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2012-08-07T22:41:59.800
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<h2>a primer on #include vs #import</h2> <p>importing (or including) a file is akin to injecting the contents of that file at the location of the #import statement. </p> <h3>Interesting details:</h3> <p>In Objective-C #import is smarter than the normal C #include in that if a file is already previously injected in an embedded #import chain, then it won't be injected again. the normal C (preprocessor) #include is not smart enough to figure that out which is why you always see constructs like</p> <pre><code> #ifndef __FOO_H #define __FOO_H .... header content here ... #endif /*__FOO_H */ </code></pre> <p>In C headers to ensure singular inclusions, whereas you don't see such constructs in objective C.</p> <p>That is all what <code>#import</code> is: a content injection mechanism.</p> <p>So, taking your own example, let's say I wanted to create an instance of the Person class, I would have to #import its definition before I could do that. :</p> <pre><code> /* main.m */ #import "Person.h" int main(int argc, char *argv[]) { Person *person = [[Person alloc] init]; //^^^^^^^^ this won't work without the #import statement [person release]; return 0; } </code></pre> <p>note that nothing is being inherited, #import just injects all the pieces needed to use the <code>Person</code> class in our main.m</p> <h1>a primer on inheritance</h1> <p>Inheritance is, as you probably already know, a mechanism to define a new class as an extension of another class. Typically, the new class is a subclass of the parent class (think animal kingdom), but not always. </p> <p>In your own example you define Employee as a sub-class of a Person.</p> <h2>So how does inheritance relate to #import?</h2> <p>Well in order to use a class for <em>anything</em> you need to #import its definition so that the compiler knows what you are talking about when you say <code>Person</code>, and that includes (pun intended) creating a derived class.</p>
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1. PODifference between importing a .h file and inheriting a class
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1. USAhmed Masud
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