Re: Implementing operator<<

From:

Ondra Holub <ondra.holub@post.cz>

Newsgroups:

comp.lang.c++

Date:

Thu, 17 Jan 2008 07:38:43 -0800 (PST)

Message-ID:

<24486ba1-24b0-4ce8-a613-5657939f3219@z17g2000hsg.googlegroups.com>

On 17 Led, 15:34, soy.h...@gmail.com wrote:

Hi all

I have a class StreamLogger which implements operator << in this way:

template <class T> StreamLogger& operator<<(const T& t)
{
  <print the stuff using fstream, etc.>

  return *this;

}

It works perfectly, using it like:
m_logger << "My ID is " << getID() << "\n";

but for some reason it does not compile this code:
m_logger << std::endl;

It gives the following error:
--------------------------------------------------------------------------------
.\src\Dealer.cpp(70) : error C2678: binary '<<' : no operator found
which takes a left-hand operand of type 'StreamLogger' (or there is no
acceptable conversion)
        C:\Program Files\Microsoft Visual Studio 8\VC\atlmfc\include
\afx.h(1428): could be 'CDumpContext &operator <<(CDumpContext
&,ATL::CTimeSpan)'
...
--------------------------------------------------------------------------------

Could you guys help me understand why it doesn't compile? (because I
dont have a clue...)

Thanks!!!

Jorge

You have to do following steps:

1. Create your own manipulator in std:
namespace std
{
    StreamLogger& endl(StreamLogger& s)
    {
        // Here do what manipulator should do
        return s;
    }

    StreamLogger& hex(StreamLogger& s)
    {
        // Here do what manipulator should do
        return s;
    }

    // Define all standard manipulators which do not have parameters
    // ...
}

2. Overload operator<< to accept function of the same type as
manipulator is:
StreamLogger& operator<<(StreamLogger& s, StreamLogger& (*manip)
(StreamLogger& s))
{
    return manip(s);
}

3. Use your manipulator in usual way:
StreamLogger sl;
sl << std::endl;

You should define all standard manipulators for your StreamLogger to
have standard interface. Do not ask me how to support anipulators with
parameters. I know it is (of course) possible, but I do not remember
the solution.