Under what conditions is @synthesize automatic in

问题:

Under what conditions is @synthesize automatic in Objective-c?

Perhaps when using LLVM 3.0 and up? From reading around the net it seems like @synthesize is unnecessary starting with Xcode 4. However I'm using Xcode 4 and receiving warnings when I don't @synthesize a property.

Some of the responses to Why don't properties get automatically synthesized seem to imply @synthesize can be omitted at some point under some circumstances.

Another (old) reference hinting that @synthesize might be automatic at some point in the future.

回答1:

As of clang 3.2 (circa February 2012), "default synthesis" (or "auto property synthesis") of Objective-C properties is provided by default. It's essentially as described in the blog post you originally read: http://www.mcubedsw.com/blog/index.php/site/comments/new_objective-c_features/ (except that that post describes the feature as "enabled, then disabled"; I don't know if that's an issue with Xcode or if the clang developers themselves have gone back and forth on the question).

As far as I know, the only case in which properties will not be default-synthesized in clang 3.2 is when those properties have been inherited from a protocol. Here's an example:

#import <Foundation/Foundation.h>

@protocol P
@property int finicky;
@end

@interface A : NSObject <P>
@property int easygoing;
@end

@implementation A
@end

int main() { A *a = [A new]; a.easygoing = 0; a.finicky = 1; }

If you compile this example, you'll get a warning:

test.m:11:17: warning: auto property synthesis will not synthesize property
      declared in a protocol [-Wobjc-protocol-property-synthesis]
@implementation A
                ^
test.m:4:15: note: property declared here
@property int finicky;
              ^
1 warning generated.

and if you run it, you'll get an error from the runtime:

objc[45820]: A: Does not recognize selector forward:: (while forwarding setFinicky:)
Illegal instruction: 4

回答2:

From the New Features in Xcode 4.4 document:

Objective-C @properties are synthesized by default when not explicitly implemented.

So @synthesize is automatic by default starting from Xcode 4.4 with the LLVM 4.0 Compiler.

回答3:

As of Xcode 4.4, if you don't write @synthesize or @dynamic for a property. the compiler acts as though you had written @synthesize property = _property.

Prior to Xcode 4.4, you must do one of the following things for each property or else the compiler will issue a warning and you will get a runtime error. In Xcode 4.4 or later, you may do any of the following things instead of letting the compiler automatically synthesize the property accessors and instance variable.

1. Use the @synthesize directive.
2. Use the @dynamic directive and somehow provide the property getter and (if necessary) setter at runtime.
3. Explicitly write the property getter method and, if the property is readwrite, the property setter method.

Note that you can use the @synthesize directive (or the @dynamic directive) and also explicitly provide the getter and/or setter methods. But @synthesize provides them if you omit them.

回答4:

Also, synthesize will not be automatic if you have implemented the setter AND getter manually. So if you wonder why you can't access _someVariable, having declared @property (...) SomeType someVariable, then it is because you have implemented the setSomeVariable: and someVariable methods.

回答5:

You can turn off the synthesize warnings by clicking on the project name in the Project Navigator on the left then click All Cobined in Build Settings and then search for synthesize. That should be set to No.