std::string _string("hello");
NSString *str= [NSString stringWithCString:_string.c_str() encoding:[NSString defaultCStringEncoding]];

NSString  * nsfaceName=@"HELLO";
const char  *  lpfaceName = [nsfaceName UTF8String];
std::string   strFaceName=  [nsfaceName UTF8String];

//NSString转换char  
  NSString * str1= @"Test";  
  const char * c1 =[str1 UTF8String];  
  //char转换NSString  
  const char * c2 ="test";  
  NSString *str2 = [NSString stringWithUTF8String:c2];