operator << must take exactly one argument

The problem is that you define it inside the class, which

a) means the second argument is implicit (this) and

b) it will not do what you want it do, namely extend std::ostream.

You have to define it as a free function:

class A { /* ... */ };
std::ostream& operator<<(std::ostream&, const A& a);

A friend function is not a member function, so the problem is that you declare operator<< as a friend of A:

 friend ostream& operator<<(ostream&, A&);

then try to define it as a member function of the class logic

 ostream& logic::operator<<(ostream& os, A& a)
          ^^^^^^^

Are you confused about whether logic is a class or a namespace?

The error is because you've tried to define a member operator<< taking two arguments, which means it takes three arguments including the implicit this parameter. The operator can only take two arguments, so that when you write a << b the two arguments are a and b.

You want to define ostream& operator<<(ostream&, const A&) as a non-member function, definitely not as a member of logic since it has nothing to do with that class!

std::ostream& operator<<(std::ostream& os, const A& a)
{
  return os << a.number;
}

I ran into this problem with templated classes. Here's a more general solution I had to use:

template class <T>
class myClass
{
    int myField;

    // Helper function accessing my fields
    void toString(std::ostream&) const;

    // Friend means operator<< can use private variables
    // It needs to be declared as a template, but T is taken
    template <class U>
    friend std::ostream& operator<<(std::ostream&, const myClass<U> &);
}

// Operator is a non-member and global, so it's not myClass<U>::operator<<()
// Because of how C++ implements templates the function must be
// fully declared in the header for the linker to resolve it :(
template <class U>
std::ostream& operator<<(std::ostream& os, const myClass<U> & obj)
{
  obj.toString(os);
  return os;
}

Now: * My toString() function can't be inline if it is going to be tucked away in cpp. * You're stuck with some code in the header, I couldn't get rid of it. * The operator will call the toString() method, it's not inlined.

The body of operator<< can be declared in the friend clause or outside the class. Both options are ugly. :(

Maybe I'm misunderstanding or missing something, but just forward-declaring the operator template doesn't link in gcc.

This works too:

template class <T>
class myClass
{
    int myField;

    // Helper function accessing my fields
    void toString(std::ostream&) const;

    // For some reason this requires using T, and not U as above
    friend std::ostream& operator<<(std::ostream&, const myClass<T> &)
    {
        obj.toString(os);
        return os;
    }
}

I think you can also avoid the templating issues forcing declarations in headers, if you use a parent class that is not templated to implement operator<<, and use a virtual toString() method.