https://stackoverflow.com/questions/3552195/how-to-convert-stdstring-to-nsstring
1. std::string 或 const char * 转 NSString
std::string stdString = "";
NSString *ocString = [NSString stringWithCString:stdString.c_str() encoding:[NSString defaultCStringEncoding]];
const char *cString = "";
NSString *ocString = [NSString stringWithCString:cString encoding:[NSString defaultCStringEncoding]];
2. NSString 转 std::string 或 const char *
NSString *ocString = @"";
std::string stdString = [ocString UTF8String];
const char *cString = [ocString UTF8String];
