Calling Objective-C method from C member function

Question

I have a class  EAGLView  which calls a member function of a C   class without problems  Now  the problem is that I need to call in that C   class a objective-C function  context renderbufferStorage GL RENDERBUFFER fromDrawable  CAEAGLLayer  self layer   which I cannot do in C   syntax   I could wrap this Objective-C call to the same Objective-C class which in the first place called the C   class  but then I need to somehow call that method from C    and I cannot figure out how to do it   I tried to give a pointer to EAGLView object to the C   member function and include the  EAGLView h  in my C   class header but I got 3999 errors     So   how should I do this  An example would be nice   I only found pure C examples of doing this

User · Answer

The easiest solution is to simply tell Xcode to compile everything as Objective C++.

Set your project or target settings for Compile Sources As to Objective C++ and recompile.

Then you can use C++ or Objective C everywhere, for example:

void CPPObject::Function( ObjectiveCObject* context, NSView* view )
{
   [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)view.layer]
}

This has the same affect as renaming all your source files from .cpp or .m to .mm.

There are two minor downsides to this: clang cannot analyse C++ source code; some relatively weird C code does not compile under C++.

User · Answer

You can compile your code as Objective-C   - the simplest way is to rename your  cpp as  mm  It will then compile properly if you include EAGLView h  you were getting so many errors because the C   compiler didn t understand any of the Objective-C specific keywords   and you can  for the most part  mix Objective-C and C   however you like

User · Answer

You can mix C   with Objective-C if you do it carefully  There are a few caveats but generally speaking they can be mixed  If you want to keep them separate  you can set up a standard C wrapper function that gives the Objective-C object a usable C-style interface from non-Objective-C code  pick better names for your files  I have picked these names for verbosity    MyObject-C-Interface h   ifndef   MYOBJECT C INTERFACE H    define   MYOBJECT C INTERFACE H       This is the C  trampoline  function that will be used    to invoke a specific Objective-C method FROM C   int MyObjectDoSomethingWith  void  myObjectInstance  void  parameter    endif   MyObject h   import  MyObject-C-Interface h      An Objective-C class that needs to be accessed from C    interface MyObject   NSObject       int someVar        The Objective-C member function you want to call from C   -  int  doSomethingWith  void    aParameter   end   MyObject mm   import  MyObject h    implementation MyObject     C  trampoline  function to invoke Objective-C method int MyObjectDoSomethingWith  void  self  void  aParameter           Call the Objective-C method using Objective-C syntax     return   id  self doSomethingWith aParameter      -  int  doSomethingWith  void    aParameter          The Objective-C function you wanted to call from C           do work here       return 21      half of 42    end   MyCPPClass cpp   include  MyCPPClass h   include  MyObject-C-Interface h   int MyCPPClass  someMethod  void  objectiveCObject  void  aParameter           To invoke an Objective-C method from C    use        the C trampoline function     return MyObjectDoSomethingWith  objectiveCObject  aParameter       The wrapper function does not need to be in the same  m file as the Objective-C class  but the file that it does exist in needs to be compiled as Objective-C code  The header that declares the wrapper function needs to be included in both CPP and Objective-C code    NOTE  if the Objective-C implementation file is given the extension   m  it will not link under Xcode   The   mm  extension tells Xcode to expect a combination of Objective-C and C    i e   Objective-C        You can implement the above in an Object-Orientented manner by using the PIMPL idiom   The implementation is only slightly different   In short  you place the wrapper functions  declared in  MyObject-C-Interface h   inside a class with a  private  void pointer to an instance of MyClass   MyObject-C-Interface h  PIMPL    ifndef   MYOBJECT C INTERFACE H    define   MYOBJECT C INTERFACE H    class MyClassImpl   public      MyClassImpl   void         MyClassImpl  void         void init  void        int  doSomethingWith  void   aParameter        void logMyMessage  char   aCStr     private      void   self       endif   Notice the wrapper methods no longer require the void pointer to an instance of MyClass  it is now a private member of MyClassImpl   The init method is used to instantiate a MyClass instance   MyObject h  PIMPL    import  MyObject-C-Interface h    interface MyObject   NSObject       int someVar     -  int   doSomethingWith  void    aParameter  -  void  logMyMessage  char    aCStr    end   MyObject mm  PIMPL    import  MyObject h    implementation MyObject  MyClassImpl  MyClassImpl  void         self  NULL          MyClassImpl   MyClassImpl  void           id self dealloc      void MyClassImpl  init  void             self     MyObject alloc  init      int MyClassImpl  doSomethingWith  void  aParameter         return   id self doSomethingWith aParameter      void MyClassImpl  logMyMessage  char  aCStr           id self doLogMessage aCStr      -  int  doSomethingWith  void    aParameter       int result              some code to calculate the result      return result     -  void  logMyMessage  char    aCStr       NSLog  aCStr        end   Notice that MyClass is instantiated with a call to MyClassImpl  init   You could instantiate MyClass in MyClassImpl s constructor  but that generally isn t a good idea   The MyClass instance is destructed from MyClassImpl s destructor   As with the C-style implementation  the wrapper methods simply defer to the respective methods of MyClass   MyCPPClass h  PIMPL    ifndef   MYCPP CLASS H    define   MYCPP CLASS H    class MyClassImpl   class MyCPPClass       enum   cANSWER TO LIFE THE UNIVERSE AND EVERYTHING   42    public      MyCPPClass   void         MyCPPClass  void         void init  void        void doSomethingWithMyClass  void     private      MyClassImpl    impl      int            myValue       endif   MyCPPClass cpp  PIMPL    include  MyCPPClass h   include  MyObject-C-Interface h   MyCPPClass  MyCPPClass  void          impl   NULL          void MyCPPClass  init  void          impl   new MyClassImpl       MyCPPClass   MyCPPClass  void         if    impl     delete  impl   impl   NULL       void MyCPPClass  doSomethingWithMyClass  void         int result    impl- gt doSomethingWith   myValue        if   result    cANSWER TO LIFE THE UNIVERSE AND EVERYTHING                  impl- gt logMyMessage   Hello  Arthur                else                impl- gt logMyMessage   Don t worry                You now access calls to MyClass through a private implementation of MyClassImpl  This approach can be advantageous if you were developing a portable application  you could simply swap out the implementation of MyClass with one specific to the other platform     but honestly  whether this is a better implementation is more a matter of taste and needs

