how implement callback from C++ function to Swift

Question

I tried do something in a cpp class and when doing the function return callback to swift.

So i do these things :

1. Creating this function in callbackClass.cpp

   int callback::run(void (*callback)(int))
    {
        for (unsigned int i = 0; i < 100; i++)
        {
            callback(i);
        }
        return 0;
    }
   

2. In callbackClass.hpp :

   class callbackClass.hpp
   {
     .
     .
     .
     public:
        int run(void (*callback)(int));
   };
   
   #endif
   

3. And for header.h‍‍ :

   int callback(void (*callback)(int));
   

It's good until logging callback in Swift side:

func callbackFunc(){

  callback({valueFromCallback in //call cpp function

    print(valueFromCallback) //Works fine

  })
}

But when try to do other stuff like :

func callbackFunc(){

  var value : String!

  callback({valueFromCallback in //call cpp function

    value = String(valueFromCallback) //It has a problem

  })
}

Xcode return this error :

A C function pointer cannot be formed from a closure that captures context

I have already seen these questions but did not help:

Swift: Pass data to a closure that captures context

How to cast self to UnsafeMutablePointer<Void> type in swift

A C function pointer cannot be formed from a closure that captures context

Answer 1

Eventually I realized it possible to pass 'closure' to c++ as argument

So at first I created a Closure in Swift class:

typealias closureCallback = (Int32) -> ()

Then passed it to cpp side :

1. In header.h‍‍

   int callback(void (^closureCallback)(int));
   

2. In callbackClass.hpp :

   class callbackClass.hpp
   {
     .
     .
     .
   
     public:
         int run(void (^closureCallback)(int));
   };
   
   #endif
   

3. And in callbackClass.cpp

   int callback::run(void (^closureCallback)(int))
    {
        for (unsigned int i = 0; i < 100; i++)
        {
            closureCallback(i);
        }
        return 0;
    }
   

At last handle it in Swift :

typealias closureCallback = (Int32) -> ()

func callbackFunc(){

  var value : String!

  let closureValue: closureCallback = {valueFromclosureCallback in
      value = String(valueFromclosureCallback)
   }

    callback(closureValue) //call cpp function

}