#ifdef replacement in the Swift language

Question

In C/C++/Objective-C you can define a macro using compiler preprocessors. Moreover, you can include/exclude some parts of code using compiler preprocessors.

#ifdef DEBUG
    // Debug-only code
#endif

Is there a similar solution in Swift?

Answer 1

XCODE 9 AND ABOVE

#if DEVELOP
    //
#elseif PRODCTN
    //
#else
    //
#endif

Answer 2

Yes you can do it.

In Swift you can still use the "#if/#else/#endif" preprocessor macros (although more constrained), as per Apple docs. Here's an example:

#if DEBUG
    let a = 2
#else
    let a = 3
#endif

Now, you must set the "DEBUG" symbol elsewhere, though. Set it in the "Swift Compiler - Custom Flags" section, "Other Swift Flags" line. You add the DEBUG symbol with the -D DEBUG entry.

As usual, you can set a different value when in Debug or when in Release.

I tested it in real code and it works; it doesn't seem to be recognized in a playground though.

You can read my original post here.

---

IMPORTANT NOTE: -DDEBUG=1 doesn't work. Only -D DEBUG works. Seems compiler is ignoring a flag with a specific value.

Answer 3

In Swift projects created with Xcode Version 9.4.1, Swift 4.1

#if DEBUG
#endif

works by default because in the Preprocessor Macros DEBUG=1 has already been set by Xcode.

So you can use #if DEBUG "out of box".

By the way, how to use the condition compilation blocks in general is written in Apple's book The Swift Programming Language 4.1 (the section Compiler Control Statements) and how to write the compile flags and what is counterpart of the C macros in Swift is written in another Apple's book Using Swift with Cocoa and Objective C (in the section Preprocessor Directives)

Hope in future Apple will write the more detailed contents and the indexes for their books.

Answer 4

Xcode 8 and above

Use Active Compilation Conditions setting in Build settings / Swift compiler - Custom flags.

• This is the new build setting for passing conditional compilation flags to the Swift compiler.
• Simple add flags like this: ALPHA, BETA etc.

Then check it with compilation conditions like this:

#if ALPHA
    //
#elseif BETA
    //
#else
    //
#endif

Tip: You can also use #if !ALPHA etc.

Answer 5

As of Swift 4.1, if all you need is just check whether the code is built with debug or release configuration, you may use the built-in functions:

• _isDebugAssertConfiguration() (true when optimization is set to -Onone)
• _isReleaseAssertConfiguration() (true when optimization is set to -O) (not available on Swift 3+)
• _isFastAssertConfiguration() (true when optimization is set to -Ounchecked)

e.g.

func obtain() -> AbstractThing {
    if _isDebugAssertConfiguration() {
        return DecoratedThingWithDebugInformation(Thing())
    } else {
        return Thing()
    }
}

Compared with preprocessor macros,

• ✓ You don't need to define a custom -D DEBUG flag to use it
• ~ It is actually defined in terms of optimization settings, not Xcode build configuration

• ✗ Undocumented, which means the function can be removed in any update (but it should be AppStore-safe since the optimizer will turn these into constants)

  • these once removed, but brought back to public to lack of @testable attribute, fate uncertain on future Swift.

• ✗ Using in if/else will always generate a "Will never be executed" warning.

Answer 6

A major change of ifdef replacement came up with Xcode 8. i.e use of Active Compilation Conditions.

Refer to Building and Linking in Xcode 8 Release note.

New build settings

New setting: SWIFT_ACTIVE_COMPILATION_CONDITIONS

“Active Compilation Conditions” is a new build setting for passing conditional compilation flags to the Swift compiler.

Previously, we had to declare your conditional compilation flags under OTHER_SWIFT_FLAGS, remembering to prepend “-D” to the setting. For example, to conditionally compile with a MYFLAG value:

#if MYFLAG1
    // stuff 1
#elseif MYFLAG2
    // stuff 2
#else
    // stuff 3
#endif

The value to add to the setting -DMYFLAG

Now we only need to pass the value MYFLAG to the new setting. Time to move all those conditional compilation values!

Please refer to below link for more Swift Build Settings feature in Xcode 8: http://www.miqu.me/blog/2016/07/31/xcode-8-new-build-settings-and-analyzer-improvements/