在实现接口时，教程和文献中的常用方法似乎是声明一个 ivar，然后设置@propertythen @synthesize。
@interface MyClass : NSObject {
NSString *myString;
}
@property (nonatomic, retain) NSString *myString;
@end
但是，省略显式声明并仅放置 @property 具有相同的效果。
@interface MyClass: NSObject {
}
@property (nonatomic, retain) NSString *myString;
@end
那么为什么大多数人都使用@property和显式声明呢？不这样做是不好的形式吗？
曾经是必须的。Objective-C 运行时有两个不同的版本：一个仅 32 位的“遗留”运行时（旧版本）和一个 32/64 位运行时（新的 32 位运行时仅用于 iOS 设备和iOS 模拟器）。
我认为唯一仍然需要这样做的地方是当您以 32 位模式（10.5 或 10.6）运行应用程序时。其他任何地方（64 位 Leopard、64 位 Snow Leopard、Lion、iOS）都使用具有“自动 ivar 合成”的更新运行时，并且生成的 ivars 被称为“合成 ivars”。
有些平台支持合成实例变量，有些不支持。显式声明实例变量使您的代码在更多地方有效，并且直到最近它一直是完全必要的，所以人们仍然这样做。几年后，他们可能不会了。
Using modern versions of Xcode (anything about 4.2 or later) there is no reason to declare a iVar in your header, EVER. Anything publicly accessible should be declared as a property.
Many forget that Objective C objects are actually pointers to C structs. As such, any iVar that is declared in your header can be accessed directly, by passing your getters and setters, using myObject->myPublicIVar. Especially in non-ARC code, that is extremely dangerous. The @private directive prevents the use the -> operator to access iVars, but still clutters the header file. There's no point in @private when there are better ways.
Anything private should be declared within your .m file. Often, this requires a class extension, like this:
// The .h file
@interface Foo : NSObject
@property (nonatomic, strong) NSString *myPublicString;
@end
// The .m file
@interface Foo ()
@property (nonatomic, strong) NSString *myPrivateString;
@end
@implementation Foo {
NSString *myPrivateIVar;
}
// Xcode 4.5 or later will not require these @synthesize
@synthesize myPublicString = _myPublicString;
@synthesize myPrivateString = _myPrivateString;
@end
An implementation like this provides a public property backed by an iVar, a private property backed by an iVar, and a private independent iVar. I've included the @synthesize directives, but they are not necessary using modern tools.
@property 仅实现访问器方法，实例变量本身必须存在。尝试忽略 ivars，它会在运行时失败，如果不是在编译时。