在做OC與C++ 混編的時候，我們可能會用到struct 與NSData的相互轉換。在這裡做一個記錄

1、struct轉換為NSData

例如如下的struct:

struct tagPackageHead		
{
	tagPackageHead()
	{
		memset(this,0,sizeof(*this));
		bStartcode = 0xFE;
	}
	Byte	bStartcode;				
	uint	iCmd;					
	uint	iXMLLen;				
	uint	iSeqNumber;            
	Byte	bExpand[10];			
};

struct tagPackageHead tagPack;
    tagPack.bStartcode = 0xFE;
    tagPack.iCmd = CMD_CLIENT_LOGIN;
    tagPack.iXMLLen = data.length;
    tagPack.iSeqNumber = 1;
 

NSData *headerData = [NSDatadataWithBytes:&tagPacklength:sizeof(tagPack)];

2、NSData轉換為struct

struct tagPackageHead tagPack;
[data getBytes:&tagPack length:sizeof(tagPack)];
NSLog(@"tagPack.iXMLLen:%d",tagPack.iXMLLen);
NSLog(@"tagPack.iCmd:%d",tagPack.iCmd);
NSLog(@"tagPack.iSeqNumber:%d",tagPack.iSeqNumber);
 

//建立XML
GDataXMLElement *rootElement = [GDataXMLNode elementWithName:@"SignInReq"];
GDataXMLElement *element = [GDataXMLNode elementWithName:@"ClientType" stringValue:@"PC"];
[rootElement addChild:element];
element = [GDataXMLNode elementWithName:@"UserName" stringValue:@"abcdef"];
[rootElement addChild:element];
element = [GDataXMLNode elementWithName:@"Password" stringValue:@"123456"];
[rootElement addChild:element];
element = [GDataXMLNode elementWithName:@"UserAgent" stringValue:@"1.01"];
[rootElement addChild:element];
GDataXMLDocument *document = [[GDataXMLDocument alloc] initWithRootElement:rootElement];
NSData *data =  [document XMLData];
 

因為伺服器返回的資料中有包頭和XML檔案兩部分，所以要先獲取包頭，然後再獲取XML檔案

struct tagPackageHead tagPack;
[data getBytes:&tagPack length:sizeof(tagPack)];
        
NSStringEncoding enc = CFStringConvertEncodingToNSStringEncoding(kCFStringEncodingGB_18030_2000);
NSString *string = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(sizeof(tagPack), tagPack.iXMLLen)] encoding:enc];
NSData *data2 = [string dataUsingEncoding:NSUTF8StringEncoding];
        
NSArray *paths=NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *path=[paths objectAtIndex:0];
NSString *areaDevice_path=[path stringByAppendingPathComponent:@"areaDevice.xml"];
//==寫入檔案
[data2 writeToFile:areaDevice_path atomically:YES];
//開始解析
NSDictionary *dict = [NSDictionary dictionaryWithXMLData:data2];