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| 4 | <title>The Macro Expansion Process</title> |
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| 12 | <td width="21"> <h1></h1></td> |
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| 13 | <td width="885"> <font face="Verdana, Arial, Helvetica, sans-serif"><b><font size="6">The |
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| 14 | Macro Expansion Process</font></b></font></td> |
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| 27 | <p>The macro expansion process described here was initially developed by <a href="mailto:pmenso57@attbi.com">Paul |
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| 28 | Mensonides</a> and is implemented in <tt>Wave</tt>. It is much more understandable |
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| 29 | as the description of the desired macro expansion algorithm provided in the |
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| 30 | C++ Standard <a href="references.html#iso_cpp">[1]</a>.</p> |
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| 31 | <p>Macro replacement proceeds left-to-right. </p> |
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| 32 | <p>If, during scanning (or rescanning) an identifier is found, it is looked up |
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| 33 | in the symbol table. If the identifier is not found in the symbol table, it |
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| 34 | is not a macro and scanning continues.</p> |
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| 35 | <p>If the identifier is found, the value of a flag associated with the identifier |
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| 36 | is used to determine if the identifier is available for expansion. If it is |
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| 37 | not, the specific token (i.e. the specific instance of the identifier) is marked |
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| 38 | as disabled and is not expanded. If the identifier is available for expansion, |
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| 39 | the value of a different flag associated with the identifier in the symbol table |
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| 40 | is used to determine if the identifier is an object-like or function-like macro. |
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| 41 | If it is an object-like macro, it is expanded. If it is a function-like macro, |
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| 42 | it is only expanded if the next token is an left parenthesis.<br> |
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| 43 | An identifier is available for expansion if it is not marked as disabled and |
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| 44 | if the the value of the flag associated with the identifier is not set, which |
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| 45 | is used to determine if the identifier is available for expansion.</p> |
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| 46 | <p>(If a macro is an object-like macro, skip past the next two paragraphs.)</p> |
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| 47 | <p>If a macro to be expanded is a function-like macro, it must have the exact |
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| 48 | number of actual arguments as the number of formal parameters required by the |
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| 49 | definition of the macro. Each argument is recursively scanned and expanded. |
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| 50 | Each parameter name found in the replacement list is replaced by the expanded |
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| 51 | actual argument after leading and trailing whitespace and all placeholder tokens |
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| 52 | are removed unless the parameter name immediately follows the stringizing operator |
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| 53 | (<tt>'#'</tt>) or is adjacent to the token-pasting operator (<tt>'##'</tt>).</p> |
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| 54 | <p>If the parameter name immediately follows the stringizing operator (<tt>'#'</tt>), |
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| 55 | a stringized version of the unexpanded actual argument is inserted. If the parameter |
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| 56 | name is adjacent to the token-pasting operator (<tt>'##'</tt>), the unexpanded |
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| 57 | actual argument is inserted after all placeholder tokens are removed.</p> |
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| 58 | <p>All concatenation takes place in the replacement list. (If a single concatenation |
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| 59 | yields multiple tokens, the behavior is undefined. Moreover, <tt>Wave</tt> in |
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| 60 | normal C++98 and C99 modes issues an error, if more then one token is produced |
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| 61 | as the result of the concatenation. In C++0x mode <tt>Wave</tt> treats token-pasting |
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| 62 | of unrelated tokens as well defined and inserts the reparsed string representation |
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| 63 | of the concatenated tokens into the replacement list.).</p> |
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| 64 | <p>The flag in the symbol table entry associated with the name of the macro being |
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| 65 | expanded is set to indicate the that the macro is not available for expansion.</p> |
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| 66 | <p>The replacement list is rescanned for further macro expansion. All leading |
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| 67 | and trailing whitespace tokens in the replacement list are removed (the placeholder |
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| 68 | tokens are left intact). </p> |
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| 69 | <p>After rescanning completes, the flag in the symbol table entry associated with |
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| 70 | the name of macro being expanded is cleared to indicate that the macro is again |
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| 71 | available for expansion, and the sequence of tokens that constitutes the rescanned |
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| 72 | replacement list is returned to the point of invocation of the macro.</p> |
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| 73 | <p>If this sequence of tokens is empty, it is replaced by a placeholder token. |
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| 74 | If a placeholder is found during scanning (or rescanning) it is ignored. (Also, |
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| 75 | if the only thing separating a parameter from the stringizing operator or token-pasting |
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| 76 | operator is placeholder, it is also ignored in that context.)</p> |
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| 77 | <p>This sequence of tokens is inserted at the original point that the macro was |
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| 78 | invoked, and scanning continues starting with the last token of the newly inserted |
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| 79 | sequence of tokens. I.e. scanning looks back a single token (possibly a placeholder |
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| 80 | token) and continues.<br> |
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| 81 | </p> |
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| 82 | <table border="0"> |
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| 83 | <tr> |
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| 84 | <td width="10"></td> |
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| 85 | <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> |
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| 86 | <td width="30"><a href="supported_pragmas.html"><img src="theme/l_arr.gif" border="0"></a></td> |
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| 87 | <td width="30"><a href="compiletime_config.html"><img src="theme/r_arr.gif" border="0"></a></td> |
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| 88 | </tr> |
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| 89 | </table> |
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| 90 | <hr size="1"> |
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| 91 | <p class="copyright">Copyright © 2003-2005 Hartmut Kaiser<br> |
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| 92 | <br> |
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| 93 | <font size="2">Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </font> </p> |
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| 94 | <span class="updated"></span> |
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| 95 | <p class="copyright"><span class="updated">Last updated: |
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| 96 | <!-- #BeginDate format:fcAm1m -->Sunday, May 15, 2005 12:23<!-- #EndDate --> |
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| 97 | </span> |
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| 98 | </p> |
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| 99 | <p> </p> |
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