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| <h3 class="section">3.2 Function-like Macros</h3> |
| |
| <p><a name="index-function_002dlike-macros-45"></a> |
| You can also define macros whose use looks like a function call. These |
| are called <dfn>function-like macros</dfn>. To define a function-like macro, |
| you use the same ‘<samp><span class="samp">#define</span></samp>’ directive, but you put a pair of |
| parentheses immediately after the macro name. For example, |
| |
| <pre class="smallexample"> #define lang_init() c_init() |
| lang_init() |
| ==> c_init() |
| </pre> |
| <p>A function-like macro is only expanded if its name appears with a pair |
| of parentheses after it. If you write just the name, it is left alone. |
| This can be useful when you have a function and a macro of the same |
| name, and you wish to use the function sometimes. |
| |
| <pre class="smallexample"> extern void foo(void); |
| #define foo() /* <span class="roman">optimized inline version</span> */ |
| ... |
| foo(); |
| funcptr = foo; |
| </pre> |
| <p>Here the call to <code>foo()</code> will use the macro, but the function |
| pointer will get the address of the real function. If the macro were to |
| be expanded, it would cause a syntax error. |
| |
| <p>If you put spaces between the macro name and the parentheses in the |
| macro definition, that does not define a function-like macro, it defines |
| an object-like macro whose expansion happens to begin with a pair of |
| parentheses. |
| |
| <pre class="smallexample"> #define lang_init () c_init() |
| lang_init() |
| ==> () c_init()() |
| </pre> |
| <p>The first two pairs of parentheses in this expansion come from the |
| macro. The third is the pair that was originally after the macro |
| invocation. Since <code>lang_init</code> is an object-like macro, it does not |
| consume those parentheses. |
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