Re: question regarding operator << overloading

From:
"Victor Bazarov" <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 22 May 2008 21:31:41 -0400
Message-ID:
<xcSdnWKZW83mg6vVnZ2dnUVZ_s_inZ2d@comcast.com>
Goran wrote:

// main.c++: In function ?std::ostream&
BADLIB::operator<<(std::ostream&, BADLIB::BadClass_t&)?:
// main.c++:110: Error: no match for ?operator<<? in ?aStream <<
BADLIB::BadClass_t::GetValue() const()?


It would be also good to know which line is 110, don't you think?

[..]
namespace GOODLIB {

  class GoodClass_t {

  public:
    GoodClass_t();
    string GetValue() const;
    void SetMode(const bool&);
    bool GetMode() const;
  private:
    string itsValue;
    bool itsMode;
  };
  // this operator works
  ostream& operator <<(ostream&, GoodClass_t&);


Note that 'GoodClass_t' object is *non-const*. Is there a reason
why this is so?

}

[..]
void
GOODLIB::
GoodClass_t::
SetMode(const bool& aMode) {

  itsMode = aMode;
}

ostream&
GOODLIB::
operator <<(ostream& aStream, GoodClass_t& aWC) {

  if(aWC.GetMode()) {
    aWC.SetMode(false);


So, while *outputting* an object you *set* its mode? I don't know.
Does not seem like a good idea. Anyway...

    aStream << "Begin Opening (" << aWC.GetValue() << ") End Opening
\n";
  }
  else {
    aWC.SetMode(true);
    aStream << "Begin Closing (" << aWC.GetValue() << ") End Closing
\n";
  }

  return aStream;
}
//// END GOOD CLASS ////

//// START BAD CLASS////

// The BadClass_t contains a private GoodClass_t Object available by
// GetValue(). So it's very simple. But in conjunction with
GoodClass_t I
// really don't know where's the failure.
using namespace std;
using namespace GOODLIB;

namespace BADLIB {

  class BadClass_t {

  public:
    BadClass_t(const GoodClass_t&);
    GoodClass_t GetValue() const;


So, this class returns an rvalue as well...

  private:
    GoodClass_t itsGoodObj;
  };
  ostream& operator <<(ostream&, /* const */ BadClass_t&);
}

[..]
ostream&
BADLIB::
operator <<(ostream& aStream, /* const */ BadClass_t& aBC) {

  aStream << aBC.GetValue() << "\nsome stuff\n" << aBC.GetValue();


So, let's examine this. 'aBC.GetValue()' returns a temporary,
which you try to pass to the op<< that expects an lvalue. The
argument of the op<< is a reference to non-const that cannot be
bound to a temporary.

You need to revise your interface.

}
//// END BAD CLASS////
[..]


V
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