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

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There is no Swift preprocessor   For one thing  arbitrary code substitution breaks type- and memory-safety    Swift does include build-time configuration options  though  so you can conditionally include code for certain platforms or build styles or in response to flags you define with -D compiler args  Unlike with C  though  a conditionally compiled section of your code must be syntactically complete  There s a section about this in Using Swift With Cocoa and Objective-C   For example    if os iOS      let color   UIColor redColor    else     let color   NSColor redColor    endif

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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

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As stated in Apple Docs     The Swift compiler does not include a preprocessor  Instead  it takes advantage of compile-time attributes  build configurations  and language features to accomplish the same functionality  For this reason  preprocessor directives are not imported in Swift    I ve managed to achieve what I wanted by using custom Build Configurations    Go to your project   select your target   Build Settings   search for Custom Flags For your chosen target set your custom flag using -D prefix  without white spaces   for both Debug and Release Do above steps for every target you have     Here s how you check for target    if BANANA     print  We have a banana    elseif MELONA     print  Melona    else     print  Kiwi    endif     Tested using Swift 2 2

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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   - gt  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

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func inDebugBuilds   code     - gt  Void        assert   code    return true          Source

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In Xcode 8  amp  above go to build setting -  search for custom flags  1  In code    if Live     print  Live        else     print  debug        endif

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isDebug Constant Based on Active Compilation Conditions  Another  perhaps simpler  solution that still results in a boolean that you can pass into functions without peppering  if conditionals throughout your codebase is to define DEBUG as one of your project build target s Active Compilation Conditions and include the following  I define it as a global constant     if DEBUG     let isDebug   true  else     let isDebug   false  endif     isDebug Constant Based on Compiler Optimization Settings  This concept builds on kennytm s answer  The main advantage when comparing against kennytm s  is that this does not rely on private or undocumented methods   In Swift 4   let isDebug  Bool         var isDebug   false        function with a side effect and Bool return value that we can pass into assert       func set debug  Bool  - gt  Bool           isDebug   debug         return isDebug              assert          Condition is only evaluated in playgrounds and -Onone builds          so isDebug is never changed to true in Release builds     assert set debug  true       return isDebug       Compared with preprocessor macros and kennytm s answer      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   Documented  which means the function will follow normal API release deprecation patterns    Using in if else will not generate a  Will never be executed  warning

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My two cents for Xcode 8   a  A custom flag using the -D prefix works fine  but     b  Simpler use   In Xcode 8 there is a new section   Active Compilation Conditions    already with two rows  for debug and release   Simply add your define WITHOUT -D

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There are some processors that take an argument and I listed them below  you can change the argument as you like   if os macOS     Checks the target operating system      if canImport UIKit     Check if a module presents      if swift  lt 5     Check the Swift version      if targetEnvironment simulator     Check envrionments like Simulator or Catalyst      if compiler  lt 7     Check compiler version     Also  You can use any custom flags like DEBUG or any other flags you defined  if DEBUG print  quot Debug mode quot    endif

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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

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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

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Moignans answer here works fine  Here is another peace of info in case it helps    if DEBUG     let a   2  else     let a   3  endif   You can negate the macros like below    if  RELEASE     let a   2  else     let a   3  endif

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This builds on Jon Willis s answer that relies upon assert  which only gets executed in Debug compilations   func Log   str  String         assert DebugLog str      func DebugLog   str  String  - gt  Bool        print str       return true     My use case is for logging print statements  Here is a benchmark for Release version on iPhone X   let iterations   100 000 000 let time1   CFAbsoluteTimeGetCurrent   for i in 0    lt  iterations       Log     unarchiveArray   fileName  memoryTime   memoryTime  count   array count      var time2   CFAbsoluteTimeGetCurrent   print   Log    time2-time1       prints    Log  0 0   Looks like Swift 4 completely eliminates the function call

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In many situations  you don t really need conditional compilation  you just need conditional behavior that you can switch on and off  For that  you can use an environment variable  This has the huge advantage that you don t actually have to recompile   You can set the environment variable  and easily switch it on or off  in the scheme editor     You can retrieve the environment variable with NSProcessInfo       let dic   NSProcessInfo processInfo   environment     if dic  TRIPLE      nil                  do secret stuff here             Here s a real-life example  My app runs only on the device  because it uses the music library  which doesn t exist on the Simulator  How  then  to take screen shots on the Simulator for devices I don t own  Without those screen shots  I can t submit to the AppStore   I need fake data and a different way of processing it  I have two environment variables  one which  when switched on  tells the app to generate the fake data from the real data while running on my device  the other which  when switched on  uses the fake data  not the missing music library  while running on the Simulator  Switching each of those special modes on   off is easy thanks to environment variable checkboxes in the Scheme editor  And the bonus is that I can t accidentally use them in my App Store build  because archiving has no environment variables

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XCODE 9 AND ABOVE   if DEVELOP         elseif PRODCTN         else         endif

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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

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After setting DEBUG 1 in your GCC PREPROCESSOR DEFINITIONS Build Settings I prefer using a function to make this calls   func executeInProduction   block     - gt  Void         if  DEBUG         block        endif     And then just enclose in this function any block that I want omitted in Debug builds   executeInProduction       Fabric with  Crashlytics self      Compiler checks this line even in Debug     The advantage when compared to    if  DEBUG     Fabric with  Crashlytics self      This is not checked  may not compile in non-Debug builds  endif   Is that the compiler checks the syntax of my code  so I am sure that its syntax is correct and builds

[swift] #ifdef replacement in the Swift language

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