ios - run-time vs. compile-time iPhone version check

Question

What's the difference between run-time, e.g., [[UIDevice currentDevice] systemVersion], and compile-time, e.g., __IPHONE_OS_VERSION_MIN_REQUIRED checking? When should you one over the other?

Is__IPHONE_OS_VERSION_MIN_REQUIRED just a variable set in Build Settings?

I've read the answers to How to target a specific iPhone version? and other related questions listed below.

But, I just noticed that __IPHONE_OS_VERSION_MIN_REQUIRED = 30200 when I build & run on iPhone (4.3.1) with Xcode 4. Why?

When you submit your code to Apple, do they compile a version of it for each version of the iPhone that exists, setting __IPHONE_OS_VERSION_MIN_REQUIRED accordingly?

That way, you could inspect __IPHONE_OS_VERSION_MIN_REQUIRED, and know what iOS version is running the code.

Related Questions:

• How to check iOS version?
• Check iOS version at runtime?
• How to check if iOS version supports library?

Answer 1

__IPHONE_OS_VERSION_MIN_REQUIRED is indeed just a build setting, which you can use in the preprocessor to modify code before compilation. No, Apple doesn't compile your code for each iPhone. They can't, since you don't give them the code. You can use compile time checking to determine, for example, if you need to compile code to emulate a new feature in older OS versions. However, you use runtime version checking to determine whether you should use the built in or emulated version of that feature.

For example, iAds were added in 4.0. You currently support 4.0 and later, but plan on adding support for 3.2 in the future. You make code to display other ads in older versions, using runtime checking to determine whether you should use iAds or other, but you don't want it to be in any releases until the rest of the application is ready for 3.2, since it will make your application larger. You use compile time checking so that the preprocessor leaves that code out of your releases. By using the minimum version macro, you can easily add the code to your release by changing the minimum version build setting.