osx - Use of undeclared identifier Objective-C Code -

i new objective-c coding ,please bear me ask if simple question

my header file:

#import <cocoa/cocoa.h>  @interface appdelegate : nsobject <nsapplicationdelegate> @property (assign) iboutlet nswindow *window;  @end 

my implementation file:

#import "appdelegate.h"  @implementation appdelegate  - (void)applicationdidfinishlaunching:(nsnotification *)anotification {         // insert code here initialize application      // listing 2-1  creating connection using nsurlconnection      // create request.     nsurlrequest *therequest=[nsurlrequest requestwithurl:[nsurl                                urlwithstring:@"http://www.apple.com/"]                    cachepolicy:nsurlrequestuseprotocolcachepolicy                                       timeoutinterval:60.0];      // create nsmutabledata hold received data.     // receiveddata instance variable declared elsewhere.     receiveddata = [nsmutabledata datawithcapacity: 0];      // create connection request     // , start loading data     nsurlconnection *theconnection=[[nsurlconnection alloc] initwithrequest:therequest                          delegate:self];      if (!theconnection) {         // release receiveddata object.         receiveddata = nil;          // inform user connection failed.     } }  @end 

at receiveddata in implementation file showing undeclared identifier. if declare variable in .h file saying cannot declare variable inside @interface , protocol

as can see in comment:

// receiveddata instance variable declared elsewhere. receiveddata = [nsmutabledata datawithcapacity: 0]; 

receiveddata should declared somewhere. try this:

nsmutabledata *receiveddata = [nsmutabledata datawithcapacity: 0];