User · Answer

Also  you can call into Objective-C runtime to call the method

User · Answer

Step 1  Create a objective c file  m file  and it s corresponding header file      Header file  We call it  ObjCFunc h     ifndef test2 ObjCFunc h  define test2 ObjCFunc h  interface myClass  NSObject - void hello  int num1   end  endif      Corresponding Objective C file We call it  ObjCFunc m     import  lt Foundation Foundation h gt   include  ObjCFunc h   implementation myClass   Your objective c code here     - void hello  int num1   NSLog   Hello             end   Step 2  Now we will implement a c   function to call the objective c function that we just       created    So for that we will define a  mm file and its corresponding header file   mm  file is to  be used here because we will be able to use both Objective C and C   coding in the file     Header file We call it  ObjCCall h     ifndef   test2  ObjCCall    define   test2  ObjCCall    include  lt stdio h gt  class ObjCCall   public  static void objectiveC Call      We define a static method to call the function directly using the class name     endif    defined   test2  ObjCCall           Corresponding Objective C   file We call it  ObjCCall mm     include  ObjCCall h   include  ObjCFunc h  void ObjCCall  objectiveC Call       Objective C code calling      myClass  obj   myClass alloc init     Allocating the new object for the objective C   class we created  obj hello  100        Calling the function we defined     Step 3  Calling the c   function which actually calls the objective c method    ifndef   HELLOWORLD SCENE H    define   HELLOWORLD SCENE H    include  cocos2d h   include  ObjCCall h  class HelloWorld   public cocos2d  Layer   public     there s no  id  in cpp  so we recommend returning the class instance pointer static cocos2d  Scene  createScene       Here s a difference  Method  init  in cocos2d-x returns bool  instead of returning   id  in cocos2d-iphone virtual bool init       a selector callback void menuCloseCallback cocos2d  Ref  pSender   void ObCCall       definition    implement the  static create    method manually CREATE FUNC HelloWorld       endif      HELLOWORLD SCENE H       Final call   include  HelloWorldScene h   include  ObjCCall h  USING NS CC  Scene  HelloWorld  createScene         scene  is an autorelease object auto scene   Scene  create        layer  is an autorelease object auto layer   HelloWorld  create       add layer as a child to scene scene- gt addChild layer      return the scene return scene       on  init  you need to initialize your instance bool HelloWorld  init                                       1  super init first if    Layer  init           return false    Size visibleSize   Director  getInstance  - gt getVisibleSize    Vec2 origin   Director  getInstance  - gt getVisibleOrigin                                      2  add a menu item with  X  image  which is clicked to quit the program       you may modify it      add a  close  icon to exit the progress  it s an autorelease object auto closeItem   MenuItemImage  create                                          CloseNormal png                                           CloseSelected png                                          CC CALLBACK 1 HelloWorld  menuCloseCallback   this     closeItem- gt setPosition Vec2 origin x   visibleSize width - closeItem- gt getContentSize   width 2                               origin y   closeItem- gt getContentSize   height 2        create menu  it s an autorelease object auto menu   Menu  create closeItem  NULL   menu- gt setPosition Vec2  ZERO   this- gt addChild menu  1                                     3  add your codes below        add a label shows  Hello World     create and initialize a label  auto label   Label  createWithTTF  Hello World    fonts Marker Felt ttf   24       position the label on the center of the screen label- gt setPosition Vec2 origin x   visibleSize width 2                          origin y   visibleSize height - label-  gt getContentSize   height       add the label as a child to this layer this- gt addChild label  1      add  HelloWorld  splash screen  auto sprite   Sprite  create  HelloWorld png       position the sprite on the center of the screen sprite- gt setPosition Vec2 visibleSize width 2   origin x  visibleSize height 2       origin y       add the sprite as a child to this layer this- gt addChild sprite  0   this- gt ObCCall        first call return true    void HelloWorld  ObCCall      Definition   ObjCCall  objectiveC Call       Final Call     void HelloWorld  menuCloseCallback Ref  pSender     if  CC TARGET PLATFORM    CC PLATFORM WP8      CC TARGET PLATFORM      CC PLATFORM WINRT  MessageBox  You pressed the close button  Windows Store Apps do not implement a close    button    Alert    return   endif Director  getInstance  - gt end     if  CC TARGET PLATFORM    CC PLATFORM IOS  exit 0    endif     Hope this works

