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| <h3 class="section">3.4 Stringification</h3> |
| |
| <p><a name="index-stringification-50"></a><a name="index-g_t_0040samp_007b_0023_007d-operator-51"></a> |
| Sometimes you may want to convert a macro argument into a string |
| constant. Parameters are not replaced inside string constants, but you |
| can use the ‘<samp><span class="samp">#</span></samp>’ preprocessing operator instead. When a macro |
| parameter is used with a leading ‘<samp><span class="samp">#</span></samp>’, the preprocessor replaces it |
| with the literal text of the actual argument, converted to a string |
| constant. Unlike normal parameter replacement, the argument is not |
| macro-expanded first. This is called <dfn>stringification</dfn>. |
| |
| <p>There is no way to combine an argument with surrounding text and |
| stringify it all together. Instead, you can write a series of adjacent |
| string constants and stringified arguments. The preprocessor will |
| replace the stringified arguments with string constants. The C |
| compiler will then combine all the adjacent string constants into one |
| long string. |
| |
| <p>Here is an example of a macro definition that uses stringification: |
| |
| <pre class="smallexample"> #define WARN_IF(EXP) \ |
| do { if (EXP) \ |
| fprintf (stderr, "Warning: " #EXP "\n"); } \ |
| while (0) |
| WARN_IF (x == 0); |
| ==> do { if (x == 0) |
| fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0); |
| </pre> |
| <p class="noindent">The argument for <code>EXP</code> is substituted once, as-is, into the |
| <code>if</code> statement, and once, stringified, into the argument to |
| <code>fprintf</code>. If <code>x</code> were a macro, it would be expanded in the |
| <code>if</code> statement, but not in the string. |
| |
| <p>The <code>do</code> and <code>while (0)</code> are a kludge to make it possible to |
| write <code>WARN_IF (</code><var>arg</var><code>);</code>, which the resemblance of |
| <code>WARN_IF</code> to a function would make C programmers want to do; see |
| <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">Swallowing the Semicolon</a>. |
| |
| <p>Stringification in C involves more than putting double-quote characters |
| around the fragment. The preprocessor backslash-escapes the quotes |
| surrounding embedded string constants, and all backslashes within string and |
| character constants, in order to get a valid C string constant with the |
| proper contents. Thus, stringifying <code>p = "foo\n";<!-- /@w --></code> results in |
| <tt>"p = \"foo\\n\";"<!-- /@w --></tt>. However, backslashes that are not inside string |
| or character constants are not duplicated: ‘<samp><span class="samp">\n</span></samp>’ by itself |
| stringifies to <tt>"\n"</tt>. |
| |
| <p>All leading and trailing whitespace in text being stringified is |
| ignored. Any sequence of whitespace in the middle of the text is |
| converted to a single space in the stringified result. Comments are |
| replaced by whitespace long before stringification happens, so they |
| never appear in stringified text. |
| |
| <p>There is no way to convert a macro argument into a character constant. |
| |
| <p>If you want to stringify the result of expansion of a macro argument, |
| you have to use two levels of macros. |
| |
| <pre class="smallexample"> #define xstr(s) str(s) |
| #define str(s) #s |
| #define foo 4 |
| str (foo) |
| ==> "foo" |
| xstr (foo) |
| ==> xstr (4) |
| ==> str (4) |
| ==> "4" |
| </pre> |
| <p><code>s</code> is stringified when it is used in <code>str</code>, so it is not |
| macro-expanded first. But <code>s</code> is an ordinary argument to |
| <code>xstr</code>, so it is completely macro-expanded before <code>xstr</code> |
| itself is expanded (see <a href="Argument-Prescan.html#Argument-Prescan">Argument Prescan</a>). Therefore, by the time |
| <code>str</code> gets to its argument, it has already been macro-expanded. |
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