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| <h2 class="chapter">4 Conditionals</h2> |
| |
| <p><a name="index-conditionals-83"></a> |
| A <dfn>conditional</dfn> is a directive that instructs the preprocessor to |
| select whether or not to include a chunk of code in the final token |
| stream passed to the compiler. Preprocessor conditionals can test |
| arithmetic expressions, or whether a name is defined as a macro, or both |
| simultaneously using the special <code>defined</code> operator. |
| |
| <p>A conditional in the C preprocessor resembles in some ways an <code>if</code> |
| statement in C, but it is important to understand the difference between |
| them. The condition in an <code>if</code> statement is tested during the |
| execution of your program. Its purpose is to allow your program to |
| behave differently from run to run, depending on the data it is |
| operating on. The condition in a preprocessing conditional directive is |
| tested when your program is compiled. Its purpose is to allow different |
| code to be included in the program depending on the situation at the |
| time of compilation. |
| |
| <p>However, the distinction is becoming less clear. Modern compilers often |
| do test <code>if</code> statements when a program is compiled, if their |
| conditions are known not to vary at run time, and eliminate code which |
| can never be executed. If you can count on your compiler to do this, |
| you may find that your program is more readable if you use <code>if</code> |
| statements with constant conditions (perhaps determined by macros). Of |
| course, you can only use this to exclude code, not type definitions or |
| other preprocessing directives, and you can only do it if the code |
| remains syntactically valid when it is not to be used. |
| |
| <p>GCC version 3 eliminates this kind of never-executed code even when |
| not optimizing. Older versions did it only when optimizing. |
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