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| <h3 class="section">8.1 <code>+load</code>: Executing code before main</h3> |
| |
| <p>The GNU Objective-C runtime provides a way that allows you to execute |
| code before the execution of the program enters the <code>main</code> |
| function. The code is executed on a per-class and a per-category basis, |
| through a special class method <code>+load</code>. |
| |
| <p>This facility is very useful if you want to initialize global variables |
| which can be accessed by the program directly, without sending a message |
| to the class first. The usual way to initialize global variables, in the |
| <code>+initialize</code> method, might not be useful because |
| <code>+initialize</code> is only called when the first message is sent to a |
| class object, which in some cases could be too late. |
| |
| <p>Suppose for example you have a <code>FileStream</code> class that declares |
| <code>Stdin</code>, <code>Stdout</code> and <code>Stderr</code> as global variables, like |
| below: |
| |
| <pre class="smallexample"> |
| FileStream *Stdin = nil; |
| FileStream *Stdout = nil; |
| FileStream *Stderr = nil; |
| |
| @implementation FileStream |
| |
| + (void)initialize |
| { |
| Stdin = [[FileStream new] initWithFd:0]; |
| Stdout = [[FileStream new] initWithFd:1]; |
| Stderr = [[FileStream new] initWithFd:2]; |
| } |
| |
| /* <span class="roman">Other methods here</span> */ |
| @end |
| |
| </pre> |
| <p>In this example, the initialization of <code>Stdin</code>, <code>Stdout</code> and |
| <code>Stderr</code> in <code>+initialize</code> occurs too late. The programmer can |
| send a message to one of these objects before the variables are actually |
| initialized, thus sending messages to the <code>nil</code> object. The |
| <code>+initialize</code> method which actually initializes the global |
| variables is not invoked until the first message is sent to the class |
| object. The solution would require these variables to be initialized |
| just before entering <code>main</code>. |
| |
| <p>The correct solution of the above problem is to use the <code>+load</code> |
| method instead of <code>+initialize</code>: |
| |
| <pre class="smallexample"> |
| @implementation FileStream |
| |
| + (void)load |
| { |
| Stdin = [[FileStream new] initWithFd:0]; |
| Stdout = [[FileStream new] initWithFd:1]; |
| Stderr = [[FileStream new] initWithFd:2]; |
| } |
| |
| /* <span class="roman">Other methods here</span> */ |
| @end |
| |
| </pre> |
| <p>The <code>+load</code> is a method that is not overridden by categories. If a |
| class and a category of it both implement <code>+load</code>, both methods are |
| invoked. This allows some additional initializations to be performed in |
| a category. |
| |
| <p>This mechanism is not intended to be a replacement for <code>+initialize</code>. |
| You should be aware of its limitations when you decide to use it |
| instead of <code>+initialize</code>. |
| |
| <ul class="menu"> |
| <li><a accesskey="1" href="What-you-can-and-what-you-cannot-do-in-_002bload.html#What-you-can-and-what-you-cannot-do-in-_002bload">What you can and what you cannot do in +load</a> |
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