Question

How do I overload the << operator? From the error I am getting, it seems that std::cout doesn’t know how to use <<.

This is in a class:

// uint64_t UPPER, LOWER;
std::ostream & operator<<(std::ostream & stream){
    if (UPPER)
        stream << UPPER;
    stream << LOWER;
    return stream;
}

I am getting error: no match for 'operator<<' in 'std::cout << test' which doesn’t seem to make sense.

edit:

Neither this:

std::ostream & operator<<(std::ostream & stream, uint128_t const & val){
    if (val.upper())
        stream << val.upper();
    stream << val.lower();
    return stream;
}

nor this:

std::ostream & operator<<(std::ostream & stream, uint128_t val){
    if (val.upper())
        stream << val.upper();
    stream << val.lower();
    return stream;
}

is changing the error.

Answer 1

The << operator takes two arguments, a left hand side, and a right hand side. Therefore you have to define the function with two parameters:

std::ostream operator<<(std::ostream &os, const MyClass &obj);

And you have to define it outside of your class definition, as far as I can remember. Inside of the class definition you can only have operators that take that class as the left hand side.