使用未声明的标识符 Objective-C 代码

Question

我是 Objective-C 编码的新手，请耐心询问这是否是一个简单的问题

我的头文件：

#import <Cocoa/Cocoa.h>

@interface AppDelegate : NSObject <NSApplicationDelegate>
@property (assign) IBOutlet NSWindow *window;

@end

我的实现文件：

#import "AppDelegate.h"

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{    
    // Insert code here to initialize your application

    // Listing 2-1  Creating a connection using NSURLConnection

    // Create the request.
    NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL    
                           URLWithString:@"http://www.apple.com/"]
                   cachePolicy:NSURLRequestUseProtocolCachePolicy
                                      timeoutInterval:60.0];

    // Create the NSMutableData to hold the received data.
    // receivedData is an instance variable declared elsewhere.
    receivedData = [NSMutableData dataWithCapacity: 0];

    // create the connection with the request
    // and start loading the data
    NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest              
           delegate:self];

    if (!theConnection) {
        // Release the receivedData object.
        receivedData = nil;

        // Inform the user that the connection failed.
    }
}

@end

在实现文件中的receivedData 处显示未声明的标识符。如果在 .h 文件中声明该变量，则表示 cannot declare variable inside @interface and protocol

Answer 1

正如你在评论中看到的那样：

// receivedData is an instance variable declared elsewhere.
receivedData = [NSMutableData dataWithCapacity: 0];

receivedData 应该在某处声明。试试这个：

NSMutableData *receivedData = [NSMutableData dataWithCapacity: 0];

Answer 2

你必须在引用它之前声明变量： NSMutableData* receivedData = [NSMutableData dataWithCapacity: 0];