Mac Forum / Programming / Mac Programming / June 2006

Consider adding a userData parameter to your fast C++ functions. Define
a struct that has a pointer to your Objective C object instance  
members. Add an initBridge Objective-C method that initializes the
struct with pointers to objective-C members. Now, your fast function
gets a pointer to the initialized struct in its userData, and can
indirect through it to get at the Obj-C data.

silly example:

// bridge struct, a C struct, holds pointers into an Obj-C instance
typedef struct BridgeStruct {
  float *mAry1;
  float *mAry2;
  float *mResult;
}BridgeStruct;

typedef void (*Operation)(BridgeStruct* bridge);

// our sample Obj-C object.
@interface Example : NSObject {
  float mAry1[1000];
  float mAry2[1000];
  float mResult[1000];
}
- (void) initBridge: (BridgeStruct*) bridge;
- (void) doFastOperation: (Operation) operation;
@end

// ordinary C function, takes a pointer to a bridge struct
void Multiply(BridgeStruct* bridge){
  int i;
  for(i = 0; i < 1000; ++i){
     bridge->mResult[i] = bridge->mAry1[i] * bridge->mAry2[i];
  }
}

@implementation Example

// initialize the bridgestruct
- (void) initBridge: (BridgeStruct*) bridge {
  bridge->mAry1 = mAry1;
  bridge->mAry2 = mAry2;
  bridge->mResult = mResult;
}

// effectively apply the operation to self.
- (void) doFastOperation: (Operation) operation {
  BridgeStruct bridge;
  [self initBridge: &bridge];
  (*operation)(&bridge);
}
@end

// example of usage.
int main(int argc, char *argv[]){
  Example *example = [[Example alloc] init];
  [example doFastOperation: Multiply];
   return 0;
}