2- A translation unit |
| 68 | +<ins>that includes a standard library header</ins> |
| 69 | +shall not `#define` or `#undef` names lexically identical to keywords, |
| 70 | +to the identifiers listed in Table 4, |
| 71 | +or to the <i>attribute-tokens</i> described in <sref ref="[dcl.attr]"/>, |
| 72 | +except that the names `likely` and `unlikely` may be defined as |
| 73 | +function-like macros (<sref ref="[cpp.replace]"/>). |
| 74 | +</p> |
| 75 | +</blockquote> |
| 76 | +</li> |
| 77 | +</ol> |
| 78 | +</resolution> |
| 79 | + |
| 80 | +</issue> |
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