std::endl is of unknown type when overloading operator<<

Question

I overloaded operator <<

template <Typename T>
UIStream& operator<<(const T);

UIStream my_stream;
my_stream << 10 << " heads";

Works but:

my_stream << endl;

Gives compilation error:

error C2678: binary '<<' : no operator found which takes a left-hand operand of type 'UIStream' (or there is no acceptable conversion)

What is the work around for making my_stream << endl work?

Answer 1

See here for better ways of extending IOStreams. (A bit outdated, and tailored for VC 6, so you will have to take it with a grain of salt)

The point is that to make functors work (and endl, which both outputs "\n" and flushes is a functor) you need to implement the full ostream interface.

Answer 2

std::endl is a function, and they way std::cout works with it, is that it has an operator<< defined to take in a function pointer with the same signature as std::endl.

In there, it calls the function, and forwards the return value.

Here is a code example:

#include <iostream>

struct MyStream
{
    template <typename T>
    MyStream& operator<<(const T& x)
    {
     std::cout << x;

     return *this;
    }


    // function that takes a custom stream, and returns it
    typedef MyStream& (*MyStreamManipulator)(MyStream&);

    // take in a function with the custom signature
    MyStream& operator<<(MyStreamManipulator manip)
    {
     // call the function, and return it's value
     return manip(*this);
    }

    // define the custom endl for this stream.
    // note how it matches the `MyStreamManipulator`
    // function signature
    static MyStream& endl(MyStream& stream)
    {
     // print a new line
     std::cout << std::endl;

     // do other stuff with the stream
     // std::cout, for example, will flush the stream
     stream << "Called MyStream::endl!" << std::endl;

     return stream;
    }

    // this is the type of std::cout
    typedef std::basic_ostream<char, std::char_traits<char> > CoutType;

    // this is the function signature of std::endl
    typedef CoutType& (*StandardEndLine)(CoutType&);

    // define an operator<< to take in std::endl
    MyStream& operator<<(StandardEndLine manip)
    {
     // call the function, but we cannot return it's value
     manip(std::cout);

     return *this;
    }
};

int main(void)
{
    MyStream stream;

    stream << 10 << " faces.";
    stream << MyStream::endl;
    stream << std::endl;

    return 0;
}

Hopefully this gives you a better idea of how these things work.

Answer 3

The std streams are not designed to be subclassed as they have no virtual methods so I don't think you'll get too far with that. You can try aggregating a std::ostream to do the work though.

To make endl work you need to implement a version of operator<< that takes a pointer-to-function as that is how the manipulators such as endl are handled i.e.

UStream& operator<<( UStream&, UStream& (*f)( UStream& ) );

or

UStream& UStream::operator<<( UStream& (*f)( UStream& ) );

Now std::endl is a function that takes and returns a reference to a std::basic_ostream so that won't work directly with your stream so you'll need to make your own version which calls through to the std::endl version in your aggregated std::iostream.

Edit: Looks likes GMan's answer is better. He gets std::endl working too!

Answer 4

The problem is that std::endl is a function template, as your operator<< is. So when you write:

my_stream << endl;

you'll like the compiler to deduce the template parameters for the operator as well as for endl. This isn't possible.

So you have to write additional, non template, overloads of operator<< to work with manipulators. Their prototype will look like:

UIStream& operator<<(UIStream& os, std::ostream& (*pf)(std::ostream&));

(there are two others, replacing std::ostream by std::basic_ios and std::ios_base, which you have also to provide if you want to allows all manipulators) and their implementation will be very similar to the one of your template. In fact, so similar that you can use your template for implementation like this:

typedef std::ostream& (*ostream_manipulator)(std::ostream&);
UIStream& operator<<(UIStream& os, ostream_manipulator pf)
{
   return operator<< <ostream_manipulator> (os, pf);
}

A final note, often writing a custom streambuf is often a better way to achieve what one try to achieve applying to technique you are using.