Writing classes which are "streamable"

Discussion in 'C++' started by Mikael, Dec 5, 2005.

  1. Mikael

    Mikael Guest

    Hi!

    I have written a logging class for my project and I want to support the
    stream syntax, that is I would like to write like this in my code:

    Log log("MYPREFIX");

    log << "This is a message, which is being sent to syslog, value ="
    << 5;

    I have created a friend function:

    Log& operator<< (Log& log, std::string& str);

    So now I can write:

    log << "Only strings are supported";

    However this doesn't work (of course):

    log << "This will create two log messages" << " why isn't this
    concatenated?";

    I wonder if someone knows about some documentation which could aid me
    in making my log class stream-compatible.


    Regards,

    Mikael
     
    Mikael, Dec 5, 2005
    #1
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  2. Mikael

    Rolf Magnus Guest

    Mikael wrote:

    > Hi!
    >
    > I have written a logging class for my project and I want to support the
    > stream syntax, that is I would like to write like this in my code:
    >
    > Log log("MYPREFIX");
    >
    > log << "This is a message, which is being sent to syslog, value ="
    > << 5;
    >
    > I have created a friend function:
    >
    > Log& operator<< (Log& log, std::string& str);


    Are you sure that your function needs to modify str? If not, why isn't it a
    const reference?

    > So now I can write:
    >
    > log << "Only strings are supported";
    >
    > However this doesn't work (of course):
    >
    > log << "This will create two log messages" << " why isn't this
    > concatenated?";


    What do you mean by "doesn't work", and why "of course"?

    > I wonder if someone knows about some documentation which could aid me
    > in making my log class stream-compatible.


    You could derive it from std::eek:stream, which will automatically give you all
    the stream operators defined for the standard output streams.
     
    Rolf Magnus, Dec 5, 2005
    #2
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  3. Mikael

    Geo Guest

    Mikael wrote:
    > Hi!
    >
    > I have written a logging class for my project and I want to support the
    > stream syntax, that is I would like to write like this in my code:
    >
    > Log log("MYPREFIX");
    >
    > log << "This is a message, which is being sent to syslog, value ="
    > << 5;
    >
    > I have created a friend function:
    >
    > Log& operator<< (Log& log, std::string& str);
    >
    > So now I can write:
    >
    > log << "Only strings are supported";
    >
    > However this doesn't work (of course):


    Post more code, what does

    Log& operator<< (Log& log, std::string& str);

    return ???
    >
    > log << "This will create two log messages" << " why isn't this
    > concatenated?";
    >
    > I wonder if someone knows about some documentation which could aid me
    > in making my log class stream-compatible.
    >
    >
    > Regards,
    >
    > Mikael
     
    Geo, Dec 5, 2005
    #3
  4. Mikael

    Mikael Guest

    The operator<< is defined as follows (I have added the const as you
    pointed out):

    Log& operator<< (Log& log, std::string const& str)
    {
    log.logMessage(str.c_str());
    return log;
    }

    Sorry if my question was too unspecific, I will try to be more verbose
    the next time.

    The above function generates two log messages when I write the
    following code:

    log << "message 1" << "message 2";

    because operator<<() calls log.logMessage() directly.

    If I derive from std::eek:stream, how do I actually get the formatted
    string into my log? If you could please clarify how it should work
    then?

    I am sorry if these are stupid questions, I am pretty new to streams (I
    have only used them sofar).


    With kind regards,

    Mikael
     
    Mikael, Dec 5, 2005
    #4
  5. Mikael

    Geo Guest

    Mikael wrote:
    > The operator<< is defined as follows (I have added the const as you
    > pointed out):
    >
    > Log& operator<< (Log& log, std::string const& str)
    > {
    > log.logMessage(str.c_str());
    > return log;
    > }
    >
    > Sorry if my question was too unspecific, I will try to be more verbose
    > the next time.
    >
    > The above function generates two log messages when I write the
    > following code:
    >
    > log << "message 1" << "message 2";
    >
    > because operator<<() calls log.logMessage() directly.
    >
    > If I derive from std::eek:stream, how do I actually get the formatted
    > string into my log? If you could please clarify how it should work
    > then?
    >
    > I am sorry if these are stupid questions, I am pretty new to streams (I
    > have only used them sofar).
    >
    >
    > With kind regards,
    >
    > Mikael


    Sorry, I'm not clear what you're question is anymore, if

    log << "message 1" << "message 2";

    generates two messages, then it would seem to be working correctly,
    what did you hope it would do ?

    You need to give an example of the output you are getting, the output
    you hoped to get and some detail of what logMessage(...) does if what
    you are seeing is not what you expected.
     
    Geo, Dec 5, 2005
    #5
  6. Mikael wrote:
    > I wonder if someone knows about some documentation which could aid me
    > in making my log class stream-compatible.


    Google for articles "James Kanze" or I have written about "streambuf".
    --
    <mailto:> <http://www.dietmar-kuehl.de/>
    <http://www.eai-systems.com> - Efficient Artificial Intelligence
     
    Dietmar Kuehl, Dec 5, 2005
    #6
  7. Mikael wrote:

    >
    > The operator<< is defined as follows (I have added the const as you
    > pointed out):
    >
    > Log& operator<< (Log& log, std::string const& str)
    > {
    > log.logMessage(str.c_str());
    > return log;
    > }
    >
    > Sorry if my question was too unspecific, I will try to be more verbose
    > the next time.
    >
    > The above function generates two log messages when I write the
    > following code:
    >
    > log << "message 1" << "message 2";


    Yes exactly, which is exactly the behaviour you are expected to get.

    > because operator<<() calls log.logMessage() directly.


