Cannot find interface declaration for 'NSObject', superclass of 'DDFileReader'

Question

I am new to OSX development and am trying to build a file parser. I found a guide on how to parse line by line in another stackoverflow thread -How to read data from NSFileHandle line by line?. I am having some issue implementing it. I created a new DDFileReader.h file to define my FileReader object, however, I keep getting the error Cannot find interface declaration for 'NSObject', superclass of 'DDFileReader'.

What does this error mean? Do I have to load/require some external library?

DDFileReader.h

@interface DDFileReader : NSObject {
    NSString * filePath;

    NSFileHandle * fileHandle;
    unsigned long long currentOffset;
    unsigned long long totalFileLength;

    NSString * lineDelimiter;
    NSUInteger chunkSize;
}