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| <h3 class="section">3.8 Undefining and Redefining Macros</h3> |
| |
| <p><a name="index-undefining-macros-68"></a><a name="index-redefining-macros-69"></a><a name="index-g_t_0023undef-70"></a> |
| If a macro ceases to be useful, it may be <dfn>undefined</dfn> with the |
| ‘<samp><span class="samp">#undef</span></samp>’ directive. ‘<samp><span class="samp">#undef</span></samp>’ takes a single argument, the |
| name of the macro to undefine. You use the bare macro name, even if the |
| macro is function-like. It is an error if anything appears on the line |
| after the macro name. ‘<samp><span class="samp">#undef</span></samp>’ has no effect if the name is not a |
| macro. |
| |
| <pre class="smallexample"> #define FOO 4 |
| x = FOO; ==> x = 4; |
| #undef FOO |
| x = FOO; ==> x = FOO; |
| </pre> |
| <p>Once a macro has been undefined, that identifier may be <dfn>redefined</dfn> |
| as a macro by a subsequent ‘<samp><span class="samp">#define</span></samp>’ directive. The new definition |
| need not have any resemblance to the old definition. |
| |
| <p>However, if an identifier which is currently a macro is redefined, then |
| the new definition must be <dfn>effectively the same</dfn> as the old one. |
| Two macro definitions are effectively the same if: |
| <ul> |
| <li>Both are the same type of macro (object- or function-like). |
| <li>All the tokens of the replacement list are the same. |
| <li>If there are any parameters, they are the same. |
| <li>Whitespace appears in the same places in both. It need not be |
| exactly the same amount of whitespace, though. Remember that comments |
| count as whitespace. |
| </ul> |
| |
| <p class="noindent">These definitions are effectively the same: |
| <pre class="smallexample"> #define FOUR (2 + 2) |
| #define FOUR (2 + 2) |
| #define FOUR (2 /* <span class="roman">two</span> */ + 2) |
| </pre> |
| <p class="noindent">but these are not: |
| <pre class="smallexample"> #define FOUR (2 + 2) |
| #define FOUR ( 2+2 ) |
| #define FOUR (2 * 2) |
| #define FOUR(score,and,seven,years,ago) (2 + 2) |
| </pre> |
| <p>If a macro is redefined with a definition that is not effectively the |
| same as the old one, the preprocessor issues a warning and changes the |
| macro to use the new definition. If the new definition is effectively |
| the same, the redefinition is silently ignored. This allows, for |
| instance, two different headers to define a common macro. The |
| preprocessor will only complain if the definitions do not match. |
| |
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