    Yes of course. The above statement is (nearly) equivalent to the following
    two:

    log << "message 1";
    log << "message 2";

    I can only guess, but I think you belive that the "<<" operator concanates
    two char arrays? right? Well it does not. It sends a char array to a
    stream, and returns the stream. So what happens when you do
    log << "message 1" << "message 2";
    is:
    Log& tmplog = operator<<(log, "message 1");
    operator<<(tmplog, "message 2");
    And not:
    operator<<(log,"message1message2")
    as you suposedly expected.


    > If I derive from std::eek:stream, how do I actually get the formatted
    > string into my log? If you could please clarify how it should work
    > then?


    If you use std::strings instead of char arrays you can use the + operator:
    log << std::string("message 1") + "message 2";

    Or if you insist on using "<<", you have to change your Log::logMessage()
    member function to accumulate everything that it gets until you give it a
    special object, like (I have not checked if this compiles, just to get the
    idea):

    struct new_log_entry {};

    Log& operator<< (Log& log, new_log_entry const& d)
    {
    log.flush();
    return log;
    }

    ....
    log << "message 1" << "message 2" << new_log_entry;


    HTH

    Fabio
     
    Fabio Fracassi, Dec 5, 2005
    #7
  8. Mikael

    Marcus Kwok Guest

    Mikael <> wrote:
    > The operator<< is defined as follows (I have added the const as you
    > pointed out):
    >
    > Log& operator<< (Log& log, std::string const& str)
    > {
    > log.logMessage(str.c_str());
    > return log;
    > }
    >
    > Sorry if my question was too unspecific, I will try to be more verbose
    > the next time.
    >
    > The above function generates two log messages when I write the
    > following code:
    >
    > log << "message 1" << "message 2";
    >
    > because operator<<() calls log.logMessage() directly.


    My guess would be to check your log.logMessage() function to see if it
    is automatically adding a newline at the end of the message.

    --
    Marcus Kwok
     
    Marcus Kwok, Dec 5, 2005
    #8
  9. Mikael

    Mikael Guest

    Okay, I will try your proposal with the special object which flushes
    the accumulated string.

    I was also wondering if I could subclass or encapsulate a certain class
    to get all the different <<-operators for free as well as all
    modifiers, so that my log class has all formatting features from the
    ostream-class.

    I will try this approach:

    class Log : std::strstream
    {
    struct endl {};

    // other members

    friend Log& operator <<(Log& log, endl const& e);
    }

    Log& operator <<(Log& log, endl const& e)
    {
    char *message = str();
    log.logMessage (message);
    delete message;

    return log;
    }

    // test

    Log log;

    log << "This is a test i=" << 5 << Log::endl;
    ^_ std::strstream<<
    ^_ std::stream<<
    ^_ prints message


    With kind regards,

    Mikael
     
    Mikael, Dec 6, 2005
    #9
  10. Mikael wrote:
    > Hi!
    >
    > I have written a logging class for my project and I want to support the
    > stream syntax, that is I would like to write like this in my code:
    >
    > Log log("MYPREFIX");
    >
    > log << "This is a message, which is being sent to syslog, value ="
    > << 5;
    >
    > I have created a friend function:
    >
    > Log& operator<< (Log& log, std::string& str);
    >
    > So now I can write:
    >
    > log << "Only strings are supported";
    >
    > However this doesn't work (of course):
    >
    > log << "This will create two log messages" << " why isn't this
    > concatenated?";
    >
    > I wonder if someone knows about some documentation which could aid me
    > in making my log class stream-compatible.
    >
    >
    > Regards,
    >
    > Mikael
    >


    What you need is a function prototype like this

    std::eek:stream &operator << (std::eek:stream &str, Log &inp);

    then handle via a member function of Log or directly by making it a
    friend of Log.

    Once you've done this Log objects can be streamed via fstream, std::cout
    and stringstream.

    Similarly for input streaming use the following:-

    std::istream &operator >> (std::eek:stream &str, Log &inp);

    JB
     
    n2xssvv g02gfr12930, Dec 6, 2005
    #10
  11. n2xssvv g02gfr12930 wrote:
    > Mikael wrote:
    >
    >> Hi!
    >>
    >> I have written a logging class for my project and I want to support the
    >> stream syntax, that is I would like to write like this in my code:
    >>
    >> Log log("MYPREFIX");
    >>
    >> log << "This is a message, which is being sent to syslog, value ="
    >> << 5;
    >>
    >> I have created a friend function:
    >>
    >> Log& operator<< (Log& log, std::string& str);
    >>
    >> So now I can write:
    >>
    >> log << "Only strings are supported";
    >>
    >> However this doesn't work (of course):
    >>
    >> log << "This will create two log messages" << " why isn't this
    >> concatenated?";
    >>
    >> I wonder if someone knows about some documentation which could aid me
    >> in making my log class stream-compatible.
    >>
    >>
    >> Regards,
    >>
    >> Mikael
    >>

    >
    > What you need is a function prototype like this
    >
    > std::eek:stream &operator << (std::eek:stream &str, Log &inp);
    >
    > then handle via a member function of Log or directly by making it a
    > friend of Log.
    >
    > Once you've done this Log objects can be streamed via fstream, std::cout
    > and stringstream.
    >
    > Similarly for input streaming use the following:-
    >
    > std::istream &operator >> (std::eek:stream &str, Log &inp);
    >
    > JB

    Just to clarify, these will enable to do things like this this:-

    Log Val;
    std::cin >> Log;
    std::cout << "Log value is " << Val << " OK" << std::endl;

    JB
     
    n2xssvv g02gfr12930, Dec 6, 2005
    #11
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