objective c - Invalid Summary in NSString assignment -

i trying implement simple calculator have update current operation (+, -, * etc) in variable of type nsstring. current operation received model parameter.

 -(double)dooperation:(nsstring *)operation withoperand:(double)operand  {      if (!currentoperation)      {          anoperand = operand;      }          currentoperation = operation; //invalid summary in currentoperation; + in operation  } 

what wrong in this? if direct assignment of pointers not allowed nsstring, alternate method?

edit

1. to more precise, legal in ios assign nsstring = sign?
2. if not, way go.

note: currentoperation private variable in controller class , operation parameter method.

here complete code

@interface calculatorbrain : nsobject {     double anoperand;     nsstring * currentoperaion; }  - (double) dooperation: (nsstring *) operation             withoperand: (double) operand;  @end    @implementation calculatorbrain   - (double) dooperation: (nsstring *) operation             withoperand: (double) operand {     if (!currentoperaion)     {         anoperand = operand;     }     else if([currentoperaion isequal: @"+"])     {         anoperand += operand;     }     else if([currentoperaion isequal: @"-"])     {         anoperand -= operand;     }     else if([currentoperaion isequal: @"*"])     {         anoperand *= operand;     }     else if([currentoperaion isequal: @"/"])     {         anoperand /= operand;     }      currentoperaion = operation; //this statement doesn't work expected      if ([currentoperaion isequal: @"="])     {         currentoperaion = nil;     }      return anoperand; }  @end 

to more precise, legal in ios assign nsstring = sign?

yes.

what wrong in this?

you not taking ownership of string lead problems if want use instance later, see memory management guide:

you can take ownership of object using retain.
remember object may have more 1 owner. taking ownership of object way of saying need kept alive.

you -retain string, or -copy avoid mutation external code, e.g.:

// in -dooperation:: [currentoperation release]; currentoperation = [operation copy]; // copy if operation might mutable  // relinquish ownership: - (void)dealloc {     [currentoperation release];     [super dealloc]; } 

also declared properties.