User · Answer

DawidDrozd s answer above is excellent    I would add one point  Recent versions of the Clang compiler complain about requiring a  bridging cast  if attempting to use his code   This seems reasonable  using a trampoline creates a potential bug  since Objective-C classes are reference counted  if we pass their address around as a void    we risk having a hanging pointer if the class is garbage collected up while the callback is still active   Solution 1  Cocoa provides CFBridgingRetain and CFBridgingRelease macro functions which presumably add and subtract one from the reference count of the Objective-C object  We should therefore be careful with multiple callbacks  to release the same number of times as we retain      C   Module  include  lt functional gt   void cppFnRequiringCallback std  function lt void void  gt  callback            callback         Objective-C Module  import  CppFnRequiringCallback h    interface MyObj   NSObject -  void  callCppFunction  -  void  myCallbackFn   end  void cppTrampoline const void  caller            id callerObjC   CFBridgingRelease caller            callerObjC myCallbackFn       implementation MyObj -  void  callCppFunction           auto callback    self                      const void  caller   CFBridgingRetain self                   cppTrampoline caller                      cppFnRequiringCallback callback      -  void  myCallbackFn       NSLog   Received callback        end   Solution 2  The alternative is to use the equivalent of a weak reference  ie  no change to the retain count   without any additional safety   The Objective-C language provides the   bridge cast qualifier to do this  CFBridgingRetain and CFBridgingRelease seem to be thin Cocoa wrappers over the Objective-C language constructs   bridge retained and release respectively  but Cocoa does not appear to have an equivalent for   bridge    The required changes are   void cppTrampoline void  caller            id callerObjC      bridge id caller           callerObjC myCallbackFn      -  void  callCppFunction           auto callback    self                      void  caller      bridge void   self                  cppTrampoline caller                      cppFunctionRequiringCallback callback

User · Answer

Sometimes renaming  cpp to  mm is not good idea  especially when project is crossplatform  In this case for xcode project I open xcode project file throught TextEdit  found string which contents interest file  it should be like      OnlineManager cpp       isa   PBXFileReference  fileEncoding   4  lastKnownFileType   sourcecode cpp cpp  path   OnlineManager cpp  sourceTree     lt group gt         and then change file type from sourcecode cpp cpp to sourcecode cpp objcpp     OnlineManager cpp       isa   PBXFileReference  fileEncoding   4  lastKnownFileType     sourcecode cpp objcpp    path   OnlineManager cpp  sourceTree     lt group gt         It is equivalent to rename  cpp to  mm

User · Answer

You can mix C   in with Objectiv-C  Objective C     Write a C   method in your Objective C   class that simply calls  context renderbufferStorage GL RENDERBUFFER fromDrawable  CAEAGLLayer  self layer   and call it from your C     I haven t tried it before my self  but give it a shot  and share the results with us

User · Answer

You need to make your C   file be treated as Objective-C     You can do this in xcode by renaming foo cpp to foo mm   mm is the obj-c   extension    Then as others have said standard obj-c messaging syntax will work

[c++] Calling Objective-C method from C++ member function?

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