为什么会收到“函数'PerformXMLXPathQuery'的隐式声明”警告?

为什么会收到“函数'PerformXMLXPathQuery'的隐式声明”警告?

NSString *response = [request responseString];  
NSString* xPathQuery = [NSString stringWithFormat:@"//ExtendedData"];  
NSArray* myArray = PerformXMLXPathQuery([response dataUsingEncoding:NSUTF8StringEncoding ],xPathQuery);