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copied!<p>Yes, there are benefits to using <code>instancetype</code> in all cases where it applies. I'll explain in more detail, but let me start with this bold statement: Use <code>instancetype</code> whenever it's appropriate, which is whenever a class returns an instance of that same class.</p> <p>In fact, here's what Apple now says on the subject:</p> <blockquote> <p>In your code, replace occurrences of <code>id</code> as a return value with <code>instancetype</code> where appropriate. This is typically the case for <code>init</code> methods and class factory methods. Even though the compiler automatically converts methods that begin with “alloc,” “init,” or “new” and have a return type of <code>id</code> to return <code>instancetype</code>, it doesn’t convert other methods. <strong>Objective-C convention is to write <code>instancetype</code> explicitly for all methods.</strong></p> </blockquote> <ul> <li>Emphasis mine. Source: <a href="https://developer.apple.com/library/ios/releasenotes/ObjectiveC/ModernizationObjC/AdoptingModernObjective-C/AdoptingModernObjective-C.html#//apple_ref/doc/uid/TP40014150-CH1-SW11" rel="nofollow noreferrer">Adopting Modern Objective-C</a></li> </ul> <p>With that out of the way, let's move on and explain why it's a good idea.</p> <p>First, some definitions:</p> <pre><code> @interface Foo:NSObject - (id)initWithBar:(NSInteger)bar; // initializer + (id)fooWithBar:(NSInteger)bar; // class factory @end </code></pre> <p>For a class factory, you should <strong>always</strong> use <code>instancetype</code>. The compiler does not automatically convert <code>id</code> to <code>instancetype</code>. That <code>id</code> is a generic object. But if you make it an <code>instancetype</code> the compiler knows what type of object the method returns.</p> <p>This is <strong>not</strong> an academic problem. For instance, <code>[[NSFileHandle fileHandleWithStandardOutput] writeData:formattedData]</code> will generate an error on Mac OS X (<strong>only</strong>) <strong>Multiple methods named 'writeData:' found with mismatched result, parameter type or attributes</strong>. The reason is that both NSFileHandle and NSURLHandle provide a <code>writeData:</code>. Since <code>[NSFileHandle fileHandleWithStandardOutput]</code> returns an <code>id</code>, the compiler is not certain what class <code>writeData:</code> is being called on.</p> <p>You need to work around this, using either:</p> <pre><code>[(NSFileHandle *)[NSFileHandle fileHandleWithStandardOutput] writeData:formattedData]; </code></pre> <p>or:</p> <pre><code>NSFileHandle *fileHandle = [NSFileHandle fileHandleWithStandardOutput]; [fileHandle writeData:formattedData]; </code></pre> <p>Of course, the better solution is to declare <code>fileHandleWithStandardOutput</code> as returning an <code>instancetype</code>. Then the cast or assignment isn't necessary.</p> <p>(Note that on iOS, this example won't produce an error as only <code>NSFileHandle</code> provides a <code>writeData:</code> there. Other examples exist, such as <code>length</code>, which returns a <code>CGFloat</code> from <code>UILayoutSupport</code> but a <code>NSUInteger</code> from <code>NSString</code>.)</p> <p><strong><em>Note</strong>: Since I wrote this, the macOS headers have been modified to return a <code>NSFileHandle</code> instead of an <code>id</code>.</em></p> <p>For initializers, it's more complicated. When you type this:</p> <pre><code>- (id)initWithBar:(NSInteger)bar </code></pre> <p>…the compiler will pretend you typed this instead:</p> <pre><code>- (instancetype)initWithBar:(NSInteger)bar </code></pre> <p>This was necessary for ARC. This is described in Clang Language Extensions <a href="http://clang.llvm.org/docs/LanguageExtensions.html#related-result-types" rel="nofollow noreferrer">Related result types</a>. This is why people will tell you it isn't necessary to use <code>instancetype</code>, though I contend you should. The rest of this answer deals with this.</p> <p>There's three advantages:</p> <ol> <li><strong>Explicit.</strong> Your code is doing what it says, rather than something else.</li> <li><strong>Pattern.</strong> You're building good habits for times it does matter, which do exist.</li> <li><strong>Consistency.</strong> You've established some consistency to your code, which makes it more readable.</li> </ol> <h2>Explicit</h2> <p>It's true that there's no <em>technical</em> benefit to returning <code>instancetype</code> from an <code>init</code>. But this is because the compiler automatically converts the <code>id</code> to <code>instancetype</code>. You are relying on this quirk; while you're writing that the <code>init</code> returns an <code>id</code>, the compiler is interpreting it as if it returns an <code>instancetype</code>.</p> <p>These are <em>equivalent</em> to the compiler:</p> <pre><code>- (id)initWithBar:(NSInteger)bar; - (instancetype)initWithBar:(NSInteger)bar; </code></pre> <p>These are not equivalent to your eyes. At best, you will learn to ignore the difference and skim over it. <strong>This is not something you should learn to ignore.</strong></p> <h2>Pattern</h2> <p>While there's no difference with <code>init</code> and other methods, there <strong>is</strong> a difference as soon as you define a class factory.</p> <p>These two are not equivalent:</p> <pre><code>+ (id)fooWithBar:(NSInteger)bar; + (instancetype)fooWithBar:(NSInteger)bar; </code></pre> <p>You want the second form. If you are used to typing <code>instancetype</code> as the return type of a constructor, you'll get it right every time.</p> <h2>Consistency</h2> <p>Finally, imagine if you put it all together: you want an <code>init</code> function and also a class factory.</p> <p>If you use <code>id</code> for <code>init</code>, you end up with code like this:</p> <pre><code>- (id)initWithBar:(NSInteger)bar; + (instancetype)fooWithBar:(NSInteger)bar; </code></pre> <p>But if you use <code>instancetype</code>, you get this:</p> <pre><code>- (instancetype)initWithBar:(NSInteger)bar; + (instancetype)fooWithBar:(NSInteger)bar; </code></pre> <p>It's more consistent and more readable. They return the same thing, and now that's obvious.</p> <h2>Conclusion</h2> <p>Unless you're intentionally writing code for old compilers, you should use <code>instancetype</code> when appropriate.</p> <p>You should hesitate before writing a message that returns <code>id</code>. Ask yourself: Is this returning an instance of this class? If so, it's an <code>instancetype</code>.</p> <p>There are certainly cases where you need to return <code>id</code>, but you'll probably use <code>instancetype</code> much more frequently.</p>

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