StackOverflow2013

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POIn Objective C, why am I allowed to assign an NSArray to an NSMutableArray without error or warning?
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copied!<p>I'm disturbed by a weird behavior, illustrated by the following example:</p> <pre><code>NSMutableArray *a1 = [[NSMutableArray alloc] init]; // fine NSMutableArray *a2 = [NSMutableArray array]; // fine, too // compiler reports incompatible pointer types; good: NSMutableArray *a3 = [[NSArray alloc] init]; // compiler says nothing and is happy to assign this! NSMutableArray *a4 = [NSArray array]; </code></pre> <p>Both <code>init</code> and <code>array</code> method of both the <code>NSArray</code> and <code>NSMutableArray</code> classes return <code>id</code>. However, the behavior when I call these methods is simply not the same, and clang lets me happily assign an empty <code>NSArray</code> to an <code>NSMutableArray</code> variable!</p> <p>It turns out that <a href="http://clang.llvm.org/docs/LanguageExtensions.html#objc_instancetype" rel="nofollow">clang will automatically change the return type of some methods, including the <code>init</code> family, to <code>instancetype</code></a>, and thus be able to determine at compile time that <code>[[NSArray alloc] init]</code> returns an <code>NSArray *</code> and not an <code>NSMutableArray *</code>. But this check simply doesn't work with the <code>array</code> method.</p> <p>Why? Shouldn't lines like my last example generate <em>at least</em> a warning? Why aren't all these methods declared as returning <code>instancetype</code>? Will it change in the future?</p> <hr/> <p><strong>Update</strong></p> <p>Good news: as of iOS 7, <code>[NSArray array]</code> returns <code>instancetype</code>, so the assignment to <code>a4</code> above also yields a warning. Other methods like <code>arrayWithContentsOfFile:</code> or <code>arrayWithContentsOfURL</code> still return <code>id</code>, though…</p>

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